How to possibly prevent cylinder scoring
Discussion
Although our low temperature thermostat is something that we think could make these engines less likely to score bores - I have also been trying hard to work out what driving conditions might be more likely to lead to scored bores in the first place – to be able to offer some advice on how best to avoid the consequences. Feedback suggests it is not people driving around in track situations that are particularly any more likely to experience this problem than anyone else – so it does not seem to be purely down to revs and hard driving entirely.
Furthermore – for those only ever driving on public roads - for some to fail so soon while others never seem to – or when some owners never have the problem while others do – there must be some differences - and if we could identify them (and back them up with technical/scientific support) it could possibly help avoid the consequences altogether.
I think I may have worked out a typical scenario – best avoided – here is the evidence and connected technical issues.
Scoring was more commonly caused by full piston seizures in which the piston grew too big to fit in the bore with some clearance and the resulting additional force between the piston and cylinder wall squeezed out the oil film resulting in metal to metal contact – the increased friction causing local melting of the piston surface and scoring both piston and bore on both sides. Dynamometer tests I have previously carried out to destruction (to measure temperatures etc) showed this cycle took about 4-5 seconds.
The odd thing about this current problem is that one side of the piston and bore looks exactly like a fully scored seized piston but the other side is perfectly OK and untouched. It is therefore clear that when the scoring occurs – one side of the piston is sufficiently cool to lubricate the piston/bore face and support the piston whereas the other side has become excessively hot and the oil film cannot support the loads applied – which result in metal to metal contact – overheating of the surface and scoring the bores and piston on one side only.
It is always the thrust side that scores (proving the additional load between the piston and cylinder wall is the problem) but it cannot be the piston temperature overall – or the other side would also seize. Although the thermostat controls overall coolant speed (from the radiator efficiency) the coolant flow in individual parts of these blocks is much slower than traditional engine designs (because it is split into 12 separate flows streams instead of one (each therefore at least 1/12 of the speed) – the 6 through the block being very much slower than the 6 through the cylinder heads and the individual speed past any one cylinder many times slower) so the temperature of the coolant as it slowly passes over the outer cylinder wall increases more giving a greater temperature rise in any one area. It is almost always bank 2 (because on that bank the thrust face is on the hottest side of the block – as the coolant is about to leave the cylinder having picked up all the heat on its travel through the block - whereas on bank 1 the coolest coolant enters on the thrust side).
For the cylinder wall to get so hot on one side that the engine piston scores must require (IMHO) some bubbling of the coolant on the surface of the outer cylinder wall and this usually only occurs when coolant speeds would be very slow indeed and bubbles would naturally rise to the top (the thrust side on bank 2 – not on bank 1). But it would also need high piston to cylinder wall forces as well to squeeze out the oil film and make hard contact between the piston and Lokasil bore.
Centrifugal coolant pumps have relatively low pumping speeds until they reach a critical speed after which they suddenly increase flow dramatically.
Radiators cool more when the road speed is higher (when their efficiency increases).
The engines in question (3.4 Cayman S, 996 and 997 3.6 and 3.8) produce more torque at low revs than their predecessors.
There is a piston cooling affect from inlet charge when the throttle is open momentarily lost as the throttle is closed.
Spray jets in the block lubricate the cylinder wall but have ball valves that require some oil pressure to open and the oil pressure and delivery is low at low revs – rising until the pressure relief valve opens when it is constant at much higher pressure and delivery providing much better cylinder wall lubrication.
Crankshaft speed also distributes more cylinder oil spray at higher speeds.
The relatively long distance from the engine to the radiators reduces cooling reaction time.
The thermostat is on the way back to the engine so if the engine is ticking over after a run the cooler coolant that has just come from the radiator will slow down as it reaches the thermostat and for a while – will not encourage it to open more – in fact it would probably start to shut a little.
Putting all this together – if we had a scenario in which the engine was under quite high load and road speed but then it suddenly went to tickover speed for a short while – at this point the piston would be very hot (no inlet charge), the coolant speed would be very low (pump slow), the heat passing from the cylinder outer wall to the coolant would still be high (probably leading to bubbling which is very poor for heat dissipation), the oil spray cylinder wall jets output and crankshaft rotational speed would be very low (so reduced cylinder wall lubrication) and while the coolant in the block would be very hot the coolant near the thermostat would be quite cool.
If just then the car was driven off under high torque/full or nearly full throttle (not necessarily at high revs) the pistons would instantaneously heat up, the dynamic forces between the piston and the cylinder wall would be at their highest, the cylinder wall and oil film temperature would be very high, the thermostat would not be fully open, the car would not yet be at a high enough road speed for the radiator to cool the coolant effectively and the oil spray jet and crankshaft speed splash lubrication would not yet be good.
So – put simply – I think that if the owner of one of these models – on a spirited drive - could not then resist the temptation to floor the car away from standstill (at say lights or junctions) - then I think we could have the worst possible scenario for piston/cylinder scoring at that precise moment.
This would also explain why – once the models are up to temperature and having had the speed gradually increased around say a circuit or long unrestricted roads (say in Germany) they seem to be OK despite the extra strain put on the engines (because thermostats are more open, radiator cooling is good, engine speed is high so oil spray etc is good et)c.
I agree such a proposition is almost madness – I can almost hear you all shouting “what the bl**dy hell do you think I bought it for” - why would anyone buy such a car if they cannot at least feel that acceleration and enjoy that performance (not to mention the satisfaction of showing everyone just how quick the car is when in reality none of us can safely or legally demonstrate anything like the cars top speeds anywhere in the UK – but we can out accelerate most others). Surely that exact scenario is partly why we bought it and high on the list of the pleasure we get from driving it and possibly the only occasion we can stuff it to other drivers! I am not against that – in fact I also love it myself (truth be known).
So I am not proposing you all drive around like old men in a Ford Fiesta – (I am of that age – but still not a slow driver either)but I am just suggesting that – it makes sense to me – that weighing up all the evidence and connecting it to solid technical analysis - this is the most likely moment of failure and therefore – it could be that if you could simply accelerate from a stationary rest - a little more gently – under slightly less throttle opening -for a few seconds until the coolant speed, oil pressure and thermostat opening times and the balance of the oil and lubrication system have equalised again – it may just save the expense and disappointment of an engine failure.
Baz
Furthermore – for those only ever driving on public roads - for some to fail so soon while others never seem to – or when some owners never have the problem while others do – there must be some differences - and if we could identify them (and back them up with technical/scientific support) it could possibly help avoid the consequences altogether.
I think I may have worked out a typical scenario – best avoided – here is the evidence and connected technical issues.
Scoring was more commonly caused by full piston seizures in which the piston grew too big to fit in the bore with some clearance and the resulting additional force between the piston and cylinder wall squeezed out the oil film resulting in metal to metal contact – the increased friction causing local melting of the piston surface and scoring both piston and bore on both sides. Dynamometer tests I have previously carried out to destruction (to measure temperatures etc) showed this cycle took about 4-5 seconds.
The odd thing about this current problem is that one side of the piston and bore looks exactly like a fully scored seized piston but the other side is perfectly OK and untouched. It is therefore clear that when the scoring occurs – one side of the piston is sufficiently cool to lubricate the piston/bore face and support the piston whereas the other side has become excessively hot and the oil film cannot support the loads applied – which result in metal to metal contact – overheating of the surface and scoring the bores and piston on one side only.
It is always the thrust side that scores (proving the additional load between the piston and cylinder wall is the problem) but it cannot be the piston temperature overall – or the other side would also seize. Although the thermostat controls overall coolant speed (from the radiator efficiency) the coolant flow in individual parts of these blocks is much slower than traditional engine designs (because it is split into 12 separate flows streams instead of one (each therefore at least 1/12 of the speed) – the 6 through the block being very much slower than the 6 through the cylinder heads and the individual speed past any one cylinder many times slower) so the temperature of the coolant as it slowly passes over the outer cylinder wall increases more giving a greater temperature rise in any one area. It is almost always bank 2 (because on that bank the thrust face is on the hottest side of the block – as the coolant is about to leave the cylinder having picked up all the heat on its travel through the block - whereas on bank 1 the coolest coolant enters on the thrust side).
For the cylinder wall to get so hot on one side that the engine piston scores must require (IMHO) some bubbling of the coolant on the surface of the outer cylinder wall and this usually only occurs when coolant speeds would be very slow indeed and bubbles would naturally rise to the top (the thrust side on bank 2 – not on bank 1). But it would also need high piston to cylinder wall forces as well to squeeze out the oil film and make hard contact between the piston and Lokasil bore.
Centrifugal coolant pumps have relatively low pumping speeds until they reach a critical speed after which they suddenly increase flow dramatically.
Radiators cool more when the road speed is higher (when their efficiency increases).
The engines in question (3.4 Cayman S, 996 and 997 3.6 and 3.8) produce more torque at low revs than their predecessors.
There is a piston cooling affect from inlet charge when the throttle is open momentarily lost as the throttle is closed.
Spray jets in the block lubricate the cylinder wall but have ball valves that require some oil pressure to open and the oil pressure and delivery is low at low revs – rising until the pressure relief valve opens when it is constant at much higher pressure and delivery providing much better cylinder wall lubrication.
Crankshaft speed also distributes more cylinder oil spray at higher speeds.
The relatively long distance from the engine to the radiators reduces cooling reaction time.
The thermostat is on the way back to the engine so if the engine is ticking over after a run the cooler coolant that has just come from the radiator will slow down as it reaches the thermostat and for a while – will not encourage it to open more – in fact it would probably start to shut a little.
Putting all this together – if we had a scenario in which the engine was under quite high load and road speed but then it suddenly went to tickover speed for a short while – at this point the piston would be very hot (no inlet charge), the coolant speed would be very low (pump slow), the heat passing from the cylinder outer wall to the coolant would still be high (probably leading to bubbling which is very poor for heat dissipation), the oil spray cylinder wall jets output and crankshaft rotational speed would be very low (so reduced cylinder wall lubrication) and while the coolant in the block would be very hot the coolant near the thermostat would be quite cool.
If just then the car was driven off under high torque/full or nearly full throttle (not necessarily at high revs) the pistons would instantaneously heat up, the dynamic forces between the piston and the cylinder wall would be at their highest, the cylinder wall and oil film temperature would be very high, the thermostat would not be fully open, the car would not yet be at a high enough road speed for the radiator to cool the coolant effectively and the oil spray jet and crankshaft speed splash lubrication would not yet be good.
So – put simply – I think that if the owner of one of these models – on a spirited drive - could not then resist the temptation to floor the car away from standstill (at say lights or junctions) - then I think we could have the worst possible scenario for piston/cylinder scoring at that precise moment.
This would also explain why – once the models are up to temperature and having had the speed gradually increased around say a circuit or long unrestricted roads (say in Germany) they seem to be OK despite the extra strain put on the engines (because thermostats are more open, radiator cooling is good, engine speed is high so oil spray etc is good et)c.
I agree such a proposition is almost madness – I can almost hear you all shouting “what the bl**dy hell do you think I bought it for” - why would anyone buy such a car if they cannot at least feel that acceleration and enjoy that performance (not to mention the satisfaction of showing everyone just how quick the car is when in reality none of us can safely or legally demonstrate anything like the cars top speeds anywhere in the UK – but we can out accelerate most others). Surely that exact scenario is partly why we bought it and high on the list of the pleasure we get from driving it and possibly the only occasion we can stuff it to other drivers! I am not against that – in fact I also love it myself (truth be known).
So I am not proposing you all drive around like old men in a Ford Fiesta – (I am of that age – but still not a slow driver either)but I am just suggesting that – it makes sense to me – that weighing up all the evidence and connecting it to solid technical analysis - this is the most likely moment of failure and therefore – it could be that if you could simply accelerate from a stationary rest - a little more gently – under slightly less throttle opening -for a few seconds until the coolant speed, oil pressure and thermostat opening times and the balance of the oil and lubrication system have equalised again – it may just save the expense and disappointment of an engine failure.
Baz
Some great positive and intelligent suggestions recently - most of which would/could make a difference and many would probably solve the problem - however all involve major changes to cars that do not yet have the scoring problem and the trouble is that most owners I would not expect to incurr the expense and inconvenience until it is too late when they realise perhaps they should have done. Many are blinded by the product's reputation and cannot believe the reported danger until it hits them.
Indeed most don't seem to find out about all this until it is too late and research who can fix it.
The take up of the low temperature thermostat has been extremely small (despite the obvious advantages, low cost and reported benefits) - so on that basis I was trying to think of something simple - that costs nothing - that owners might just hear about and manage to follow to help avoid damage - just simply to restrain themselves from too much enthusiasm on opening imediately up after a spell at rest when the engine had been used hard before the stop.
Baz
Indeed most don't seem to find out about all this until it is too late and research who can fix it.
The take up of the low temperature thermostat has been extremely small (despite the obvious advantages, low cost and reported benefits) - so on that basis I was trying to think of something simple - that costs nothing - that owners might just hear about and manage to follow to help avoid damage - just simply to restrain themselves from too much enthusiasm on opening imediately up after a spell at rest when the engine had been used hard before the stop.
Baz
Hi Rockster, sorry I don't know your background but you have managed to put together a load of scientific facts that are basically right and then allowed the conclusion to be the opposite of both common and technical sense and practical experience.
I explained exactly under what circumstances a stop start could occur (you didn't read it all did you?) and even explained why they are OK in racing conditions - so please don't muddy the waters by suggesting otherwise.
What test is it that shows that oil is better at higher temperatures - everyone knows it degrades, is thinner and doesn't do its job as well when engines run too hot - that's why they suffer crankshaft and bore failures when the engines overheat and not when everything is nice and cool.
You correctly confirm what I said that the coolant is going more slowly through the engine - can you not manage to work out that this must mean that it picks up more temperature than if it was going faster and therefore the temperature rise is greater (which is my point). if therefore the thermostat is set higher to start with AND the coolant is going slowly - the output temperature must be much higher and therefore so must the top of the bank 2 cylinder wall.
Out tests showed that the coolant boils under pressure at about 25 to 30 degrees above atmospheric boiling point - but this is anyway not really relevant because if fan speed is higher this cools the coolant in the system more and therefore lowers the entry temperature to the engine through the thermostat. It is impossible to argue against the fact that if the inlet temperature is controlled by a thermostat - AND the amount of cooling to drop the coolant to that temperature (which it does as it is controlled by the coolant flow through the thermostat) then the outlet temperature from the engine simply must be higher. Our tests also showed that although it boils at a higher temperature it starts bubbling much sooner at lower temperatures - largely because of localised heat.
Now lets ask another practical question - what do you do when your baked beans start bubbling in the pan - stir it, what do you do is heating milk starts to boil - stir it, there is no doubt that the slower the coolant speed across the surface the more it is likely to bubble. This is exactly what will happen when you stop for a short while and the engine slows down (and coolant pump) - there can be no argument to support the idea that just then the coolant speeds up - it obviously does not - hence bubbling more likely.
You are also ignoring the fact that the overall temperature of the coolant is the result of it mixing back together from the head and the block - without doing the tests we did - you would not know what temperature the coolant from the block or head is before it mixes back together - I do - and what happens when we stop and start and drive fast etc - because we have tested just that with two cars witted with multiple temperature sensors in the engine and recording on different gauges - Have you done this?
Once a piston has scored it no longer produces the same compression pressure before ignition and therefore runs much cooler - hence the oil temperature at the surface of the piston to cylinder wall interface is actually lower and the rings have less to do.
Your comments that scoring just doesn't actually happen makes me wonder if you are working for Porsche under instruction to mix a load of technical rights with a load of nonsense conclusions to dilute the truth from being established. You seem to be living in a theoretical dream world in which you can persuade yourself that reality doesn't exist if your technical knowledge suggests to you it is impossible - but without practical experience and tests your conclusions are anyway totally flawed. Your posting seems to me to be from an academic with little practical experience and it is only that practical hands on work combined with technical knowledge that enables engineers to work out how they fit together and what actually happens inside the engine.
Your last sentence is in all honesty disgraceful and I hope you are never confronted by the hundreds of owners faced with a financial disaster because they expected a Porsche to be better engineered than it obviously is - or for the manufacturers to do something to help them when faced with an obvious technical design shortcoming.
I suppose at least you have managed to manipulate me to spending more time defending my advice than providing more (perhaps that was the aim and I fell for it) so at risk of allowing more similar nonsense to be posted and the resulting confusion from limiting the public's understanding - I am going to ignore your further posts and concentrate on advising those that can see through these and similar posts (and I am grateful to those supporting me with similar common sense practical points and experiences that prove that these engines have a bore scoring problem).
I am still not sure why anyone would be motivated to discredit advice intended to help avoid damage - that can do no harm whatsoever to the car and could only help - unless there are more sinister motives - or they are the rantings of someone who has no practical experience and cannot bear to see someone else appearing to know the score and hold a place on the internet as an expert.
I find it very sad that someone so obviously knowledgeable can spend so long discrediting someone with proven knowledge, experience and who is fully technically qualified, who has out-designed the might of the Japanese motorcycle industry and who clearly has resolved many of the existing problems and is simply offering some advice that might help. Presumably you prefer people to thrash their cars immediately after being forced to sit stationary for a while (because you think that could not cause scoring) rather than consider that there might be some good reasoning in my advice (combined with actual practical road tests and temperature readings) and therefore - as no harm could possibly be dome by my suggestions - just keep out of it.
Baz
I explained exactly under what circumstances a stop start could occur (you didn't read it all did you?) and even explained why they are OK in racing conditions - so please don't muddy the waters by suggesting otherwise.
What test is it that shows that oil is better at higher temperatures - everyone knows it degrades, is thinner and doesn't do its job as well when engines run too hot - that's why they suffer crankshaft and bore failures when the engines overheat and not when everything is nice and cool.
You correctly confirm what I said that the coolant is going more slowly through the engine - can you not manage to work out that this must mean that it picks up more temperature than if it was going faster and therefore the temperature rise is greater (which is my point). if therefore the thermostat is set higher to start with AND the coolant is going slowly - the output temperature must be much higher and therefore so must the top of the bank 2 cylinder wall.
Out tests showed that the coolant boils under pressure at about 25 to 30 degrees above atmospheric boiling point - but this is anyway not really relevant because if fan speed is higher this cools the coolant in the system more and therefore lowers the entry temperature to the engine through the thermostat. It is impossible to argue against the fact that if the inlet temperature is controlled by a thermostat - AND the amount of cooling to drop the coolant to that temperature (which it does as it is controlled by the coolant flow through the thermostat) then the outlet temperature from the engine simply must be higher. Our tests also showed that although it boils at a higher temperature it starts bubbling much sooner at lower temperatures - largely because of localised heat.
Now lets ask another practical question - what do you do when your baked beans start bubbling in the pan - stir it, what do you do is heating milk starts to boil - stir it, there is no doubt that the slower the coolant speed across the surface the more it is likely to bubble. This is exactly what will happen when you stop for a short while and the engine slows down (and coolant pump) - there can be no argument to support the idea that just then the coolant speeds up - it obviously does not - hence bubbling more likely.
You are also ignoring the fact that the overall temperature of the coolant is the result of it mixing back together from the head and the block - without doing the tests we did - you would not know what temperature the coolant from the block or head is before it mixes back together - I do - and what happens when we stop and start and drive fast etc - because we have tested just that with two cars witted with multiple temperature sensors in the engine and recording on different gauges - Have you done this?
Once a piston has scored it no longer produces the same compression pressure before ignition and therefore runs much cooler - hence the oil temperature at the surface of the piston to cylinder wall interface is actually lower and the rings have less to do.
Your comments that scoring just doesn't actually happen makes me wonder if you are working for Porsche under instruction to mix a load of technical rights with a load of nonsense conclusions to dilute the truth from being established. You seem to be living in a theoretical dream world in which you can persuade yourself that reality doesn't exist if your technical knowledge suggests to you it is impossible - but without practical experience and tests your conclusions are anyway totally flawed. Your posting seems to me to be from an academic with little practical experience and it is only that practical hands on work combined with technical knowledge that enables engineers to work out how they fit together and what actually happens inside the engine.
Your last sentence is in all honesty disgraceful and I hope you are never confronted by the hundreds of owners faced with a financial disaster because they expected a Porsche to be better engineered than it obviously is - or for the manufacturers to do something to help them when faced with an obvious technical design shortcoming.
I suppose at least you have managed to manipulate me to spending more time defending my advice than providing more (perhaps that was the aim and I fell for it) so at risk of allowing more similar nonsense to be posted and the resulting confusion from limiting the public's understanding - I am going to ignore your further posts and concentrate on advising those that can see through these and similar posts (and I am grateful to those supporting me with similar common sense practical points and experiences that prove that these engines have a bore scoring problem).
I am still not sure why anyone would be motivated to discredit advice intended to help avoid damage - that can do no harm whatsoever to the car and could only help - unless there are more sinister motives - or they are the rantings of someone who has no practical experience and cannot bear to see someone else appearing to know the score and hold a place on the internet as an expert.
I find it very sad that someone so obviously knowledgeable can spend so long discrediting someone with proven knowledge, experience and who is fully technically qualified, who has out-designed the might of the Japanese motorcycle industry and who clearly has resolved many of the existing problems and is simply offering some advice that might help. Presumably you prefer people to thrash their cars immediately after being forced to sit stationary for a while (because you think that could not cause scoring) rather than consider that there might be some good reasoning in my advice (combined with actual practical road tests and temperature readings) and therefore - as no harm could possibly be dome by my suggestions - just keep out of it.
Baz
Although I could get into a lengthy discussion about cooling and heat transfer etc I am sorry but I don't see the relevance. The only things we can change with these cars without stripping and rebuilding them (and even then we cannot change the material or much of the design) is the temperature of the coolant sitting next to the cylinder wall and the speed that it circulates.
There must be no one who really thinks that slowing it down is going to make the surface between the piston and the cylinder wall cooler and no one who thinks that if it is hotter when it goes into the engine it will somehow cool the cylinders down more than if it were cooler.
So lets confine our thoughts to these simple but totally relevant issues and accept that anything that reduces the initial temperature of the coolant going into the engine will lower the temperature at the cylinder wall and as the thermostat is on the way in and controls the in going temperature by adjusting the flow rate - the only way to get it to enter cooler is to run a lower temperature thermostat.
Secondly, there is surely no one that thinks that after driving fast and suddenly stopping - the engine internals suddenly cool by themselves - I think everyone realises they see a rise - so increasing engine revs while driving off again must help speed up circulation to the radiators and back to the engine = quicker cooling while increasing revs provide more oil spray to cool the piston and crankshaft splash to lubricate the cylinder walls (all with oil that has gone through the oil cooler so is cooler than before when it sat inside the engine).
Finally the scoring only takes place when one side (the thrust side) of the piston loses the oil film strength keeping it away from the cylinder wall. Is there anyone that thinks opening the throttle full open somehow provides less thrust force on that very face.
That being the case, using a lower temperature thermostat and/or not giving the engine full throttle immediately after an enforced rest and keeping the revs up a bit as you drive off - can only help.
All other thoughts - while interesting theoretically and no doubt interesting to some - have no bearing on what owners can practically do to help reduce the incidences of cylinder scoring - which is what I opened the posting about and perhaps are more at home in a new subject about the theory of heat transfer.
Baz
There must be no one who really thinks that slowing it down is going to make the surface between the piston and the cylinder wall cooler and no one who thinks that if it is hotter when it goes into the engine it will somehow cool the cylinders down more than if it were cooler.
So lets confine our thoughts to these simple but totally relevant issues and accept that anything that reduces the initial temperature of the coolant going into the engine will lower the temperature at the cylinder wall and as the thermostat is on the way in and controls the in going temperature by adjusting the flow rate - the only way to get it to enter cooler is to run a lower temperature thermostat.
Secondly, there is surely no one that thinks that after driving fast and suddenly stopping - the engine internals suddenly cool by themselves - I think everyone realises they see a rise - so increasing engine revs while driving off again must help speed up circulation to the radiators and back to the engine = quicker cooling while increasing revs provide more oil spray to cool the piston and crankshaft splash to lubricate the cylinder walls (all with oil that has gone through the oil cooler so is cooler than before when it sat inside the engine).
Finally the scoring only takes place when one side (the thrust side) of the piston loses the oil film strength keeping it away from the cylinder wall. Is there anyone that thinks opening the throttle full open somehow provides less thrust force on that very face.
That being the case, using a lower temperature thermostat and/or not giving the engine full throttle immediately after an enforced rest and keeping the revs up a bit as you drive off - can only help.
All other thoughts - while interesting theoretically and no doubt interesting to some - have no bearing on what owners can practically do to help reduce the incidences of cylinder scoring - which is what I opened the posting about and perhaps are more at home in a new subject about the theory of heat transfer.
Baz
I have measured on a dyno what happens when an engine is about to seize and then seizes by changing a parameter suddenly (mixture - coolant flow etc) It takes about 3-4 seconds from starting to overheat to the piston dramatically overheating and causing seizures. Whatever people theorise about that is a fact I have checked.
Now on one forum someone had exactly the failure I suggested immediately after that stop and then flat out start scenario.
Bubbles can appear very quickly and if bubbles appear at the cylinder wall the subsequent cooling affect in that small area is effectively zero.
If 996 3.4's are OK but cayman S, 996 3.6 and 997 3.6 and 3.8 models fail we have to ask ourselves what are the differences.
General design - no, piston material and surface finish - no, cylinder bore material or thickness - no, general cooling design - no, more force on the piston face at low speed full throttle driving - yes, higher coolant temperature inside the cylinder block - yes.
Ask yourselves what link there could possibly be between higher piston face forces and temperatures and failures - it can only be the oil film going beyond some safe limit - what is different - the temperature and pressure. Nothing else in this scenario really matters - as many others have said if you turbo charge a standard engine you can get the same problem for which a period of revs without load is usually a suitable solution.
996 3.4's will do this scoring IF there is something wrong with the coolant flow or the coolant pressure going low (due to radiator leaks or pressure cap or expansion tank cracks or leaks - does this give us a clue - I would have thought so.
Why so many people have to argue about a simple suggestion to help protect engines (that is actually perfectly good practice anyway) I don't know. At every step I supported reasoning with several different simple physical or technical facts about engines and cooling - there was nothing contradictory about it.
Yes the top of the RH bank 2 is the thrust face and is hotter because the coolant enters at the bottom of both banks and the bank 1 thrust face is on that bottom. Again simple common sense links the failures being on bank 2 to this simple observation and logical technical link.
Although (as someone somewhere stated) the coolant is travelling slower this also means it picks up a greater proportion of the heat - so bank 2 top will be hotter than bank 1 top.
What is it about linking the thrust face being on the hot side of bank 2, the temperatures being higher than the previous almost identical engines that were OK and the thrust face forces being higher - that is difficult to follow and although plenty say I am wrong - where is their similar logical progressive link through several factors that I have measured and proven that they come up with as a plausable explanation for the failures - or is it simply - like some people say about the wartime horrors - that they simply didn't
happen. Heads well in the sand, bums in the air - frankly some deserve to be kicked.
This is not just a question of opinion that we all have a right to express - it is a fact that a lot are scoring bores - there is no opinion about that. If one person analyses factors involved and others dissagree - where are their alternatives that I can contradict - or is it just impossible for some people to just say - that's interesting - thanks.
Amongst critics there are those that state the thermostat is on exit, that the hotter the engine is the better the oil protects it, it is gravity that causes the problem etc etc - well intentioned may be but totally off the track and simply adding confusion to those not sure about such issues.
To all those kind and sensible people who have supported me on this a big thanks and appreciation.
I might well not have it all right but why try and stop people from simply taking it a bit easier for a short while when leaving a stationary period - when there is a good case made out to suggest it might help - when in so doing it may well damage their engines?
Baz
Now on one forum someone had exactly the failure I suggested immediately after that stop and then flat out start scenario.
Bubbles can appear very quickly and if bubbles appear at the cylinder wall the subsequent cooling affect in that small area is effectively zero.
If 996 3.4's are OK but cayman S, 996 3.6 and 997 3.6 and 3.8 models fail we have to ask ourselves what are the differences.
General design - no, piston material and surface finish - no, cylinder bore material or thickness - no, general cooling design - no, more force on the piston face at low speed full throttle driving - yes, higher coolant temperature inside the cylinder block - yes.
Ask yourselves what link there could possibly be between higher piston face forces and temperatures and failures - it can only be the oil film going beyond some safe limit - what is different - the temperature and pressure. Nothing else in this scenario really matters - as many others have said if you turbo charge a standard engine you can get the same problem for which a period of revs without load is usually a suitable solution.
996 3.4's will do this scoring IF there is something wrong with the coolant flow or the coolant pressure going low (due to radiator leaks or pressure cap or expansion tank cracks or leaks - does this give us a clue - I would have thought so.
Why so many people have to argue about a simple suggestion to help protect engines (that is actually perfectly good practice anyway) I don't know. At every step I supported reasoning with several different simple physical or technical facts about engines and cooling - there was nothing contradictory about it.
Yes the top of the RH bank 2 is the thrust face and is hotter because the coolant enters at the bottom of both banks and the bank 1 thrust face is on that bottom. Again simple common sense links the failures being on bank 2 to this simple observation and logical technical link.
Although (as someone somewhere stated) the coolant is travelling slower this also means it picks up a greater proportion of the heat - so bank 2 top will be hotter than bank 1 top.
What is it about linking the thrust face being on the hot side of bank 2, the temperatures being higher than the previous almost identical engines that were OK and the thrust face forces being higher - that is difficult to follow and although plenty say I am wrong - where is their similar logical progressive link through several factors that I have measured and proven that they come up with as a plausable explanation for the failures - or is it simply - like some people say about the wartime horrors - that they simply didn't
happen. Heads well in the sand, bums in the air - frankly some deserve to be kicked.
This is not just a question of opinion that we all have a right to express - it is a fact that a lot are scoring bores - there is no opinion about that. If one person analyses factors involved and others dissagree - where are their alternatives that I can contradict - or is it just impossible for some people to just say - that's interesting - thanks.
Amongst critics there are those that state the thermostat is on exit, that the hotter the engine is the better the oil protects it, it is gravity that causes the problem etc etc - well intentioned may be but totally off the track and simply adding confusion to those not sure about such issues.
To all those kind and sensible people who have supported me on this a big thanks and appreciation.
I might well not have it all right but why try and stop people from simply taking it a bit easier for a short while when leaving a stationary period - when there is a good case made out to suggest it might help - when in so doing it may well damage their engines?
Baz
Look chaps - I don't have a lot of time for all this - I spend hours trying to solve problems created by a much bigger and wealthier organisation than mine and then more time trying to advise people what I find out - only to have - in some cases - very ill informed questioning requiring me (they think) to spend even more time explaining why their opinion may just be out of line - or completely wrong - or indeed exactly right.
I honestly don't have the time and when the questions clearly demonstrate very little knowledge of the engine or little practical experience - or - are just put there to glorify someone else's ego - I do get tired of being expected to patiently advise all and sundry - yes I should be more patient - yes I should spend more time just advising people that know very very little anyway - yes I should be less touchy - but I am also trying to find out much more advanced issues, design solutions, develop tests and answers and run a busy business - one of the few doing anything at all about a serious design error.
So - I am sorry I am such a grumpy old man - sorry I think I know more than most people - but honestly - if most just took on board what I offer and resisted arguing all the time (mostly with frankly very poor technical comment or completely inaccurate points) - I could get time to offer a lot more advice that might be more useful to readers than fighting silly arguments and answering questions raised by people for whom the answer will not help them, not change their mind but just take my time.
Now after that genuine apology - the S2 piston has little taper and the cylinder bore area (in common with the later liquid cooled engines) has relatively shallow coolant block depth. It does run tight and can seize at the top - but ALL these scored bores occur down the bottom of the piston - on the thrust side - they have nothing to do with ignition or fuelling - and they are not a full seizure - only on one side.
This means that while the piston is pushing hard against the thrust side (like the piston had expanded too much like most seizures) the other side is not hard against the bore and is lubricated perfectly OK.
The thrust load is not all that high but the temperature is and therefore the oil is extremely thin and watery - poor support = scoring.
The thicker the cylinder wall the less heat can be transferred through it quickly - but the problem is really one of bubbles in my opinion.
If the cylinder was upright (or almost upright) bubbles naturally rise away from the hot spot and new coolant rushes in its place - but in a horizontal engine with slow coolant flow - it is trapped. The outside of the block is almost exactly the same size as the original 2.5 Boxster and has been increased through the Boxster S, 3.4 and 3.8 engines until there is very little clearance between the outer cylinder wall and the block - so very little distance for bubbles to float away and be replaced by coolant. This also restricts the thermal current that allows hotter coolant to swap places with cooler coolant in the same area of the block in engines with more coolant space.
Furthermore the block height (from centre of crank to cylinder head) is exactly the same in all these engines despite two increases in stroke length. The consequence of this is shorter rods and/or (depending on the engine) moving the gudgeon pin position upwards in the piston - both adding load to the piston skirt (from identical cylinder pressure on a longer block height).
It just seems to me that in the pursuit of more power and torque (through bigger bores and strokes and introducing variable valve lift) the engines have been pushed closer to a safe working limit while little or nothing was done to improve basic cooling etc to compensate - in fact in some areas it appears to have been made worse. It is therefore perfectly expected that a higher number will suffer failures (and entirely common with many other previously reliable engines that were basically bored out or stroked for extra performance without the benefit of complete redesign compensation).
Obviously a small engine increased in capacity - makes a very fast car but reliability can suffer.
I would challenge anyone to make out a case that by offering both the low temperature thermostat (at a lower price than from the USA) and advice on how I think owners maybe able to avoid scored bores - I can be trying to increase my businesses profitability. It should be obvious to an idiot that I would be better off keeping quiet and just posting about their shortcomings and fixing more failures.
If my motivation is clearly not commercial (in fact goes against our income potential) and I have backed up my advice with a list of technical explanations that hold up - why so many arguments and why do so many people think I should then have to explain in detail the answers to all their questions when often they are so far out from reality I would need to spend hours just teaching those same people about engineering etc before they could understand the answer.
Of course I will not always be right and of course it is interesting to read about other suggestions when they are realistic and relate to the actual engine and genuine engineering principles - but they are few and far between and I just don't have time to explain to everyone in detail the why's and wherefores of every post I submit - sorry guys!
On a purely personal note - I hate having to act so arrogantly - those that know me well know that I always discuss ideas with staff and encourage comment and disagreement - and I would prefer to just be known for my knowledge and helpful advice - but it becomes difficult to find time to respond to the level of feedback on the Internet and at times I simply have to resort to blunt facts that make me appear extremely bog headed (I know). I hate it but seem trapped by time constraints into that somewhat inaccurate image. It is difficult to get away from that situation when a person does know their stuff.
Give us a break for a while please.
Baz
I honestly don't have the time and when the questions clearly demonstrate very little knowledge of the engine or little practical experience - or - are just put there to glorify someone else's ego - I do get tired of being expected to patiently advise all and sundry - yes I should be more patient - yes I should spend more time just advising people that know very very little anyway - yes I should be less touchy - but I am also trying to find out much more advanced issues, design solutions, develop tests and answers and run a busy business - one of the few doing anything at all about a serious design error.
So - I am sorry I am such a grumpy old man - sorry I think I know more than most people - but honestly - if most just took on board what I offer and resisted arguing all the time (mostly with frankly very poor technical comment or completely inaccurate points) - I could get time to offer a lot more advice that might be more useful to readers than fighting silly arguments and answering questions raised by people for whom the answer will not help them, not change their mind but just take my time.
Now after that genuine apology - the S2 piston has little taper and the cylinder bore area (in common with the later liquid cooled engines) has relatively shallow coolant block depth. It does run tight and can seize at the top - but ALL these scored bores occur down the bottom of the piston - on the thrust side - they have nothing to do with ignition or fuelling - and they are not a full seizure - only on one side.
This means that while the piston is pushing hard against the thrust side (like the piston had expanded too much like most seizures) the other side is not hard against the bore and is lubricated perfectly OK.
The thrust load is not all that high but the temperature is and therefore the oil is extremely thin and watery - poor support = scoring.
The thicker the cylinder wall the less heat can be transferred through it quickly - but the problem is really one of bubbles in my opinion.
If the cylinder was upright (or almost upright) bubbles naturally rise away from the hot spot and new coolant rushes in its place - but in a horizontal engine with slow coolant flow - it is trapped. The outside of the block is almost exactly the same size as the original 2.5 Boxster and has been increased through the Boxster S, 3.4 and 3.8 engines until there is very little clearance between the outer cylinder wall and the block - so very little distance for bubbles to float away and be replaced by coolant. This also restricts the thermal current that allows hotter coolant to swap places with cooler coolant in the same area of the block in engines with more coolant space.
Furthermore the block height (from centre of crank to cylinder head) is exactly the same in all these engines despite two increases in stroke length. The consequence of this is shorter rods and/or (depending on the engine) moving the gudgeon pin position upwards in the piston - both adding load to the piston skirt (from identical cylinder pressure on a longer block height).
It just seems to me that in the pursuit of more power and torque (through bigger bores and strokes and introducing variable valve lift) the engines have been pushed closer to a safe working limit while little or nothing was done to improve basic cooling etc to compensate - in fact in some areas it appears to have been made worse. It is therefore perfectly expected that a higher number will suffer failures (and entirely common with many other previously reliable engines that were basically bored out or stroked for extra performance without the benefit of complete redesign compensation).
Obviously a small engine increased in capacity - makes a very fast car but reliability can suffer.
I would challenge anyone to make out a case that by offering both the low temperature thermostat (at a lower price than from the USA) and advice on how I think owners maybe able to avoid scored bores - I can be trying to increase my businesses profitability. It should be obvious to an idiot that I would be better off keeping quiet and just posting about their shortcomings and fixing more failures.
If my motivation is clearly not commercial (in fact goes against our income potential) and I have backed up my advice with a list of technical explanations that hold up - why so many arguments and why do so many people think I should then have to explain in detail the answers to all their questions when often they are so far out from reality I would need to spend hours just teaching those same people about engineering etc before they could understand the answer.
Of course I will not always be right and of course it is interesting to read about other suggestions when they are realistic and relate to the actual engine and genuine engineering principles - but they are few and far between and I just don't have time to explain to everyone in detail the why's and wherefores of every post I submit - sorry guys!
On a purely personal note - I hate having to act so arrogantly - those that know me well know that I always discuss ideas with staff and encourage comment and disagreement - and I would prefer to just be known for my knowledge and helpful advice - but it becomes difficult to find time to respond to the level of feedback on the Internet and at times I simply have to resort to blunt facts that make me appear extremely bog headed (I know). I hate it but seem trapped by time constraints into that somewhat inaccurate image. It is difficult to get away from that situation when a person does know their stuff.
Give us a break for a while please.
Baz
Lots of interesting responses - needing some comment.
mats
th yes that's it but it is difficult to bleed the system. Better get a specialist with vacuum bleeding equipment to do it for you - but not impossible on your own.
bcnrml - I was new to Internet chat before and prematurely offended by criticism etc - but more used to it now - generally a thicker skin and try to be more remote - still regretably get occasionally irritated and sound off - wish I could resist it more.
Air cooled engines - while generally running hotter had Nikasil bores which are very smooth and ran hotter but as a result expanded more so less tight piston fits - plus - of course - no sudden bubbling to locally overheat a small area.
cmoose your cynical comments about numbers and failure ratios against profit seem right to me.
Radiators fail because they are pure alloy with no protective coating - so corrode - they have to have air passing through them and air with rain and dirt will form crud that sticks and rots them away. After this of course - once they start leaking slightly - the system pressure drops and the temperature that the coolant previously didn't bubble at - now starts it happening and so the internal engine cooling suffers without the owner noticing the difference.
Moving the thermostat to the outlet would be a great idea (that I hadn't given any thought to - more power to the forums!) - because the inlet temperature would be much lower and the outlet temperature would be the same as the inlet temperature used to be. It would also be more expensive - but I am now working on a solution for that that will be affordable - watch this space - not my idea at all - thanks for the contributions.
Mike (Sports and classic) yes you are of course right - the gradual ovality increases blow by and burns the oil but the thrust face is also hard against the bore so it usually blows by on the other side of the piston where it is not under such stress and high level pressure. It also lowers the compression on that cylinder so the combustion pressure at ignition is lower and the power and heat produced is reduced.
Would it help to understand what cylinder scoring is caused by. It is only evident because the surface of the piston has basically melted in a small local area and thrown/stuck/rolled a piece of allow into a space too small for it. It is all about load over area. If you put oil under a plate and stand on it the plate will slide because your weight is distributed over a large area and insufficient to break through the oil film. If you put a small pebble under the plate - the plate will stick and if you tried to roll it along the pebble would score the underlaying floor. The heavier the person the more it will stick.
It is a bit like this with a piston, the hotter the oil film and the greater the pressure the more likely a small "pebble" will break thought the oil film and put a score in the bore. The surface of the piston is allowing heat from the crown (directly in the flame path) to pass across the oil film and into the cylinder wall, then through that to the coolant outside.
The piston crown temperature is mainly affected by the recent throttle opening. Weak mixtures can melt the crown - but that is not our problem. The temperature at the cylinderwall/piston face is mainly influenced by that heat coming down from the crown and the rate at which the coolant is removing the heat on the other side.
Oil spray jets cool the crown and also add cooler oil to the cylinder wall - but this reduces on tickover. The coolant flow reduces on tickover - so the cylinder wall temporarily gets hotter, the oil thinner and the surface of the piston hotter - but with little load/force on it at this point it should survive that.
If however there is an immediate increase in load before the rest of the oil and coolant cooling cycle has caught up - there is a small moment in time when everything is against the piston face surviving. Basically a small piece of piston alloy melts and forms a ball that rolls up the piston and scores the bore.
A very relevant factor is that the molycote on the pistons gradually flakes off (or if not eventually wears down). Wear is not too much of a problem but flaking leaves the area of the piston resisting the loads - smaller so the localised force/unit area is higher and the heat transfer reduced - so this also adds to the failures over time as the piston coating diminishes.
Anything that reduces running temperatures will reduce the flaking or wear on the coating and IMHO the most critical area for scoring is - as previously stated - just after resting from a fast drive and then driving off fast again.
I am not telling anyone how to drive - please yourself - I will repair more if you ignore my advice - I am not bothered as long as you are aware of the dangers.
This is typical of all good well meant advice based by technical proof and experience. Humans can please themselves but not if they have not been informed of the risks.
All I have been doing with this post is to explain what I consider to be a significant risk and suggesting a simple solution. If you don't agree - that's just fine - if you agree but cannot be bothered and are prepared to take the risk - good for you - you know the risk - it is on your head entirely.
But if I didn't inform others of my concerns I think I would be failing my customers (and owners generally) and I could not do that (although by the amount of work it has created - I sometimes wished I had kept my own countenance).
Remember these engines are different, the temperature gauge fools you, many have scored bores and don't even know it yet - others could (just could) possibly avoid it - regardless of whose fault it is or what other solutions may help, regardless of opinion etc or who is right or wrong - if you want to avoid the problem do't give it full throttle after a standing period with a hot engine.
Baz
mats
th yes that's it but it is difficult to bleed the system. Better get a specialist with vacuum bleeding equipment to do it for you - but not impossible on your own.bcnrml - I was new to Internet chat before and prematurely offended by criticism etc - but more used to it now - generally a thicker skin and try to be more remote - still regretably get occasionally irritated and sound off - wish I could resist it more.
Air cooled engines - while generally running hotter had Nikasil bores which are very smooth and ran hotter but as a result expanded more so less tight piston fits - plus - of course - no sudden bubbling to locally overheat a small area.
cmoose your cynical comments about numbers and failure ratios against profit seem right to me.
Radiators fail because they are pure alloy with no protective coating - so corrode - they have to have air passing through them and air with rain and dirt will form crud that sticks and rots them away. After this of course - once they start leaking slightly - the system pressure drops and the temperature that the coolant previously didn't bubble at - now starts it happening and so the internal engine cooling suffers without the owner noticing the difference.
Moving the thermostat to the outlet would be a great idea (that I hadn't given any thought to - more power to the forums!) - because the inlet temperature would be much lower and the outlet temperature would be the same as the inlet temperature used to be. It would also be more expensive - but I am now working on a solution for that that will be affordable - watch this space - not my idea at all - thanks for the contributions.
Mike (Sports and classic) yes you are of course right - the gradual ovality increases blow by and burns the oil but the thrust face is also hard against the bore so it usually blows by on the other side of the piston where it is not under such stress and high level pressure. It also lowers the compression on that cylinder so the combustion pressure at ignition is lower and the power and heat produced is reduced.
Would it help to understand what cylinder scoring is caused by. It is only evident because the surface of the piston has basically melted in a small local area and thrown/stuck/rolled a piece of allow into a space too small for it. It is all about load over area. If you put oil under a plate and stand on it the plate will slide because your weight is distributed over a large area and insufficient to break through the oil film. If you put a small pebble under the plate - the plate will stick and if you tried to roll it along the pebble would score the underlaying floor. The heavier the person the more it will stick.
It is a bit like this with a piston, the hotter the oil film and the greater the pressure the more likely a small "pebble" will break thought the oil film and put a score in the bore. The surface of the piston is allowing heat from the crown (directly in the flame path) to pass across the oil film and into the cylinder wall, then through that to the coolant outside.
The piston crown temperature is mainly affected by the recent throttle opening. Weak mixtures can melt the crown - but that is not our problem. The temperature at the cylinderwall/piston face is mainly influenced by that heat coming down from the crown and the rate at which the coolant is removing the heat on the other side.
Oil spray jets cool the crown and also add cooler oil to the cylinder wall - but this reduces on tickover. The coolant flow reduces on tickover - so the cylinder wall temporarily gets hotter, the oil thinner and the surface of the piston hotter - but with little load/force on it at this point it should survive that.
If however there is an immediate increase in load before the rest of the oil and coolant cooling cycle has caught up - there is a small moment in time when everything is against the piston face surviving. Basically a small piece of piston alloy melts and forms a ball that rolls up the piston and scores the bore.
A very relevant factor is that the molycote on the pistons gradually flakes off (or if not eventually wears down). Wear is not too much of a problem but flaking leaves the area of the piston resisting the loads - smaller so the localised force/unit area is higher and the heat transfer reduced - so this also adds to the failures over time as the piston coating diminishes.
Anything that reduces running temperatures will reduce the flaking or wear on the coating and IMHO the most critical area for scoring is - as previously stated - just after resting from a fast drive and then driving off fast again.
I am not telling anyone how to drive - please yourself - I will repair more if you ignore my advice - I am not bothered as long as you are aware of the dangers.
This is typical of all good well meant advice based by technical proof and experience. Humans can please themselves but not if they have not been informed of the risks.
All I have been doing with this post is to explain what I consider to be a significant risk and suggesting a simple solution. If you don't agree - that's just fine - if you agree but cannot be bothered and are prepared to take the risk - good for you - you know the risk - it is on your head entirely.
But if I didn't inform others of my concerns I think I would be failing my customers (and owners generally) and I could not do that (although by the amount of work it has created - I sometimes wished I had kept my own countenance).
Remember these engines are different, the temperature gauge fools you, many have scored bores and don't even know it yet - others could (just could) possibly avoid it - regardless of whose fault it is or what other solutions may help, regardless of opinion etc or who is right or wrong - if you want to avoid the problem do't give it full throttle after a standing period with a hot engine.
Baz
The route is from the water pump it splits internally into two tubes and goes to the bottom of each bank of cylinders. It then goes along a channel each side in which there are three round holes and three small slots. The head gasket also has three round holes in it to match the three holes in the block.
Most of the coolant passes into the heads through the three round holes while a much smaller amount passes through the slots into the cylinders.
This goes on in the bottom of each bank.
The coolant passing through the heads meets the coolant coming from the block on the top side and joins up. This disguises the temperatures in each individual part (the head and block) as it is mixed back together again.
The flow therefore is pump, crankcase, SPLIT - most into heads - much less into cylinder block - join up again - off to radiators - back to thermostat, pump and into the common block entry to split back to each side and later between each head and cylinder.
Lower flow into the cylinders is right because the surface area is much greater - but if you go too far the slow coolant volume of flow results in higher temperature differences across the block - which can be dangerous.
The entry into the cylinders is at the bottom both sides but bank 1 this is the thrust side (so cooler) bank 2 it is the top that is the thrust side (so hotter).
An electric pump would keep temperatures lower during low revs but the thermostat being on the entry at the bottom would have to open enough to allow the increased flow and I am not sure it would.
The right place for a thermostat (IMHO) is on the outlet (as it always used to be in years of successful engine designs).
Baz
Most of the coolant passes into the heads through the three round holes while a much smaller amount passes through the slots into the cylinders.
This goes on in the bottom of each bank.
The coolant passing through the heads meets the coolant coming from the block on the top side and joins up. This disguises the temperatures in each individual part (the head and block) as it is mixed back together again.
The flow therefore is pump, crankcase, SPLIT - most into heads - much less into cylinder block - join up again - off to radiators - back to thermostat, pump and into the common block entry to split back to each side and later between each head and cylinder.
Lower flow into the cylinders is right because the surface area is much greater - but if you go too far the slow coolant volume of flow results in higher temperature differences across the block - which can be dangerous.
The entry into the cylinders is at the bottom both sides but bank 1 this is the thrust side (so cooler) bank 2 it is the top that is the thrust side (so hotter).
An electric pump would keep temperatures lower during low revs but the thermostat being on the entry at the bottom would have to open enough to allow the increased flow and I am not sure it would.
The right place for a thermostat (IMHO) is on the outlet (as it always used to be in years of successful engine designs).
Baz
Yes Globs - that's right - it will warm up quicker (confusing to others isn't it all?) but if the temperature rise in the block is say 10 degrees then running the same thermostat on entry as that used in other engines on top - will result in the engine running hotter (hence us lowering it back again).
The 924 had the thermostat in the top on the outlet but the 944 and 968 made the change to entry positioning.
Also an outlet thermostat can lead to cold shock when it finally opens on a cold day and the almost freezing cold coolant in the radiator flows for the first time into the now hot engine (often minimised by fitting a by pass or small hole in the thermostat). Some sort of dual thermostat would I suppose be best - low temp on the way in and higher on the way out.
Following my last post I should have added that the round holes that allow most of the coolant to flow into the heads (and bypass the cylinders) that are restricted in size and position with 996 3.4 (and previous smaller Boxster engines) by the head gasket having progressively larger holes as the flow reduces as it flows down the feeder tube - balances the flow and temperatures through the block - but requires 2 different head gaskets (because the heads are fitted opposite way around on each side but the feed is from the same end).
The 3.6, 3.8 and Cayman S 3.4 engines used one single head gasket with the same sized holes (so it could be fitted either side) and this not only changed the balance of coolant through the engine but also increased further the proportion going through the heads and reduced the amount going through the cylinders (where the problem is). Cost cutting seems to have over-ridden technicality.
All this can be clearly seen in photos and explanations in our buyers guide available free on www.hartech.org, section 5.
Anyone used to engines that looks at those pictures and sees the small slot for the block coolant will realise that with so little coolant passing relatively slowly through a block that needs to dissipate more heat (in later engines) - that this is where the problem starts.
If you have been driving fast then suddenly slow right down to tickover - the heat in the cylinders is still passing through the metal and the coolant on the outside has slowed even more - so it must get hotter. The thermostat will not see this because the coolant has slowed though the long route forwards and through the radiators, and if bubbles start the slow flow will not dissipate them - forming a pocket and failing to cool those hot cylinder walls. Meanwhile the oil between the cylinders is heating up and getting thinner (because the flow from the oil spray jets and crankshaft has reduced)and it is probably slowly sliding down round the pistons (under gravity) - OK for the bank 1 but bad news for bank 2 since the thrust side is then on the top.
Suddenly open up the engine and I still think you have a bad scenario for a few seconds (I never said anything like 45 minutes) until the coolant can flow again, the oil can cool again and everything settles down.
Whichever way you look at it - running the relatively high temperature thermostat on the inlet to the engine and combining this with a long routs to the radiators and back - is bound to run very hot engine temperatures indeed - that seemed OK in the earlier engines (with the more balanced flow directed by the dissimilar head gaskets) but that at the same time as increasing the load on the cylinder walls from the piston, they reduced the amount of coolant present to cool it and this seems to have pushed the engine so near the safe limit that under some circumstances and some driving styles - as they age - just momentarily can go past the point of no return and score a piston and hence a bore.
Baz
The 924 had the thermostat in the top on the outlet but the 944 and 968 made the change to entry positioning.
Also an outlet thermostat can lead to cold shock when it finally opens on a cold day and the almost freezing cold coolant in the radiator flows for the first time into the now hot engine (often minimised by fitting a by pass or small hole in the thermostat). Some sort of dual thermostat would I suppose be best - low temp on the way in and higher on the way out.
Following my last post I should have added that the round holes that allow most of the coolant to flow into the heads (and bypass the cylinders) that are restricted in size and position with 996 3.4 (and previous smaller Boxster engines) by the head gasket having progressively larger holes as the flow reduces as it flows down the feeder tube - balances the flow and temperatures through the block - but requires 2 different head gaskets (because the heads are fitted opposite way around on each side but the feed is from the same end).
The 3.6, 3.8 and Cayman S 3.4 engines used one single head gasket with the same sized holes (so it could be fitted either side) and this not only changed the balance of coolant through the engine but also increased further the proportion going through the heads and reduced the amount going through the cylinders (where the problem is). Cost cutting seems to have over-ridden technicality.
All this can be clearly seen in photos and explanations in our buyers guide available free on www.hartech.org, section 5.
Anyone used to engines that looks at those pictures and sees the small slot for the block coolant will realise that with so little coolant passing relatively slowly through a block that needs to dissipate more heat (in later engines) - that this is where the problem starts.
If you have been driving fast then suddenly slow right down to tickover - the heat in the cylinders is still passing through the metal and the coolant on the outside has slowed even more - so it must get hotter. The thermostat will not see this because the coolant has slowed though the long route forwards and through the radiators, and if bubbles start the slow flow will not dissipate them - forming a pocket and failing to cool those hot cylinder walls. Meanwhile the oil between the cylinders is heating up and getting thinner (because the flow from the oil spray jets and crankshaft has reduced)and it is probably slowly sliding down round the pistons (under gravity) - OK for the bank 1 but bad news for bank 2 since the thrust side is then on the top.
Suddenly open up the engine and I still think you have a bad scenario for a few seconds (I never said anything like 45 minutes) until the coolant can flow again, the oil can cool again and everything settles down.
Whichever way you look at it - running the relatively high temperature thermostat on the inlet to the engine and combining this with a long routs to the radiators and back - is bound to run very hot engine temperatures indeed - that seemed OK in the earlier engines (with the more balanced flow directed by the dissimilar head gaskets) but that at the same time as increasing the load on the cylinder walls from the piston, they reduced the amount of coolant present to cool it and this seems to have pushed the engine so near the safe limit that under some circumstances and some driving styles - as they age - just momentarily can go past the point of no return and score a piston and hence a bore.
Baz
Guys - when discussing coolant temperatures and graphs etc (as interesting and valid as they are - thanks) please remember you are not measuring the temperature of the coolant in the block - only the temperature of the coolant that went through the block after it has mixed back with the coolant passing through the cylinder head (which is the majority).
It is specifically the temperature inside the block that causes the problem.
Our thermostat tends to run the engine at a constant 80 to 81 degrees.
To anyone thinking Porsche know best - that's OK - just keep with what they fitted and bear the responsibility if it goes wrong. Perhaps also answer the logic of that position when Porsche did have the guts to admit to initial design errors when they later modified synchromesh systems, cylinder head and cylinder designs, and the many changes they fitted inside these engines (IMS bearing and spindle, RMS design etc) and why they then changed from the open deck design to a closed deck design in later engines - they cannot have been right in ALL those areas IMHO.
Finally - sorry I cannot see inside the block of the dfi engine to see how they have adjusted the coolant flow - presume the thermostat is still on the return side?
Baz
It is specifically the temperature inside the block that causes the problem.
Our thermostat tends to run the engine at a constant 80 to 81 degrees.
To anyone thinking Porsche know best - that's OK - just keep with what they fitted and bear the responsibility if it goes wrong. Perhaps also answer the logic of that position when Porsche did have the guts to admit to initial design errors when they later modified synchromesh systems, cylinder head and cylinder designs, and the many changes they fitted inside these engines (IMS bearing and spindle, RMS design etc) and why they then changed from the open deck design to a closed deck design in later engines - they cannot have been right in ALL those areas IMHO.
Finally - sorry I cannot see inside the block of the dfi engine to see how they have adjusted the coolant flow - presume the thermostat is still on the return side?
Baz
Temperature sensors were fitted to the narrow inlet and outlet of the cylinders and heads on both sides both before and after modifying the coolant flow and changing the thermostat. They therefore collected information about the change in cylinder temperature at different speeds (engine and road), different driving conditions and different set ups - from this our reasonable conclusions have been based.
The temperature difference is around 10 degrees - not sure what the problem with this is - if the sun or smoking is bad for you is it only a worthwhile solution if you stop smoking all together or use factor 50 all day every day - or is a reduction in smoking and some sun cream better than nothing? we cannot redesign the engine only try to find a simple inexpensive way to reduce the problem and have never claimed anything else.
A faster impellor would make no difference to the overall cooling because the thermostat controls the flow rate until it matches the setting of the thermostat temperature and whatever temperature it is set at - it would simply end up with the same coolant speed and therefore temperature difference. If the thermostat was on the outlet it could help in situations where the engine suddenly got hotter (so the thermostat opened more) and then you drove off quickly and got better coolant flow before the thermostat shut down again. But with it set at the inlet - while slowing down with the throttle shut the coolant in the system will momentarily cool and shut the thermostat down - while inside the block the temperatures will rise as the flow is reduced and the latent heat in the metal continues to try and find a way out. Then suddenly driving off fast is bad news because there will be no maximum flow restored for a while at the very time the load on the piston and cylinder wall are highest again.
The dfi engine does seem to have more radial width for the coolant to flow around (which is a good thing) as some conduction takes place between the cooler coolant and the hotter coolant nearer the cylinder wall and acts as a buffer to slow temperature changes and minimise hot spots.
Baz
The temperature difference is around 10 degrees - not sure what the problem with this is - if the sun or smoking is bad for you is it only a worthwhile solution if you stop smoking all together or use factor 50 all day every day - or is a reduction in smoking and some sun cream better than nothing? we cannot redesign the engine only try to find a simple inexpensive way to reduce the problem and have never claimed anything else.
A faster impellor would make no difference to the overall cooling because the thermostat controls the flow rate until it matches the setting of the thermostat temperature and whatever temperature it is set at - it would simply end up with the same coolant speed and therefore temperature difference. If the thermostat was on the outlet it could help in situations where the engine suddenly got hotter (so the thermostat opened more) and then you drove off quickly and got better coolant flow before the thermostat shut down again. But with it set at the inlet - while slowing down with the throttle shut the coolant in the system will momentarily cool and shut the thermostat down - while inside the block the temperatures will rise as the flow is reduced and the latent heat in the metal continues to try and find a way out. Then suddenly driving off fast is bad news because there will be no maximum flow restored for a while at the very time the load on the piston and cylinder wall are highest again.
The dfi engine does seem to have more radial width for the coolant to flow around (which is a good thing) as some conduction takes place between the cooler coolant and the hotter coolant nearer the cylinder wall and acts as a buffer to slow temperature changes and minimise hot spots.
Baz
Well you are right and wrong all at the same time.
Of course a thermostat helps reduce warm up times but the only time it does not control the driving temperature is when it is fully open and the system as a whole cannot manage to cool the engine to the thermostat setting.
So - if the thermostat is missing - say - the engine takes longer to warm up and will usually run cooler than the thermostat original setting - much cooler on cold days (as the air temperature going through the radiator is lower) and hotter on very hot days (for the reverse reason) - or it may fluctuate a little with road speed (due to interference with the air speed through the rad and the resulting pressure drop).
Without a thermostat - the running temperature will be seen to vary with the ambient conditions and the speed and type of driving - usually running cooler (as the system should be designed for the most hot conditions which rarely occur).
Providing the situation and car is capable of it - the thermostat will then slow the coolant flow and by doing so it will control the temperature - this is how it works. Temperature cooler - thermostat closes a bit and slows flow - coolant heats up. Because the coolant heats up the thermostat opens a bit more - flow increases - coolant gets cooler.
It is designed not to react too quickly - matched to the expectations of normal driving and usually results in stable temperatures. The only times it fails (apart from mechanical failures) is when it has been hot and you suddenly slow down (which slows or stops the airflow rate through the radiator) - the coolant gets hotter and the thermostat is fully open so you see the temperature go up - but shortly after this (as a fail safe) the radiator fans cut in to increase the air flow you just lost by slowing down the air flow and the thermostat can cut in again - together with the radiator fan switch working between two settings for on and off - both slightly higher than the thermostat setting.
Taking time to thoroughly warm up the engine is to make sure the heat has soaked through all the metal parts and the thermostat etc has stabilised so clearances are all at their designed level. The clearances in the cylinders increase quite quickly with age (due to the cylinders going oval) - so after the first few thousand miles are larger and continue to grow as the years and miles pass by.
This is particularly quick in these engines due to the open deck design and the Lokasil material creeping more than Alusil or aluminium plated with Nikasil.
You may have noticed that some manufacturers recognise the benefit of changing oil grades in the summer and winter (due to the system running slightly cooler in the winter. Many also recognise that for arduous conditions (like racing) thicker oil is beneficial (as bearing and cylinder support are more important then than ecconomy).
When a manufacturer makes a product with an unacceptable failure rate (you can decide if this is such a case or not - I believe it is) and then fails to do anything about it - people seem to fall into 2 categories - those that believe the manufacturer can do no wrong (even though they may be faced with an unacceptable failure) and those that recognise such things can happen and hope someone somewhere has the skill to find a solution.
Stangely people often change exhausts, wheels, tyres, and accessories - air filters, oil types etc but still believe everything else must be perfect and cannot be improved upon.
Even when the manufacturer tries different designs (as with the IMS bearing) but fails to modify the fault that engineers around the world agree on - some people are blinded by the original design - even though the manufacturer - by trying different solutions - has openly admitted the first design was not best. It is well documented that once people accept something as right - they are very difficult to persuade differently (flat earth etc) even to the point of life threatening issues and of course if two different groups each think they are right - wars frequently follow.
We are a strange race in this regard and there are always a minority who seem able to follow a logical and well researched practical argument and accept an improvement may be possible. I have no problem with all that - it doesn't harm me one bit if people dissagree with me - it is just a shame that more of them will have to have their engines rebuilt or replaced as a result.
Like the medics that first realised smoking kills and the Sun can as well - it can be unpopular to air such views but over time - some are proven right - some are not.
To some extent time may be needed before my suggestions are more accepted (and the level of technical missunderstandings revealed in the various replies and questions are quite astonishing to me - arguing about issues they clearly have no background to support and from entirely opposite views to reality and science a lot of the time) - but again - why should I worry?
Like those that warned of health issues that later became accepted - I am happy to wait my time to find that just like when we warned about the 944 S, S2 and 968 cam chain weakness (the first to do so and not accepted at first) that advice has since become accepted practice with those models Worldwide (and saved owners a small fortune as a result).
Similar changes have been advised by us all over many models (and I am not claiming just by us - by many many more Worldwide) and become the benefit you get from dealing with people in a closed specialist topic.
It is a free World and all those that want to ignore this advice are free to do so - no problem - time will tell who is right.
But - if you cannot make up your own mind - remember we advised about many weaknesses in these engines and supported our findings with photos and words in our buyers guides - right at the beginning - many years ago
- putting our money where our mouth was and invested in appropriate soloutions.
Most now have become accepted worldwide (and some since appeared in the latest engines - not that I am claiming they copied us but just becasue the original design was not best and the newer one was better - just as we reported originally) so were we all wrong then or not?
This advice is therefore just to help others avoid or reduce failures - and as I benefit more than most - if that advice is ignored - please follow it or ignore it as you please - at least you have been properly warned and backed up with both technical reasoning, actual practical rebuilds and cars and a business that (likemay others inmany other walks of life) has previously proven to be capable of providing such early advice correctly - to the benefit of the majority.
Baz
Of course a thermostat helps reduce warm up times but the only time it does not control the driving temperature is when it is fully open and the system as a whole cannot manage to cool the engine to the thermostat setting.
So - if the thermostat is missing - say - the engine takes longer to warm up and will usually run cooler than the thermostat original setting - much cooler on cold days (as the air temperature going through the radiator is lower) and hotter on very hot days (for the reverse reason) - or it may fluctuate a little with road speed (due to interference with the air speed through the rad and the resulting pressure drop).
Without a thermostat - the running temperature will be seen to vary with the ambient conditions and the speed and type of driving - usually running cooler (as the system should be designed for the most hot conditions which rarely occur).
Providing the situation and car is capable of it - the thermostat will then slow the coolant flow and by doing so it will control the temperature - this is how it works. Temperature cooler - thermostat closes a bit and slows flow - coolant heats up. Because the coolant heats up the thermostat opens a bit more - flow increases - coolant gets cooler.
It is designed not to react too quickly - matched to the expectations of normal driving and usually results in stable temperatures. The only times it fails (apart from mechanical failures) is when it has been hot and you suddenly slow down (which slows or stops the airflow rate through the radiator) - the coolant gets hotter and the thermostat is fully open so you see the temperature go up - but shortly after this (as a fail safe) the radiator fans cut in to increase the air flow you just lost by slowing down the air flow and the thermostat can cut in again - together with the radiator fan switch working between two settings for on and off - both slightly higher than the thermostat setting.
Taking time to thoroughly warm up the engine is to make sure the heat has soaked through all the metal parts and the thermostat etc has stabilised so clearances are all at their designed level. The clearances in the cylinders increase quite quickly with age (due to the cylinders going oval) - so after the first few thousand miles are larger and continue to grow as the years and miles pass by.
This is particularly quick in these engines due to the open deck design and the Lokasil material creeping more than Alusil or aluminium plated with Nikasil.
You may have noticed that some manufacturers recognise the benefit of changing oil grades in the summer and winter (due to the system running slightly cooler in the winter. Many also recognise that for arduous conditions (like racing) thicker oil is beneficial (as bearing and cylinder support are more important then than ecconomy).
When a manufacturer makes a product with an unacceptable failure rate (you can decide if this is such a case or not - I believe it is) and then fails to do anything about it - people seem to fall into 2 categories - those that believe the manufacturer can do no wrong (even though they may be faced with an unacceptable failure) and those that recognise such things can happen and hope someone somewhere has the skill to find a solution.
Stangely people often change exhausts, wheels, tyres, and accessories - air filters, oil types etc but still believe everything else must be perfect and cannot be improved upon.
Even when the manufacturer tries different designs (as with the IMS bearing) but fails to modify the fault that engineers around the world agree on - some people are blinded by the original design - even though the manufacturer - by trying different solutions - has openly admitted the first design was not best. It is well documented that once people accept something as right - they are very difficult to persuade differently (flat earth etc) even to the point of life threatening issues and of course if two different groups each think they are right - wars frequently follow.
We are a strange race in this regard and there are always a minority who seem able to follow a logical and well researched practical argument and accept an improvement may be possible. I have no problem with all that - it doesn't harm me one bit if people dissagree with me - it is just a shame that more of them will have to have their engines rebuilt or replaced as a result.
Like the medics that first realised smoking kills and the Sun can as well - it can be unpopular to air such views but over time - some are proven right - some are not.
To some extent time may be needed before my suggestions are more accepted (and the level of technical missunderstandings revealed in the various replies and questions are quite astonishing to me - arguing about issues they clearly have no background to support and from entirely opposite views to reality and science a lot of the time) - but again - why should I worry?
Like those that warned of health issues that later became accepted - I am happy to wait my time to find that just like when we warned about the 944 S, S2 and 968 cam chain weakness (the first to do so and not accepted at first) that advice has since become accepted practice with those models Worldwide (and saved owners a small fortune as a result).
Similar changes have been advised by us all over many models (and I am not claiming just by us - by many many more Worldwide) and become the benefit you get from dealing with people in a closed specialist topic.
It is a free World and all those that want to ignore this advice are free to do so - no problem - time will tell who is right.
But - if you cannot make up your own mind - remember we advised about many weaknesses in these engines and supported our findings with photos and words in our buyers guides - right at the beginning - many years ago
- putting our money where our mouth was and invested in appropriate soloutions.
Most now have become accepted worldwide (and some since appeared in the latest engines - not that I am claiming they copied us but just becasue the original design was not best and the newer one was better - just as we reported originally) so were we all wrong then or not?
This advice is therefore just to help others avoid or reduce failures - and as I benefit more than most - if that advice is ignored - please follow it or ignore it as you please - at least you have been properly warned and backed up with both technical reasoning, actual practical rebuilds and cars and a business that (likemay others inmany other walks of life) has previously proven to be capable of providing such early advice correctly - to the benefit of the majority.
Baz
Yes Globs - it is quite staggering that someone would state that a thermostat doesn't control flow and stays open once hot and that the system then controls the temperature by itself - so unbelievably wrong and misleading I am gobsmacked (good job it wasn't your suggestion or it would have to be Globsmacked).
One of the first laws anyone interested in science will learn is that the rate of heat transfer is roughly propotional to the temperature difference. i.e. double the temperature difference = you double the amount of heat removable (assuming other factors like flow etc constant).
Therefore with a thermostat running @ say 90 degrees in air @ 40 degrees there is a potential 50 degrees of temperature difference at the radiator. Now in air at say -10 degrees there is a temperature difference of 100 degrees (twice the temperature difference) - how anyone imagines the engine would would not run cooler unless the only other parameter (the coolant flow) was altered - defeats me and is just not worth listening to.
This is however the problem with thermostats because in the above freezing cold scenario - although the thermostat would close more to reduce the flow (if the coolant was much colder passing by it) - if the thermostat was only on the exit from the engine - the coolant flow would be lower but the temperature rise in the block would be higher - so although the temperature at outlet would be the same the average temperature inside the engine would be lower - so the thermostat would not be controlling the average temperature it should ideally run at.
It would do this better if the thermostat was on the inlet to the engine but then - as the engine works harder the outlet temperature rises and takes time to equalise with such a remote radiator and slow acting inlet thermostat.
You see the whole subject is a lot more complicated than people realise even though the principles are dead simple.
Probably it needs thermostats at both ends with different settings and I agree with whoever posted the idea of a variable flow rate pump (perhaps electric) - with clever digital programming this would certainly do a great job of flow control in different scenarios.
If (as I suspect) the present engines run close to the limit of cylinder lubrication safety and temperatures - then as the engine ages - the block surface material gets coated, the radiators get clogged (inside and out), the pump blades deteriorate, the pump bearing wears (and critical rear impellor clearance increases), the piston clearance goes up, the Molycote surface finish wears or flakes off and with such long service intervals - the oil quality deteriorates - some that are then subjected to more severe use - just go past that limit for a few seconds and subsequently fail.
It doesn't help that they are so dammed torquey a lot of fast driving can be achieved without revving the engine high but then having reduced coolant and oil spray flow (the oil contributing greatly to overall cooling - but since it then goes into an oil/coolant cooler - increases in coolant temperature don't help cool it as much as perhaps and air/oil cooler might (being independent of the coolant temp).
We support the position to slightly thicken the oil grade as the engines wear and also to lower back the running temperature through fitting a lower temperature thermostat - all good common sense changes to prolong the life of an engine running close to its safety limit.
When we rebuild them we also re-round the cylinders and convert them to a closed deck construction, increase the proportion of coolant round the cylinders and lower the running temperatures - all far better than doing nothing.
Most engines used to run thermostats around 70 to 82 degrees - but at the outlet (so average internal temperatures around 75 degrees) these have the thermostat at the outlet so average internal temperatures are higher - lowering the thermostat setting changes that.
Just on final cynical thought - are all these completely wrong technical statements thrown at me (that I then try to correct (or the sake of not confusing or miss-informing readers) a clever ploy by someone with vested interests to reduce the amount of good quality criticism of the product and instead spend too much time stating the obvious. What do you think? should I ignore them or is it necessary to correct such howlers?
Baz
One of the first laws anyone interested in science will learn is that the rate of heat transfer is roughly propotional to the temperature difference. i.e. double the temperature difference = you double the amount of heat removable (assuming other factors like flow etc constant).
Therefore with a thermostat running @ say 90 degrees in air @ 40 degrees there is a potential 50 degrees of temperature difference at the radiator. Now in air at say -10 degrees there is a temperature difference of 100 degrees (twice the temperature difference) - how anyone imagines the engine would would not run cooler unless the only other parameter (the coolant flow) was altered - defeats me and is just not worth listening to.
This is however the problem with thermostats because in the above freezing cold scenario - although the thermostat would close more to reduce the flow (if the coolant was much colder passing by it) - if the thermostat was only on the exit from the engine - the coolant flow would be lower but the temperature rise in the block would be higher - so although the temperature at outlet would be the same the average temperature inside the engine would be lower - so the thermostat would not be controlling the average temperature it should ideally run at.
It would do this better if the thermostat was on the inlet to the engine but then - as the engine works harder the outlet temperature rises and takes time to equalise with such a remote radiator and slow acting inlet thermostat.
You see the whole subject is a lot more complicated than people realise even though the principles are dead simple.
Probably it needs thermostats at both ends with different settings and I agree with whoever posted the idea of a variable flow rate pump (perhaps electric) - with clever digital programming this would certainly do a great job of flow control in different scenarios.
If (as I suspect) the present engines run close to the limit of cylinder lubrication safety and temperatures - then as the engine ages - the block surface material gets coated, the radiators get clogged (inside and out), the pump blades deteriorate, the pump bearing wears (and critical rear impellor clearance increases), the piston clearance goes up, the Molycote surface finish wears or flakes off and with such long service intervals - the oil quality deteriorates - some that are then subjected to more severe use - just go past that limit for a few seconds and subsequently fail.
It doesn't help that they are so dammed torquey a lot of fast driving can be achieved without revving the engine high but then having reduced coolant and oil spray flow (the oil contributing greatly to overall cooling - but since it then goes into an oil/coolant cooler - increases in coolant temperature don't help cool it as much as perhaps and air/oil cooler might (being independent of the coolant temp).
We support the position to slightly thicken the oil grade as the engines wear and also to lower back the running temperature through fitting a lower temperature thermostat - all good common sense changes to prolong the life of an engine running close to its safety limit.
When we rebuild them we also re-round the cylinders and convert them to a closed deck construction, increase the proportion of coolant round the cylinders and lower the running temperatures - all far better than doing nothing.
Most engines used to run thermostats around 70 to 82 degrees - but at the outlet (so average internal temperatures around 75 degrees) these have the thermostat at the outlet so average internal temperatures are higher - lowering the thermostat setting changes that.
Just on final cynical thought - are all these completely wrong technical statements thrown at me (that I then try to correct (or the sake of not confusing or miss-informing readers) a clever ploy by someone with vested interests to reduce the amount of good quality criticism of the product and instead spend too much time stating the obvious. What do you think? should I ignore them or is it necessary to correct such howlers?
Baz
Actually a high speed - small diameter - centrifugal pump is quite clever because if the flow is restricted it kind of free wheels (without displacing any coolant) without absorbing much power and yet once the speed is high enough and the flow is allowed to increase - it pumps greater and greater volumes even at the same engine speeds. It is therefore quite ideally matched to a flow control thermostat.
The only down side is the low flow at very low revs (but which is usually OK as most engines are not very powerful at such low revs - but more recent advances in variable cam timing and valve lift have changed that significantly). Up until now most very powerful engines have been turbo charged and only produced high outputs at higher revs anyway (so the pumps would be OK at those revs) - so I think this new generation of torquey engines has introduced a new potential problem at low revs but high piston loads.
Of course it could be that the Lokasil bores don't like the combination of high temperatures and loads but supercharged versions of the 996 3.4 worked OK in that regard - and all early engines are OK - it only seems to be those that also have the change from the handed head gaskets to the single version (that fits both sides but allows less coolant by proportion to pass through the cylinders and more through the heads and in a different balance between them than before) that are scoring - so I still think the problem is temperature related and therefore thicker oil, lower coolant temperatures and time to flow the coolant before full throttle - seems an appropriate safeguard.
I agree that speeding up the coolant pump may help (will also look into that possibility).
Baz
The only down side is the low flow at very low revs (but which is usually OK as most engines are not very powerful at such low revs - but more recent advances in variable cam timing and valve lift have changed that significantly). Up until now most very powerful engines have been turbo charged and only produced high outputs at higher revs anyway (so the pumps would be OK at those revs) - so I think this new generation of torquey engines has introduced a new potential problem at low revs but high piston loads.
Of course it could be that the Lokasil bores don't like the combination of high temperatures and loads but supercharged versions of the 996 3.4 worked OK in that regard - and all early engines are OK - it only seems to be those that also have the change from the handed head gaskets to the single version (that fits both sides but allows less coolant by proportion to pass through the cylinders and more through the heads and in a different balance between them than before) that are scoring - so I still think the problem is temperature related and therefore thicker oil, lower coolant temperatures and time to flow the coolant before full throttle - seems an appropriate safeguard.
I agree that speeding up the coolant pump may help (will also look into that possibility).
Baz
Statistically we see slightly more tiptronics with cracked liners or scored bores. I can only put this down to a point I observed and posted some months ago - basically that I find I drive a manual at slightly higher revs (so in any gear it is about right to accelerate away comfortably) whereas I noticed the revs I plod about at in a tiptronic are usually lower and then - when I want to move a bit quicker - I don't kick down (since this would require flooring it - something an engineer with some mechanical sympathy rarely does unless he has to) but do use a lot of throttle - which makes the torque converter allow the revs to rise quite a lot in the same gear - so generally accelerate at lower revs than in a manual = more load on the cylinder wall and more cylinder combustion pressure.
However this is only an observation and really it is impossible to use statistics unless you have full information on all the test vehicles and rank them in driving styles, ambient conditions, type of roads etc etc.
The best we can do is reflect an impression of what we see the most of.
Right now this is definitely scored bores - being more common than either IMS bearing failures - or cracked cylinders - used to be - but were more of these sold - no doubt someone will come along with som euseful statistics about the sales of 3.6, 3.8 and Cayman S cars compared to the older 996 3.4's
Baz
However this is only an observation and really it is impossible to use statistics unless you have full information on all the test vehicles and rank them in driving styles, ambient conditions, type of roads etc etc.
The best we can do is reflect an impression of what we see the most of.
Right now this is definitely scored bores - being more common than either IMS bearing failures - or cracked cylinders - used to be - but were more of these sold - no doubt someone will come along with som euseful statistics about the sales of 3.6, 3.8 and Cayman S cars compared to the older 996 3.4's
Baz
OK so there is a general agreement now that in slow moving traffic or standstill, engine temperatures rise and the fans often kick in. I think it is also obvious that if you have been driving very fast just before the slow or stop - then the engine will be even hotter.
However it is also clear that if you don't drive off immediately with full throttle - no damage has been done.
Perhaps I could now then bring things back to the point I made at the outset - basically that if you have a 3.6, 3.8 or Cayman S (in which the cylinder block temperature will probably be higher than earlier models) and you drive fast then slow down or stop for while (say at lights) then drive off in a racing start drag race - this is possibly the most likely time to score a bore because the oil film between the piston and bore is the hottest and the load between the piston and the bore is the highest - so I just suggested a short period of less than full throttle - to help protect the engines.
A lot of interesting comment has now been posted and various linked subjects but within it there now does seem to me to be general agreement that my suggestion seems valid.
Smaller engines have lighter pistons, relatively longer rods, less pressure on the piston face at lower revs and seem to survive the cylinder scoring problem OK.
Hopefully - if you follow this advice - you may avoid cylinder scoring and in the meantime I feel sure the debate will go on about other solutions that might help to the benefit of owners everywhere.
Baz
However it is also clear that if you don't drive off immediately with full throttle - no damage has been done.
Perhaps I could now then bring things back to the point I made at the outset - basically that if you have a 3.6, 3.8 or Cayman S (in which the cylinder block temperature will probably be higher than earlier models) and you drive fast then slow down or stop for while (say at lights) then drive off in a racing start drag race - this is possibly the most likely time to score a bore because the oil film between the piston and bore is the hottest and the load between the piston and the bore is the highest - so I just suggested a short period of less than full throttle - to help protect the engines.
A lot of interesting comment has now been posted and various linked subjects but within it there now does seem to me to be general agreement that my suggestion seems valid.
Smaller engines have lighter pistons, relatively longer rods, less pressure on the piston face at lower revs and seem to survive the cylinder scoring problem OK.
Hopefully - if you follow this advice - you may avoid cylinder scoring and in the meantime I feel sure the debate will go on about other solutions that might help to the benefit of owners everywhere.
Baz
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