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
anonymous said:
[redacted]
I'd be surprised if :A. The orignal piston to bore specifications were that tight.
and
B. Modern production tolerances were that large.
Sounds more like an intrinsic design fault that rears its head in certain circumstances.
Are the engines being run too lean for emissions reasons ? which in turn means the pistons are running too hot and their rate of thermal expansion is inconsistent and excessive.
Edited by Slippydiff on Tuesday 9th August 13:44
I think your theory is plausible.
One thing you could look at next time you get the chance, is whether any ensuing air-pockets in the water jacket will be trapped against the bore-wall - for example if the openings in the head-gasket are not matched to the water core in the block, could they prevent the air bleeding out, particularly at idle, were the coolant isn't flowing fast enough to drag the air with it.
Assuming this is Porsche's first attempt at water-cooling a flat engine, it's something they may have overlooked.
One thing you could look at next time you get the chance, is whether any ensuing air-pockets in the water jacket will be trapped against the bore-wall - for example if the openings in the head-gasket are not matched to the water core in the block, could they prevent the air bleeding out, particularly at idle, were the coolant isn't flowing fast enough to drag the air with it.
Assuming this is Porsche's first attempt at water-cooling a flat engine, it's something they may have overlooked.
Are you sure the thermostat is on the coolant return to the engine.
A thermostat should allow restricted flow of coolant when cool to allow the engine to warm up quickly, and also regulate engine temperature in cold weather. If the thermostat is on the return the coolant passing through the bleed hole will take absolutly agees to get hot as it has to pass through the rads in the front of the car and then back to the thermostat. Full circulation and cooling will only start once the thermostat is open, and by then the coolant temp within the engine would be really hot.
Could a remote thermostat be fitted in the correct position, ie on the flow pipe from the engine where it will respond to temperature change immediatly.
A thermostat should allow restricted flow of coolant when cool to allow the engine to warm up quickly, and also regulate engine temperature in cold weather. If the thermostat is on the return the coolant passing through the bleed hole will take absolutly agees to get hot as it has to pass through the rads in the front of the car and then back to the thermostat. Full circulation and cooling will only start once the thermostat is open, and by then the coolant temp within the engine would be really hot.
Could a remote thermostat be fitted in the correct position, ie on the flow pipe from the engine where it will respond to temperature change immediatly.
Sounds like I need to start driving my 3.4 Cayman S like my sister would - get up to a terminal velocity then maintain that speed for the entire journey, oblivious to roundabout priority, junctions, traffic lights, prevailing speed limits, pedestrians, etc. Drive it like a slow but lethal train, right?
Hi,
I did suggest a electronically controlled thermostat around 12 months... Like BMW use in some of their models they have what looks like a heater plug directly in front of the thermostat... When needed it superheats the coolant and forces the stat to open...would this be of some use. If you want I could offer some electrical assistance. Worth considering if you think it maybe of some use.
Mike
I did suggest a electronically controlled thermostat around 12 months... Like BMW use in some of their models they have what looks like a heater plug directly in front of the thermostat... When needed it superheats the coolant and forces the stat to open...would this be of some use. If you want I could offer some electrical assistance. Worth considering if you think it maybe of some use.
Mike
Interesting reading Baz, well done for taking the time to explain it to us mere mortals.
There is a great deal to digest, but would it be to simplistic to think that if a thicker oil was specified by Porsche then the problems would of been lessened?
When you see oil being drained from a hot 996/7 (or cayman etc) then you can see how thin it gets when up to temperature, and therefore decreasing the engine oil pressure. I personally don't think the service schedule on the later cars helps the situation either, 6 monthly oil changes with a 10/60 would surely help, no?
Cheers.
There is a great deal to digest, but would it be to simplistic to think that if a thicker oil was specified by Porsche then the problems would of been lessened?
When you see oil being drained from a hot 996/7 (or cayman etc) then you can see how thin it gets when up to temperature, and therefore decreasing the engine oil pressure. I personally don't think the service schedule on the later cars helps the situation either, 6 monthly oil changes with a 10/60 would surely help, no?
Cheers.
Baz excellent perfect storm scenario,I had a car that had on its last memorable drive before failure (just asked the owner about his last journey)been driven hard on the motorway then got caught in jam or slow moving traffic on a hot sunny day then had a bit more WOT on the way home arrived all fine.
However next morning from cold start up ticking noise from one bank new engine required!
NB We all appreciate these posts Baz they are so helpfull!
However next morning from cold start up ticking noise from one bank new engine required!
NB We all appreciate these posts Baz they are so helpfull!
hartech said:
I agree and have always promoted the use of a thicker oil - despite creating a huge wave of posts dissagreeing with me.
I suppose I would actually be better off ignoring publishing my concerns for owners and just fixing the increasing numbers that are failing.
Baz
From what I'm told you already have enough work to see you into retirement ;-)I suppose I would actually be better off ignoring publishing my concerns for owners and just fixing the increasing numbers that are failing.
Baz
hartech said:
I agree and have always promoted the use of a thicker oil - despite creating a huge wave of posts dissagreeing with me.
I suppose I would actually be better off ignoring publishing my concerns for owners and just fixing the increasing numbers that are failing.
Baz
I for one think for a relatively small outfit all things considered are doing a fantastic job ! I don't know of anyone more knowledgable than you in this field.I suppose I would actually be better off ignoring publishing my concerns for owners and just fixing the increasing numbers that are failing.
Baz
Mike
hartech said:
I agree and have always promoted the use of a thicker oil - despite creating a huge wave of posts dissagreeing with me.
I suppose I would actually be better off ignoring publishing my concerns for owners and just fixing the increasing numbers that are failing.
Baz
Please don't be so melodramatic Baz - many of us out here really do enjoy a lengthy technical post in terms that we can understand and also always have our independents of choice fill our cars with 10w60... I suppose I would actually be better off ignoring publishing my concerns for owners and just fixing the increasing numbers that are failing.
Baz
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