How to possibly prevent cylinder scoring
Discussion
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
Here is a bit of text from Wikipedia with some relevant content reinforcing the Hartech analysis. Baz can you help me understand, is the thrust face in question ie bank 2, the upper side of the right of the engine as you look at the rear?
[edit]Coolant and heat transfer agent
The major use of ethylene glycol is as a medium for convective heat transfer in, for example, automobiles and liquid cooled computers. Ethylene glycol is also commonly used in chilled water air conditioning systems that place either the chiller or air handlers outside, or systems that must cool below the freezing temperature of water. In geothermal heating/cooling systems, ethylene glycol is the fluid that transports heat through the use of a geothermal heat pump. The ethylene glycol either gains energy from the source (lake, ocean, water well) or dissipates heat to the source, depending if the system is being used for heating or cooling.
Pure ethylene glycol has a specific heat capacity about one half that of water. So, while providing freeze protection and an increased boiling point, ethylene glycol lowers the specific heat capacity of water mixtures relative to pure water. A 50/50 mix by mass has a specific heat capacity of about 0.75 BTU/lb F, thus requiring increased flow rates in same system comparisons with water. Additionally, the increase in boiling point over pure water inhibits nucleate boiling on heat transfer surfaces thus reducing heat transfer efficiency in some cases, such as gasoline engine cylinder walls. Therefore, pure ethylene glycol should not be used as an engine coolant in most cases.
[edit]Coolant and heat transfer agent
The major use of ethylene glycol is as a medium for convective heat transfer in, for example, automobiles and liquid cooled computers. Ethylene glycol is also commonly used in chilled water air conditioning systems that place either the chiller or air handlers outside, or systems that must cool below the freezing temperature of water. In geothermal heating/cooling systems, ethylene glycol is the fluid that transports heat through the use of a geothermal heat pump. The ethylene glycol either gains energy from the source (lake, ocean, water well) or dissipates heat to the source, depending if the system is being used for heating or cooling.
Pure ethylene glycol has a specific heat capacity about one half that of water. So, while providing freeze protection and an increased boiling point, ethylene glycol lowers the specific heat capacity of water mixtures relative to pure water. A 50/50 mix by mass has a specific heat capacity of about 0.75 BTU/lb F, thus requiring increased flow rates in same system comparisons with water. Additionally, the increase in boiling point over pure water inhibits nucleate boiling on heat transfer surfaces thus reducing heat transfer efficiency in some cases, such as gasoline engine cylinder walls. Therefore, pure ethylene glycol should not be used as an engine coolant in most cases.
More reading on the low-temp thermostat (interesting comments regarding DFI and GT/TT engines):
http://www.lnengineering.com/lowtemperaturethermos...
http://www.lnengineering.com/lowtemperaturethermos...
anonymous said:
[redacted]
I think we all agree this would be over the top, so it is a good thing Baz didn't say this.What he did say was:
hartech said:
Your last sentence is in all honesty disgraceful
referring to this:Rockster said:
There can be reasons to limit hard launches with any car including Porches, but piston/cylinder wall scoring is not one of them.
Since the evidence suggests this can cause scoring I find it hard not to agree with Baz here.Whilst I am not an automotive engineer I am a trained scientist and professional engineer in a different discipline. Baz has put forward a very plausible hypothesis to my mind, some of the negative comments with regards to things such as journalistic testing of vehicles beggars belief. Yeah right you can just expect to jump in and drive the car flat out away from the lights each and every time in any conditions and never expect a failure. Its hard to take comments like that even remotely seriously.
OlberJ said:
Do you have a picture of the coolant pump in question? Particularly of the blades/vanes side of the pump?
Reason i ask, have had problems before pumping coolant round a similar length system when a big engine is involved.We found the impeller blades of certain aftermarket pumps were not up to the job.
If you can find/develop a better pump you may find that solves the stagnant coolant issue.
Thanks for the info Baz, spot on as always.
While I cannot explain what was going through Porsche engineers minds when they put the thermostat on the return side on the engine. how easy is it to modify the system to put the thermostat onto the correct side?
Also some months ago there was a thread where we discussed putting a smaller pulley onto the water pump - for the same reason, to speed up coolant flow at idle to stop boiling, have you had any thoughts about modified pulleys etc?
While I cannot explain what was going through Porsche engineers minds when they put the thermostat on the return side on the engine. how easy is it to modify the system to put the thermostat onto the correct side?
Also some months ago there was a thread where we discussed putting a smaller pulley onto the water pump - for the same reason, to speed up coolant flow at idle to stop boiling, have you had any thoughts about modified pulleys etc?
anonymous said:
[redacted]
Oh come on you must do better....anyone with ANY race or tunning/engine building skills knows after a period of hot running (either stationary in traffic on a hot day or after a high speed run) if you were to turn off the car the temp inside the engine continues to rise for a time hence with tuned turbo engines they sometimes have turbo timers ect thats just known basics,I dont think your qualified to speak on the same page as Baz,both as a pun and a fact!Gary11 said:
Oh ok "Regardless of the scenario"
you simply cant be serious?
G
"I don't think five seconds of revving off the line is enough to conduct heat through the metal and in turn overheat the cooling system"you simply cant be serious?
G
We were discussing potitial causes of hot spots not overheating the cooling system in the normal sense,the basis of this (I think) is its normaly similar bore pick up on similar pistons with piston material deposited on bores.
Again (I may be wrong) based around mechanical "inneficientcies" present in the design and sometimes poor construction by the manufacturer?
Baz in another thread has measured the realtime engine temp being a lot hotter than the temp gauge.
Edited by Gary11 on Sunday 14th August 13:23
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
Baz I am grateful that you share your experiences and observations here, particularly on this very matter as, when my car is returned from its engine rebuild (due to suffering from scored bores) - hopefully this coming week, it's very useful to read any tips and pointers on avoiding it recurring.
Baz, just a side question if you don't mind - and not especially related to the flat 6 engines.
Under full throttle, what sort of figure should the coolant temp not exceed on outer cylinder walls?
As far as I know coolant temp varies between 80°C and 100°C under "normal" operating conditions, but I'm wondering what is usually the highest acceptable temp differential between the radiator and outer cylinders walls?
Under full throttle, what sort of figure should the coolant temp not exceed on outer cylinder walls?
As far as I know coolant temp varies between 80°C and 100°C under "normal" operating conditions, but I'm wondering what is usually the highest acceptable temp differential between the radiator and outer cylinders walls?
anonymous said:
[redacted]
Im so glad Baz stepped in! I can see NO help or function to the content of any of your posts questioning helpfull ideas made by someone who does this as a freebie (posting on here) cynicaly an engine rebuilder could advocate WOT at all times.I have spoken with many owners looked at mamy damaged engines and therefore feel more comfotable knowing why a particular engine is prone to fail and the repair proccess thereon.
Im sure the temp monitoring was a bit more sophisticated than the cars own gauge!
Its is reasurring as an owner to know there is a modification rather than like for like replacement of a flawed part.
That is the purpose of (I hope Im right)Bazs informative posts IE a functioning PH forum.
What use is your reply to anyone? and what "hands on" experiance have you to qualify your reply chap?
ISTR this is certainly not a new problem for Porsche either. Baz can expand on the details but ISTR way back on the 944 S2 they could seize up very quickly if allowed to overheat and thus score the bores, although I always thought the tight piston to cylinder wall gap on that engine was part of the problem. More recently Baz had spotted this was also due to the HG deteriating on what are now mostly very old cars and allowing a coolant flow path effectively allowing one part of the block to overheat. Certainly not unique to Porsche either, I had a cylinder head crack on another car due to a hotspot most likely caused by an air trap in the coolant.
Wow that is some back tracking. You said you didn't believe a few seconds of power could heat up the metal enough to damage an engine yet Baz then said from his own testing an engine will fail after 3 to 4 seconds.
Yes its fine to question the things those in the business say on here but at no point have you offered a counter argument based on either a credible analysis or experience.
Yes its fine to question the things those in the business say on here but at no point have you offered a counter argument based on either a credible analysis or experience.
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