996-997 wet-sump engine reliability: enter your stats here!
Discussion
for someone looking to buy a 996(997 are too dear) whats the easiest way to reduce the risk of ending up with some of the problems that are stated? i know turbos and gt series are good but again they are out of range. ive had to have a rebuild on my previous car(not porsche) and want to try and avoid this issue again but love 911's and want to go for one.
from what i see both 3,4 and 3.6 both have ssues but which is the safer option? and is a low mile car going to be safer or is one with full porsche service history better to go for?
im not looking till probaly feb next year and im still undecided on manual(always had manuals) or tip or even coupe or cabrio(never had a cab)
cheers neil
from what i see both 3,4 and 3.6 both have ssues but which is the safer option? and is a low mile car going to be safer or is one with full porsche service history better to go for?
im not looking till probaly feb next year and im still undecided on manual(always had manuals) or tip or even coupe or cabrio(never had a cab)
cheers neil
IMO tips by definition have an easier life regarding over revs and also put the power down much more smoothly and are more gentle in respect of reverse tourque and missed gears ect ect so have therefore an easier life,so I would say the failures of tips in general seem less accross all models but thats only my view.Some one like Baz may know differently?
I would have expected a Tiptronic to have an easier life but unfortuntaley there doesn't seem to be that correlation from the evidence when connecting to cylinder failures.
This seems illogical until to realise two important issues.
(1) When you drive a Tip - in normal driving you are usually running at less revs than you do in a manual - in the gear selected - then when you accelerate - unless you really hit the throttle hard - relativley fast acceleration can be achieved from those low revs - and this means you are using the torque at lower revs. Now the force on the piston that is pushing it against the cylinder wall and the force inside the cylinder (or hoop stress) derived from the fuel burning - is a function of that torque - so the result is that in a tip - you may be more often using higher cylinder loads than in a manual over the lifetime of the car and this may explain why they seem to fail just as often (in fact 3.4s probably more often).
(2) The 3.6 and 3.8 engines produce a lot more torque at low revs (due to the new camshaft variocam systems in use) so this once again gives the opportunity to use even more torque at low revs and again a Tip is more often than not used in this mode without kicking down even for quite fast road work.
I know many people will imagine that the highest loads come about through the highest revs - but although the dynamic loads on the masses flying about are highest then - usually the highest cylinder hoop stresses and load stresses are closer to maximum torque - which is often suiprisingly low down the rev scale.
Baz
This seems illogical until to realise two important issues.
(1) When you drive a Tip - in normal driving you are usually running at less revs than you do in a manual - in the gear selected - then when you accelerate - unless you really hit the throttle hard - relativley fast acceleration can be achieved from those low revs - and this means you are using the torque at lower revs. Now the force on the piston that is pushing it against the cylinder wall and the force inside the cylinder (or hoop stress) derived from the fuel burning - is a function of that torque - so the result is that in a tip - you may be more often using higher cylinder loads than in a manual over the lifetime of the car and this may explain why they seem to fail just as often (in fact 3.4s probably more often).
(2) The 3.6 and 3.8 engines produce a lot more torque at low revs (due to the new camshaft variocam systems in use) so this once again gives the opportunity to use even more torque at low revs and again a Tip is more often than not used in this mode without kicking down even for quite fast road work.
I know many people will imagine that the highest loads come about through the highest revs - but although the dynamic loads on the masses flying about are highest then - usually the highest cylinder hoop stresses and load stresses are closer to maximum torque - which is often suiprisingly low down the rev scale.
Baz
This is correct, of course. The torque an engine delivers is a function of revs and importantly of throttle opening. Peak torque (recorded on full throttle) might be at 4600rpm in a 3.4 and the tip will normally be operating below that BUT on greater throttle opening like for like. Tips deliver the same motive force (assuming the driver is going as quickly) but at lower revs, accordingly torque must be higher ... hoop stress higher. Prodding the throttle in the tip will take you torwards peak torque, prodding the throttle in a manual will take you towards the rev limit.
Edited by ScienceTeacher on Monday 7th November 12:37
ScienceTeacher said:
This is correct, of course. The torque an engine delivers is a function of revs and importantly of throttle opening. Peak torque (recorded on full throttle) might be at 4600rpm in a 3.4 and the tip will normally be operating below that BUT on greater throttle opening like for like. Tips deliver the same motive force (assuming the driver is going as quickly) but at lower revs, accordingly torque must be higher ... hoop stress higher. Prodding the throttle in the tip will take you torwards peak torque, prodding the throttle in a manual will take you towards the rev limit.
I do understand the physics however just using ECU data hardly any tip cars have over rev data substatiating then that these are random failures as opposed to age ,mileage and useage related issues,as it is a fact a manual car with over revdata has had a "harder life" than a tip with none,also the lack of missed gear unloaded over revs and reverse torque loads caused by badly timed or misplaced downshiftsEdited by ScienceTeacher on Monday 7th November 12:37
must help! IMHO.
Edited by Gary11 on Monday 7th November 16:52
Yes, tip cars are unlikely to have been over-revved, but they can still have had a 'hard life', as Baz explained above. He associates scored and oval bores with hoop stress which he proposes is higher in tip cars as on average they are working at lower revs and higher torque. An over-revved manual car might also be likely to suffer too, but for different reasons.
Yes Science Teacher is right - the typical failures we see are related to the problems I mentioned and therefore hoop stress and cylinder wall loading.
Over revving is practically impossible in a Tip but although some cars show records of a few over revs - it is rare to find an actual fault that you could put down to that as the cylinder loading and hoop stress are actually relatively low at these revs and the most likely fault to be connected to an over-rev would be crankshaft bearing failure or possibly valve or tappet damage - which is very rare.
Baz
Over revving is practically impossible in a Tip but although some cars show records of a few over revs - it is rare to find an actual fault that you could put down to that as the cylinder loading and hoop stress are actually relatively low at these revs and the most likely fault to be connected to an over-rev would be crankshaft bearing failure or possibly valve or tappet damage - which is very rare.
Baz
Yes the point I was erring towards was trying to make some sense of the failures Baz so eloquently describes on here spending much of his own time to help owners understand the whys and wherefores and repair methodology,this is very helpfull and Im sure much appreciated,but what does an owner (or prospective owner) gain from this? when the root causes are debated from an engineers point of view there are MANY mechanical failures across many models in all configurations,using Baz as a barometer it seems there is no specific reason for failure,we have debated various driving styles and parameters that may help induce or cause a few of these defects,mechanically however failures are at best major at worst catastrophic and worryingly without cause other than latent bad design,it seems the reworking so far is sucessful a log of repaired engine failures (or non failures) would be very helpfull for the future.
ETA,
If it transpires a small engineering shop can re-engineer the M96/7 units and prevent even some of the total failures for future owners how disappointing Porsche havent tried on these models.
ETA,
If it transpires a small engineering shop can re-engineer the M96/7 units and prevent even some of the total failures for future owners how disappointing Porsche havent tried on these models.
I never thought I would be posting on this thread, but it looks like I will become another statistic
Five weeks of Porsche ownership and it does not look good,
Car 997.1 C2S
34500 miles
1 liter of oil in 680 miles
Left bank exhaust black, right side golden brown
Smoke for about 5secs on next day start up (left bank)
Heavy smoke both exhausts if parked for more than 5 days.
Oh deep joy, should have got the 996 turbo that I always really wanted
Five weeks of Porsche ownership and it does not look good,
Car 997.1 C2S
34500 miles
1 liter of oil in 680 miles
Left bank exhaust black, right side golden brown
Smoke for about 5secs on next day start up (left bank)
Heavy smoke both exhausts if parked for more than 5 days.
Oh deep joy, should have got the 996 turbo that I always really wanted
scarebus said:
I never thought I would be posting on this thread, but it looks like I will become another statistic
Five weeks of Porsche ownership and it does not look good,
Car 997.1 C2S
34500 miles
1 liter of oil in 680 miles
Left bank exhaust black, right side golden brown
Smoke for about 5secs on next day start up (left bank)
Heavy smoke both exhausts if parked for more than 5 days.
Oh deep joy, should have got the 996 turbo that I always really wanted
No warranty, or was it private?Five weeks of Porsche ownership and it does not look good,
Car 997.1 C2S
34500 miles
1 liter of oil in 680 miles
Left bank exhaust black, right side golden brown
Smoke for about 5secs on next day start up (left bank)
Heavy smoke both exhausts if parked for more than 5 days.
Oh deep joy, should have got the 996 turbo that I always really wanted
MTR
Sorry - did I miss something - where did the broken piston ring come from - I have never seen one on these models.
We know several different factors that if they are all combined contribute to scored bores and there is a lot of rubbish printed about it - not the least of which was a recent article proposing several totally wrong explanations and facts. It is very complicated and difficult to understand but that is no exceuse for making inaccurate claims and misleading people.
It claimed the scoring was from piston rings whereas - any engine builder worthy of any note understands that the rings cannot put pressure in one area - only spring pressure all over the rings circumference (although higher at the open ends). The rings cannot just put pressure on the thrust face of the bore on one side (where all the scoring takes place) and the rings sit in a groove that is deeper than the size of the rings so there is no extra pressure from the ring anywhere - certainly not the thrust face - in fact the ovality in the bores would reduce the thrust from the rings on the thrust face area as they resist becoming oval quite well and this leads to the opposite problem off additional blow by.
Then the article said the later engines have no problem with cylinder "D" chunk failures - not true - they do - it is just that not so many have done enough miles and then they may score 1st - but we already have fixed several higher mileage examples that have not scored but cracked or "D" chunked.
Engines with steel liners fitted by 2 different UK suppliers have also been in here to be replaced. In one case they had rotated round in the cylinder as a result of being very thin and the fact that they cannot be fitted with a high interference fit so when the block expands more than the liner they go lose. Further more they have to run bigger cold clearances as they don't expand as much and eventually there can be a problem over head sealing as the block expands about 4 thou (0.1mm) more than the liner in the vertical plain when hot and the head gasket is a thin 3 layer steel design with very little compliance - the problem made worse as the pressure sinks the liner down into the softer Loksail/aluminium support face.
It is true that Loksail is hard wearing and not as strong as steel but then neither is alusil and that worked OK for years and anyway how come the 3.4 996's don't score bores? Manufacturers did not go away from steel (which is undeniably cheaper) by mistake but because proper bore clearances and long life can result from using silicon and aluminium mixes. Lokasil worked very well in the smaller engines and is a great material if a few other factors are sorted out first.
I understand why different engineers and technicians disagree about causes and solutions to problems – especially when competitors offer different solutions. However – despite this natural tendency to colour the reasoning in your favour – these technical points should be published accurately and not mislead others – like that article entitled “Oil smoke from your 997’s exhaust” which has got several things horribly wrong and frankly therefore provides extremely uneducated reasoning and the resulting advice is inaccurate, misleading and in many areas – plainly misguided advertising (not the journalists fault by the way - they just report what they are told).
1
This is why it benefits your readers to have points like these discussed on the Internet - because there is clearly a lot of wrong information going about and it is only those with the confidence and knowlege to point out these innacuracies and argue their point (if neccessary) that enables the less knowlegeable reader to know who and what information to trust.
Baz
We know several different factors that if they are all combined contribute to scored bores and there is a lot of rubbish printed about it - not the least of which was a recent article proposing several totally wrong explanations and facts. It is very complicated and difficult to understand but that is no exceuse for making inaccurate claims and misleading people.
It claimed the scoring was from piston rings whereas - any engine builder worthy of any note understands that the rings cannot put pressure in one area - only spring pressure all over the rings circumference (although higher at the open ends). The rings cannot just put pressure on the thrust face of the bore on one side (where all the scoring takes place) and the rings sit in a groove that is deeper than the size of the rings so there is no extra pressure from the ring anywhere - certainly not the thrust face - in fact the ovality in the bores would reduce the thrust from the rings on the thrust face area as they resist becoming oval quite well and this leads to the opposite problem off additional blow by.
Then the article said the later engines have no problem with cylinder "D" chunk failures - not true - they do - it is just that not so many have done enough miles and then they may score 1st - but we already have fixed several higher mileage examples that have not scored but cracked or "D" chunked.
Engines with steel liners fitted by 2 different UK suppliers have also been in here to be replaced. In one case they had rotated round in the cylinder as a result of being very thin and the fact that they cannot be fitted with a high interference fit so when the block expands more than the liner they go lose. Further more they have to run bigger cold clearances as they don't expand as much and eventually there can be a problem over head sealing as the block expands about 4 thou (0.1mm) more than the liner in the vertical plain when hot and the head gasket is a thin 3 layer steel design with very little compliance - the problem made worse as the pressure sinks the liner down into the softer Loksail/aluminium support face.
It is true that Loksail is hard wearing and not as strong as steel but then neither is alusil and that worked OK for years and anyway how come the 3.4 996's don't score bores? Manufacturers did not go away from steel (which is undeniably cheaper) by mistake but because proper bore clearances and long life can result from using silicon and aluminium mixes. Lokasil worked very well in the smaller engines and is a great material if a few other factors are sorted out first.
I understand why different engineers and technicians disagree about causes and solutions to problems – especially when competitors offer different solutions. However – despite this natural tendency to colour the reasoning in your favour – these technical points should be published accurately and not mislead others – like that article entitled “Oil smoke from your 997’s exhaust” which has got several things horribly wrong and frankly therefore provides extremely uneducated reasoning and the resulting advice is inaccurate, misleading and in many areas – plainly misguided advertising (not the journalists fault by the way - they just report what they are told).
1
This is why it benefits your readers to have points like these discussed on the Internet - because there is clearly a lot of wrong information going about and it is only those with the confidence and knowlege to point out these innacuracies and argue their point (if neccessary) that enables the less knowlegeable reader to know who and what information to trust.
Baz
"[quote=hartech]Sorry - did I miss something - where did the broken piston ring come from - I have never seen one on these models."
Baz ,
I was ust trying to explain the "sort" of damage covered by most dealer supplied MBI policys ie a component needs to mechanicaly fail resulting in a clear broken part anything looking remotely wear and tear orientated is usually upon inspection a rejected claim,its only specialist policys (possibly like yours) that will cover scored bores ect,I am fully aware there is not a broken piston ring .....but if there were and it scored the bores then it would be perhaps covered,
Baz ,
I was ust trying to explain the "sort" of damage covered by most dealer supplied MBI policys ie a component needs to mechanicaly fail resulting in a clear broken part anything looking remotely wear and tear orientated is usually upon inspection a rejected claim,its only specialist policys (possibly like yours) that will cover scored bores ect,I am fully aware there is not a broken piston ring .....but if there were and it scored the bores then it would be perhaps covered,
Edited by Gary11 on Monday 14th November 21:19
Edited by Gary11 on Tuesday 15th November 10:17
I understand - sorry - lots on at the moment and speed reading made me miss your point - but it is still worth mentioning that the article also said overheating plainly cannot be the cause yet the cause they advised is complete nonsense and there is evidence and proof that reducing running temperatures helps.
The article states that no one definitively knows the cause of scored bores whereas everyone knows that we do and suggests that after fitting steel liners you could use the original pistons whereas anyone with any experience of engines knows that any scored bore is scored because the piston face has melted at the surface and the piston is always the first part to fail.
There are a number of contributory factors leading to a scored bore (not one) and we already fix several of them (with excellent results) and are currently testing yet another potential improvement - to see if it makes the engine last even longer.
I do understand that people have different opinions and often there is more than one solution but I have never read a technical article so full of nonsense portrayed as "THE ANSWER" that contains so many fundamental errors that most true engine experts would see through imediately. Furthermore to state that fitting steel liners is the ultimate answer - is like going back years - but I admit - it is a cheaper option at the moment - but not for much longer - watch this space!
Baz
The article states that no one definitively knows the cause of scored bores whereas everyone knows that we do and suggests that after fitting steel liners you could use the original pistons whereas anyone with any experience of engines knows that any scored bore is scored because the piston face has melted at the surface and the piston is always the first part to fail.
There are a number of contributory factors leading to a scored bore (not one) and we already fix several of them (with excellent results) and are currently testing yet another potential improvement - to see if it makes the engine last even longer.
I do understand that people have different opinions and often there is more than one solution but I have never read a technical article so full of nonsense portrayed as "THE ANSWER" that contains so many fundamental errors that most true engine experts would see through imediately. Furthermore to state that fitting steel liners is the ultimate answer - is like going back years - but I admit - it is a cheaper option at the moment - but not for much longer - watch this space!
Baz
Gary, I don't like doing that - in fact I keep quiet about a lot of misleading publicity that is published - mainly because it is not the journalists fault and not the magazines fault and we don't want to fall out with everyone involved in our type of business - nor be accused of thinking we are the only ones that know anything (which we are not and don't claim to be).
We also don't want to harm other businesses (who are often doing their best and providing a useful alternative) - don't want to be accused of being a clever dick or a big head - it is just that every now and again something is published that is so full of technical innacuracies and poor logic or information - it worries me what the public make of it all. It would be so easy for them to be mislead into a poor decision - and the Internet is indeed a good place to argue out the relevant points with knowlegable people form all over the World (something we are not afraid to do).
It is a high profile mag and it just shows how little many people know (or understand these days) that no one else has picked up on it.
When I was an engineering student (OK 45 or so years ago) we all fixed our own cars and motorbikes (we had no alternative) engines, clutches etc - but even some workshop technicians rarely get to see the inside of an engine these days let alone experiment and understand what is going on inside.
I think - gradually - those of us that understand engines work out who knows enough to take seriously and who is just a wind bag, egotist or pure accademic - but that is no help to the general public (or it seems to technical journalists) who are increasingly at the mercy of those promoting a business issue but actually talking rubbish.
I wouldn't worry so much if the reporters made it clear it was the business representatives opinion they were reporting on instead of making broad statements - as if they have their approval and were verifyably a matter of fact and therefore correct.
Baz
We also don't want to harm other businesses (who are often doing their best and providing a useful alternative) - don't want to be accused of being a clever dick or a big head - it is just that every now and again something is published that is so full of technical innacuracies and poor logic or information - it worries me what the public make of it all. It would be so easy for them to be mislead into a poor decision - and the Internet is indeed a good place to argue out the relevant points with knowlegable people form all over the World (something we are not afraid to do).
It is a high profile mag and it just shows how little many people know (or understand these days) that no one else has picked up on it.
When I was an engineering student (OK 45 or so years ago) we all fixed our own cars and motorbikes (we had no alternative) engines, clutches etc - but even some workshop technicians rarely get to see the inside of an engine these days let alone experiment and understand what is going on inside.
I think - gradually - those of us that understand engines work out who knows enough to take seriously and who is just a wind bag, egotist or pure accademic - but that is no help to the general public (or it seems to technical journalists) who are increasingly at the mercy of those promoting a business issue but actually talking rubbish.
I wouldn't worry so much if the reporters made it clear it was the business representatives opinion they were reporting on instead of making broad statements - as if they have their approval and were verifyably a matter of fact and therefore correct.
Baz
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