997 Gen 2 Engine Failure/Bore Scoring
Discussion
Well we are certainly not the causes of that as we always state when asked that the numbers failing are still very low and they are well worth the risk - however - as engineers seeing and measuring inside countless failed engines it is perfectly clear that as they age and mileages increase - so will failure rates.
I agree Gen 2 should be much much more reliable - but some have failed and some have scored bores and when the earlier Gen 1 models started with problems it was equally hidden under a mist of miss-information for a long time and it was only really engineers that could assess the internals and judge the designs that correctly predicted the likely outcome and fortunately for many used that assessment to invest in better solutions that have helped many to obtain a better rebuild at a more modest cost.
Keeping our eye on the potential problem (or lack of it) has helped people with the earlier models but the later ones (with the same process) look brilliant - except for some question mark over possibly the coating on some of the pistons - something we are trying to establish the answer for.
Baz
I agree Gen 2 should be much much more reliable - but some have failed and some have scored bores and when the earlier Gen 1 models started with problems it was equally hidden under a mist of miss-information for a long time and it was only really engineers that could assess the internals and judge the designs that correctly predicted the likely outcome and fortunately for many used that assessment to invest in better solutions that have helped many to obtain a better rebuild at a more modest cost.
Keeping our eye on the potential problem (or lack of it) has helped people with the earlier models but the later ones (with the same process) look brilliant - except for some question mark over possibly the coating on some of the pistons - something we are trying to establish the answer for.
Baz
Baz I don't know if this has ever been asked but do you have a feel for the comparative risk of bore scoring in the 996/997 era of cars compared to for example the front engined cars 944/968 era. People forget that those cars could bore score, it was rare but they do happen sometimes.
I am no engineer, but is there a possibility that this whole issue of bore scoring, points to a fundamental problem with all horizontally orientated pistons?
In a conventional engine, with the pistons mounted vertically, any muck or swarf in the bore, is going to drop out the bottom into the sump where it will be filtered out.
With the horizontally mounted piston layout, this same muck or swarf, cannot fall out and will be held in the bore for longer, before being dispersed.
This theory is based on little engine knowledge but it would be interesting to look at other horizontally laid out engines ( eg Alfa, Subaru etc) to see if they too have had this bore scoring issue.
Like many, I find it incredible that you almost need to factor in an £8-10K engine re- build, before considering a second hand 996 or 997!
In a conventional engine, with the pistons mounted vertically, any muck or swarf in the bore, is going to drop out the bottom into the sump where it will be filtered out.
With the horizontally mounted piston layout, this same muck or swarf, cannot fall out and will be held in the bore for longer, before being dispersed.
This theory is based on little engine knowledge but it would be interesting to look at other horizontally laid out engines ( eg Alfa, Subaru etc) to see if they too have had this bore scoring issue.
Like many, I find it incredible that you almost need to factor in an £8-10K engine re- build, before considering a second hand 996 or 997!
NJH said:
Always a risk though of oil pooling on the bottom face of the cylinder, whereas in pretty much every other design (V, I4, I6 etc) it won't. I have no idea if this makes much difference or is a contributory factor.
Thats what causes flat engines to smoke a little on start up, ie a little oil seeps past the rings.Its no problem though.
To return to your question Andrew - if I was in your shoes, given the inherently better reliability of the DFI enginess I'd probably decide NOT to fork out on the OPC warranty, and instead build a small pot of cash on a monthly basis for the 'in case' scenario. That way you keep the dough not Porsche Finance, and you can use it anyway you wish.
If you treat these cars sensibly (ie with some mechanical sympathy!) and have them inspected and serviced annually, I'd be AMAZED if people were to encounter any serious issues with the engine now. Surely Porsche have learned their lesson?!
Having said that - we were all pretty amazed that a celebrated manufacturer of premium sports cars like Porsche, with almost their entire reputation built on endurance racing, could have allowed the issues with M96/M97 to come to pass in the first place!
If you treat these cars sensibly (ie with some mechanical sympathy!) and have them inspected and serviced annually, I'd be AMAZED if people were to encounter any serious issues with the engine now. Surely Porsche have learned their lesson?!
Having said that - we were all pretty amazed that a celebrated manufacturer of premium sports cars like Porsche, with almost their entire reputation built on endurance racing, could have allowed the issues with M96/M97 to come to pass in the first place!
All alloy silicon enriched bore surfaces (except Nikasil) will gradually release small silicon particles from the surface. What damage they do depends upon the size of the particles, the thickness of the oil separating the cylinder bore from the piston surface and the hardness of the piston coating. Elevated temperatures reduce oil film thickness and the hardness and bonding strength of plastic coated pistons but so not affect the hardness of ferrous coated pistons.
Early ferrous coatings were first zinc, then copper than a hard iron then a tin flash. Recent ferrous coatings seem different but as yet we have not determined what the differences are although we are researching it.
The higher the mileage the greater the erosion of substrate and the more silicon particles are released.
944's and 968's ran quite cool as all the coolant entered the cylinder block. Most that scored bores were 944 S2's (because the piston taper was the least of all the models and the clearances were very small).
Usually it was cylinder 3, 4 or both. The coolant entered the block at cylinder 1 and flowed to cylinder 4 where it escaped so cylinder 3 and 4 were the hottest and therefore had the thinnest oil film but when the head gasket rotted the coolant could miss out travelling to cylinders 3 and 4 and short circuit to escape into the head at cylinder 1 or 2 and 3 and 4 got much hotter and this was usually the main contributory factor in scoring together with worn ferrous coatings.
The biggest problem with horizontal cylinders is that the thrust face of the piston is on top on bank 2 and underneath on bank 1 so when stopped oil does collect at the bottom of the pistons in both bores (as correctly pointed out) but this adds oil to the thrust face of bank 1 on start up but dries it from bank 2 - which is always starting with less oil.
Also as the coolant (now reduced just in this model range to a small flow and a tiny proportion of the whole) enters both cylinders at the bottom that is always cooler in bank 1 and therefore has a thicker oil film - hence it is bank 2 that scores bores.
The oil spray jets allow oil to enter the under piston area but under gravity it falls to the bottom as well - but we have fitted double jets and aimed them at the top of bank 2 to increase oil supply but this made no difference to piston coating wear rates - so it seems it is the running temperatures combined with piston coatings that contributes to scoring.
Gen 2 has 100% coolant into the blocks, Alusil cylinders and (it seems) a new type of ferrous coating - so should last much longer.
A thicker oil, lower coolant temperature (LTT), avoiding hard acceleration from cold and after being stationary when hot will all help extend cylinder life.
The distribution of silicon and the bonding strength in the matrix of Lokasil does not seem as good as Alusil and so there may also be a random failure depending on that distribution and security.
Once a large silicon particle has penetrated the plastic coating - if it hits another particle before escaping it can set up a sequence similar to the causes of pot holes in which the lose particles can knock out another piece and then there are two particles etc and the failure can become a classic catastrophe theory example and fail quickly.
It seems that the original ferrous coated pistons worked well enough in Lokasil but as power and torque increased with variable valve lift and bigger capacity engines (without significantly increasing coolant flow or cylinder coolant volume and space - in fact reducing it) the introduction of plastic coated pistons was a step too far for the level of reliability most expect of the marque.
Fortunately as a very high proportion of owners never drive with maximum torque or very high loads and top speeds - they neither get the piston crown too hot, nor the coolant, nor the cylinder walls and reduce thrust loads - so in that scenario they survive well.
But owners wanting to screw the cars should be able to if they want to without experiencing significantly greater failure rates - but this is where the damage is done.
Baz
Early ferrous coatings were first zinc, then copper than a hard iron then a tin flash. Recent ferrous coatings seem different but as yet we have not determined what the differences are although we are researching it.
The higher the mileage the greater the erosion of substrate and the more silicon particles are released.
944's and 968's ran quite cool as all the coolant entered the cylinder block. Most that scored bores were 944 S2's (because the piston taper was the least of all the models and the clearances were very small).
Usually it was cylinder 3, 4 or both. The coolant entered the block at cylinder 1 and flowed to cylinder 4 where it escaped so cylinder 3 and 4 were the hottest and therefore had the thinnest oil film but when the head gasket rotted the coolant could miss out travelling to cylinders 3 and 4 and short circuit to escape into the head at cylinder 1 or 2 and 3 and 4 got much hotter and this was usually the main contributory factor in scoring together with worn ferrous coatings.
The biggest problem with horizontal cylinders is that the thrust face of the piston is on top on bank 2 and underneath on bank 1 so when stopped oil does collect at the bottom of the pistons in both bores (as correctly pointed out) but this adds oil to the thrust face of bank 1 on start up but dries it from bank 2 - which is always starting with less oil.
Also as the coolant (now reduced just in this model range to a small flow and a tiny proportion of the whole) enters both cylinders at the bottom that is always cooler in bank 1 and therefore has a thicker oil film - hence it is bank 2 that scores bores.
The oil spray jets allow oil to enter the under piston area but under gravity it falls to the bottom as well - but we have fitted double jets and aimed them at the top of bank 2 to increase oil supply but this made no difference to piston coating wear rates - so it seems it is the running temperatures combined with piston coatings that contributes to scoring.
Gen 2 has 100% coolant into the blocks, Alusil cylinders and (it seems) a new type of ferrous coating - so should last much longer.
A thicker oil, lower coolant temperature (LTT), avoiding hard acceleration from cold and after being stationary when hot will all help extend cylinder life.
The distribution of silicon and the bonding strength in the matrix of Lokasil does not seem as good as Alusil and so there may also be a random failure depending on that distribution and security.
Once a large silicon particle has penetrated the plastic coating - if it hits another particle before escaping it can set up a sequence similar to the causes of pot holes in which the lose particles can knock out another piece and then there are two particles etc and the failure can become a classic catastrophe theory example and fail quickly.
It seems that the original ferrous coated pistons worked well enough in Lokasil but as power and torque increased with variable valve lift and bigger capacity engines (without significantly increasing coolant flow or cylinder coolant volume and space - in fact reducing it) the introduction of plastic coated pistons was a step too far for the level of reliability most expect of the marque.
Fortunately as a very high proportion of owners never drive with maximum torque or very high loads and top speeds - they neither get the piston crown too hot, nor the coolant, nor the cylinder walls and reduce thrust loads - so in that scenario they survive well.
But owners wanting to screw the cars should be able to if they want to without experiencing significantly greater failure rates - but this is where the damage is done.
Baz
Fl0pp3r said:
To return to your question Andrew - if I was in your shoes, given the inherently better reliability of the DFI enginess I'd probably decide NOT to fork out on the OPC warranty, and instead build a small pot of cash on a monthly basis for the 'in case' scenario. That way you keep the dough not Porsche Finance, and you can use it anyway you wish.
If you treat these cars sensibly (ie with some mechanical sympathy!) and have them inspected and serviced annually, I'd be AMAZED if people were to encounter any serious issues with the engine now. Surely Porsche have learned their lesson?!
Having said that - we were all pretty amazed that a celebrated manufacturer of premium sports cars like Porsche, with almost their entire reputation built on endurance racing, could have allowed the issues with M96/M97 to come to pass in the first place!
I went through this last year and decided not to renew the warranty: I've had my car from 13-29k miles so far and all seems well (taps coffee table hastily…). Only issue was that OPC introduced a damn great airlock into the cooling system, which stranded me halfway home in a snowstorm. Made them do a complete leakdown/compression test and all was well. After that however, I decided I'd rather put my money into a good indie, on the basis that, if it's maintained properly, it shouldn't be a high risk. But I did intentionally go for a Gen 2 for both performance and peace of mind.If you treat these cars sensibly (ie with some mechanical sympathy!) and have them inspected and serviced annually, I'd be AMAZED if people were to encounter any serious issues with the engine now. Surely Porsche have learned their lesson?!
Having said that - we were all pretty amazed that a celebrated manufacturer of premium sports cars like Porsche, with almost their entire reputation built on endurance racing, could have allowed the issues with M96/M97 to come to pass in the first place!
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