M96 Engine failure teardown this weekend
01-22-2009, 01:27 PM
#91
Race Car
01-23-2009, 11:32 PM
#93
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it was always a center cylinder. My thinking was the block was undergoing some stress/twisting and the crack happened at the weakest point.
Mr. Raby reports the failure is not limited to any cylinder but appears in all of them with no apparent pattern.
Thus this would appear that twisting of the block not to blame.
Several things come to mind: One is that the failing cylinder experiences improper combustion -- detonation or something close to it -- and the extreme pressure spikes cause a crack. I note the crack is always at the top of the cylinder. The open deck design doesn't help.
Another possibility is the cooling of the area that cracks is uneven. Perhaps low coolant or maybe coolant that has lost some of its ability to resist boiling causes an air pocket to periodically form. This allows this area to heat up and then when the pocket collapses the relatively cooler coolant heat shocks this area. If this happens, over and over again, at some point the wall fails.
The Boxster engine can suffer from oil starvation under some circumstances. One is under extreme braking in (IIRC) a left hand and high speed corner.
This forces oil under the right cylinder bank camshaft cover away from the scavenge pump located at rear of head under camshaft cover. A huge volume of oil is constantly fed to the cams and valve gear. It doesn't take much time for a considerable volume of oil (just several guarts is all it takes, maybe less if oil level a bit lower than optimum) to accumulate under the camshaft cover.
This can result in the oil level in sump going down to the point the oil pump may ingest oil with more air in it. If the oil "old" its anti-foaming additives may have been depleted too.
If the main bearings, or rod bearing don't suffer the hydraulic lifters might. With air in the oil these lifters will collapse though how this can result in engine failure I'm not sure. That is causes a drop off in power at higher rpms is a given as the valves no longer follow the desired valve action and lower lift, less duration, etc. work to reduce engine output.
If this air laden oil makes it ito any of the camshaft chain tensioners a tensioner could fail to maintain proper cam chain tension. This can be very serious cause the chain with more slack -- and at high speeds -- jump/skip a tooth and valve contact with piston can result with disasterous results.
Or the chain slams against tensioner that has lost its ability to maintain tension and breaks the tensioner/guide. Same bad result for engine.
Sincerely,
Macster.
Mr. Raby reports the failure is not limited to any cylinder but appears in all of them with no apparent pattern.
Thus this would appear that twisting of the block not to blame.
Several things come to mind: One is that the failing cylinder experiences improper combustion -- detonation or something close to it -- and the extreme pressure spikes cause a crack. I note the crack is always at the top of the cylinder. The open deck design doesn't help.
Another possibility is the cooling of the area that cracks is uneven. Perhaps low coolant or maybe coolant that has lost some of its ability to resist boiling causes an air pocket to periodically form. This allows this area to heat up and then when the pocket collapses the relatively cooler coolant heat shocks this area. If this happens, over and over again, at some point the wall fails.
The Boxster engine can suffer from oil starvation under some circumstances. One is under extreme braking in (IIRC) a left hand and high speed corner.
This forces oil under the right cylinder bank camshaft cover away from the scavenge pump located at rear of head under camshaft cover. A huge volume of oil is constantly fed to the cams and valve gear. It doesn't take much time for a considerable volume of oil (just several guarts is all it takes, maybe less if oil level a bit lower than optimum) to accumulate under the camshaft cover.
This can result in the oil level in sump going down to the point the oil pump may ingest oil with more air in it. If the oil "old" its anti-foaming additives may have been depleted too.
If the main bearings, or rod bearing don't suffer the hydraulic lifters might. With air in the oil these lifters will collapse though how this can result in engine failure I'm not sure. That is causes a drop off in power at higher rpms is a given as the valves no longer follow the desired valve action and lower lift, less duration, etc. work to reduce engine output.
If this air laden oil makes it ito any of the camshaft chain tensioners a tensioner could fail to maintain proper cam chain tension. This can be very serious cause the chain with more slack -- and at high speeds -- jump/skip a tooth and valve contact with piston can result with disasterous results.
Or the chain slams against tensioner that has lost its ability to maintain tension and breaks the tensioner/guide. Same bad result for engine.
Sincerely,
Macster.
01-24-2009, 02:29 AM
#94
Race Director
New? well, 10 years or so new. The cylinders are rather strange....
Could be the cylinder material. I've never heard of cast iron cylinders cracking although most iron block engines used a closed deck which supports the upper cylinder wall better.
Sincerely,
Macster.
01-24-2009, 02:35 AM
#95
Parts Specialist
Rennlist Member
Rennlist Member
though. I've seen pictures of broken cylinders and the material exposed looks more like ceramic than aluminum.
Could be the cylinder material. I've never heard of cast iron cylinders cracking although most iron block engines used a closed deck which supports the upper cylinder wall better.
Sincerely,
Macster.
Could be the cylinder material. I've never heard of cast iron cylinders cracking although most iron block engines used a closed deck which supports the upper cylinder wall better.
Sincerely,
Macster.
True, but have you ever cut cast iron drain pipe? All it takes a a score line and then it snaps like how one would cut a piece of glass...
I dont know what materials we have, but I just brough this up
01-24-2009, 02:36 PM
#96
Race Director
Cast iron drain pipe is brittle cause the manufacturer...
Engine blocks are a different iron and have received some heat treatment to reduce internal stresses. Also, machining works to remove these as well.
I don't necessarily disagree with your point about the composition of the M96 engine cylinders being a contributor maybe the sole contributor to the cracked cylinders.
Any casting process can introduce stresses or weak points from inclusions of some contaminate. The cylinder is actually a rather porous insert with a high content of silicon that is fitted into the engine block mold and then during the casting process molten aluminum flows into this insert. If the insert is banged around during shipment, assembly, etc., it could have a crack that will cause it to fail later when the engine's in service. We do not have access to the Porsche factory failure analysis of the engines that fail so we don't know how big (or small) a percentage of early engine troubles/failures arise from this area.
Until something is clearly identified as the cause just about anything is suspect.
Sincerely,
Macster.
01-25-2009, 12:37 AM
#97
Burning Brakes
Any cylinder can fail... We have seen more outboard cylinder failures than center cylinder failures...
We are just starting to gather enough trend data to make accurate hypothesis about whats occuring and why... It takes direct interface with a high number of failed engines to see this and prior to our program the factory was seeing the engines so things were swept under the carpet.
More effort has gone into why and how things fail than anything.. The solutions to the issues only required money to create and thats the easy part coupled to somne intense motivation, dedication and application.
We are just starting to gather enough trend data to make accurate hypothesis about whats occuring and why... It takes direct interface with a high number of failed engines to see this and prior to our program the factory was seeing the engines so things were swept under the carpet.
More effort has gone into why and how things fail than anything.. The solutions to the issues only required money to create and thats the easy part coupled to somne intense motivation, dedication and application.
01-25-2009, 01:00 AM
#98
Drifting
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Any cylinder can fail... We have seen more outboard cylinder failures than center cylinder failures...
We are just starting to gather enough trend data to make accurate hypothesis about whats occuring and why... It takes direct interface with a high number of failed engines to see this and prior to our program the factory was seeing the engines so things were swept under the carpet.
More effort has gone into why and how things fail than anything.. The solutions to the issues only required money to create and thats the easy part coupled to somne intense motivation, dedication and application.
We are just starting to gather enough trend data to make accurate hypothesis about whats occuring and why... It takes direct interface with a high number of failed engines to see this and prior to our program the factory was seeing the engines so things were swept under the carpet.
More effort has gone into why and how things fail than anything.. The solutions to the issues only required money to create and thats the easy part coupled to somne intense motivation, dedication and application.
