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Originally Posted by JFP in PA
"Quod Erat Demonstrandum". If there was no intermixing of oil and water, how did engine metal shrapnel get into and destroy the pump? Short answer: It didn’t, the pump failed.
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let's think this out from a failure mode standpoint. let's say you are correct and that, somehow, every one of the impeller blades sheared off and entered the cooling system. let's also consider that, somehow, even though we lost EVERY impeller blade, the water temp did not rise prior to engine failure. in the motor, there are four general places the coolant will go:
1. left head
2. right head
3. water jacket (bottom end)
4. oil cooler
if the debris enters either head, it can plug up one or more of the SMALL passages designed to cool the head. if this occurs, water will continue to circulate in the system and the dash will read normal temperatures. this, incidentally, is why this type of failure is so dangerous. at any rate, once a small passage is plugged up, a LOCAL hot spot will occur. this causes uneven expansion of the metal and a crack in the head forms. this failure allows intermix of oil and coolant & loss of compression. engine is dead.
if the debris enters the water jacket, it will probably exit back out. this area is comparatively large. if it DOES happen to plug up a portion of the water jacket, we will again see a localized temperature rise in the metal matrix composite (MMC) cylinder wall material. if this occurs, we would expect to see a crack / failure in the MMC itself. this would be the D-chunk type failure. engine is dead.
if debris were to enter the radiators & restrict flow, we would expect to see elevated water temperatures. eventually, if left unchecked, oil temperatures would also rise. this would lead to a collapse of the viscous layer between metal parts (like rod bearings & crankshaft journals), allowing metal to metal contact and eventual failure.
finally, we get to the ONLY viable failure mode i can think of where we'd lose the BOTTOM end of the motor as the result of debris in the cooling system: debris enteres the oil cooler. were this to occur, i think it is possible for coolant temps to remain stable while oil temps are allowed to rise. in this situation, it would be possible for the oil to lose viscosity & allow contact between the rod bearings & crank journals. i have to admit, i am unsure as to the exact passage the coolant follows leading into the oil cooler & am not 100% sure that the debris wouldn't get filtered out by this point.
so, theoritically, were the impeller to disintegrate, it MAY be possible to kill the bottom end of the motor without seeing higher coolant temps and without intermix.
now, let's look at byron's actual case. he had EXTREME detonation of the bottom end. rod caps were hurled through the case. this was VIOLENT. even if it was not the cause of the failure, i would be shocked if the MMC of one or more cylinders wasn't cracked, if not outright compromised. further, the chances of shedding EVERY impeller blade simultaneously with no debris contact is, well, almost zero. it simply doesn't happen.
finally, let's look at what we know of the M96's shortfalls and what we know of WHERE byron was on the track. the M96 is known to suffer oil starvation issues that are caused by cavitating the scavenge pumps in the cylinder heads. byron's failure occured shortly after the infamous turn 12. this is a sustained, high-g, right hand corner. the boxster has known issues with sustained high-g right hand corners: the oil pools toward the back of the driver's side cylinder head, AWAY from the scavenge pump. this allows cavitation of the sump & a drop of oil pressure. the result is metal to metal contact in the motor's bottom end, resulting in catastrophic failure similar to what byron experienced.
byron - can we take this thing apart this summer?
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