Will a Compression Test Screen a Cayenne for the Piston Scoring Defect?
04-01-2014, 09:25 PM
#31
Three Wheelin'
Proper oil. No WOT driving until oil is up to temp. Don't start it and let it idle to warm up - start and drive off like normal. But, these are the habits of choice for anyone in a cold climate driving any vehicle.
04-01-2014, 09:51 PM
#32
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Thx. Stupid question. Why drive to warm-up vs. idle to warm up?
04-01-2014, 11:20 PM
#33
Three Wheelin'
Most engine wear occurs during startup and the warming phase. If you start the car and let it idle, you're spending a lot of time in that phase. If you drive, you get it up to temp a lot faster and spend less time in that high-wear temperature. This is why the manual suggests you don't let it idle (along with the emissions aspect).
No WOT driving is important because you don't want to strain the motor when things haven't reached that dynamic equilibrium in regards to thermal expansion. It's especially important in the bearing housing on the turbo. At WOT, the turbo is spinning somewhere around 300,000 RPMs, and lubrication is critical. If the oil viscosity isn't correct, this will lead to a lot of wear. Same goes for the piston/bore relationship - especially since CTT pistons are forged and (in general) aren't as thermally stable (may expand faster than the cylinder bore). Got WOT and really heat up that piston, and you will end up with cylinder scoring (though not quite the same type as what happens in the cayenne S).
Likewise - it's important to let the motor cool down a bit if you've spent any appreciable amount of time in boost (climbing a mountain road, towing, at a track, etc). Heat soak can coke (burn) the oil in the turbo after shutdown, and this can quickly lead to turbo failure. This is much more critical in a journal-type bearing that you see in smaller cars (why some use a turbo timer), since the film of oil is what's acting as the "bearing", but the turbo in the CTT is a ball bearing type and a bit more resilient in that regard. The electric coolant pump that cycles with the "rest" function on the HVAC unit, also provides flow after you shutdown to prevent oil coking in the turbo, as well as allowing the whole motor to cool down a bit more evenly. I believe this pump runs with the fans after shut-down. On a lot of other VAG cars, it runs up to 15 minutes after shutdown, or until the coolant as reached a certain temp (whichever happens first), but I'm not sure how long it runs on the CTT.
No WOT driving is important because you don't want to strain the motor when things haven't reached that dynamic equilibrium in regards to thermal expansion. It's especially important in the bearing housing on the turbo. At WOT, the turbo is spinning somewhere around 300,000 RPMs, and lubrication is critical. If the oil viscosity isn't correct, this will lead to a lot of wear. Same goes for the piston/bore relationship - especially since CTT pistons are forged and (in general) aren't as thermally stable (may expand faster than the cylinder bore). Got WOT and really heat up that piston, and you will end up with cylinder scoring (though not quite the same type as what happens in the cayenne S).
Likewise - it's important to let the motor cool down a bit if you've spent any appreciable amount of time in boost (climbing a mountain road, towing, at a track, etc). Heat soak can coke (burn) the oil in the turbo after shutdown, and this can quickly lead to turbo failure. This is much more critical in a journal-type bearing that you see in smaller cars (why some use a turbo timer), since the film of oil is what's acting as the "bearing", but the turbo in the CTT is a ball bearing type and a bit more resilient in that regard. The electric coolant pump that cycles with the "rest" function on the HVAC unit, also provides flow after you shutdown to prevent oil coking in the turbo, as well as allowing the whole motor to cool down a bit more evenly. I believe this pump runs with the fans after shut-down. On a lot of other VAG cars, it runs up to 15 minutes after shutdown, or until the coolant as reached a certain temp (whichever happens first), but I'm not sure how long it runs on the CTT.
