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Originally Posted by MNBoxster
Hi,
There are lots of other factors. An engine never achieves complete combustion, if that were true, we'd never need emissions gear.
But, there is an RPM range in each engine where the combustion is most complete, and from the sound of it, it's in the 4k range for the M96 engine.
But other factors contributing to long life, maybe even the dominant factors, are that the Crankspeed dependent Oil and Coolant pumps (run off the crank-driven serpentine belt) may be in their most efficient range here as well.
All the engineer was speaking about was Carbon build-up, but a steady supply of Oil and Coolant will mean more to longevity than the buildup of a little Carbon. Carbon is softer than steel or aluminum and actually acts as a lubricant and sealant. You need some, but too much is a problem because it causes hotspots (retains heat) in the valvetrain and interferes with heat transfer to the head where the coolant can carry it away.
And, you need to put this into perspective. Incomplete combustion takes places at either end of the Tach. At some place north of 4k rpm, the piston doesn't remain at the top of the Compression Stroke long enough to combust all the fuel, even with an altered timing curve. This is why you often hear an exhaust popping on deceleration from a hard run, because unburnt fuel has accumulated in the exhaust and ignites there instead of in the cylinder where it's supposed to.
The interesting thing to me is that Porsche doesn't gear the Tiptronic or the Control Unit to shift at 4k rpm, I wonder why they wouldn't do that? And, the Tip cars generally have a more trouble-free life than the Manuals...
Happy Motoring!... Jim'99
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Carbon can also build up on the intake side. As fuel is exposed to a hot intake valve, it can solidify and build up. The build up within the ring seals is also partially irrespective of combustion. The faster engine speed gives the fuel less time to sit on hot surfaces as it is raced into the combustion chamber.
Also, don't forget about fuel quality and the presence of oil, which are also contributors.