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      03-05-2010, 01:21 PM   #10
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Drives: 2011 M3 COUPE
Join Date: May 2008
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Originally Posted by TrackRat View Post
The expansion stroke losses all practical push beyond 90 degrees after TDC because the change in cylinder volume by virtue of the downward piston motion increases faster than the pressure rise of the burning charge. The charge is still burning when the exhaust valve opens but the pressure rise has dropped long before that. Slow burn fuels can allow more energy to go out the exhaust valve compared to fast burn fuels.

In all situations a slow burning fuel of the same octane and energy content delivers less power than a fast burning fuel. In addition to the increased pressure build-up before TDC required with a slow burn fuel, the heat loss (rejection) to the cooling system increases also resulting in an additional tiny loss of power. If the engine could mechanically stand the forces, you'd want a really fast burn rate but if it's too high you get crank rumble as everything starts bending and vibrating from the extremely fast pressure rise. If the burn rate is excessive it's like hitting the piston with a sledge hammer at TDC.
Awesome info... Thanks