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      02-22-2008, 01:15 PM   #163
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Drives: E30 M3; Expedition
Join Date: Apr 2007
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Originally Posted by bruce.augenstein@comcast. View Post
There may or may not be SAE papers which could shed *some* light on the subject (available for some $$), but I'm wearing a big "EMPTY" sign on my forehead at the moment.

I'd say, max power output, durability, and emissions being on par, the engine that consistently has a higher thermal efficiency across the power output range is the "better" design since thermal efficiency is the ultimate measure of how effective an engine is in converting stored chemical energy to do mechanical work.

Generally speaking, without knowing the values of engine specific parameters for the other variables such as combustion characteristics, friction, volumetric efficiency, mass of pistons and rods, the theoretical maximum for the thermal efficiency of an IC engine is dictated by the compression ratio (specific heat of the working fluid matters as well, but that can be assumed to be the same). So whoever consistently can pull off near complete combustion with as little friction as possible and with as much air sucked into the cylinder as possible at a higher compression ratio over the power output range will have the better design.

The only piece of info we have on these engines is the compression ratio. For the 2008 Corvette, it is 10.7:1, and for the E9X M3, it is 12.0:1. But that doesn't mean too much by itself, and I doubt we will see data on all of the other variables. Regardless, the M3's higher compression ratio clearly makes some kind of a technological statement.