Originally Posted by thenewrick
Ethanol is used to increase effective octane rating. In turbo cars it's especially noticeable. You can run more boost and advance timings as well as produce less heat while increasing exhaust volume(faster turbo spool).
I realize it increases knock resistance, which I hadn't thought of the implications of for a forced induction engine, that makes sense. But then at the same time, there's only so far you can advance timings and increase compression ratio. That said, I suppose if you've got control over boost pressure, that's fair, you can increase it by taking advantage of the higher KR, and that's not necessarily an engine-controlled parameter.
I guess, mostly, it seems to me that there has to be some pronounced crossover point where the increased knock resistance and following performance gain possibility gets mitigated by the fact that you're effectively removing a lot of energy density from the gas - forcing more air in is great, but if you get twice the charge with gas that's got half the energy in that charge, it's a wash.
I suppose I have no idea where that crossover is, though.
Increased exhaust volume for faster turbo spool makes sense, but do you really get substantially increased volume for less-energy-dense input?
I guess the point is, increased effective octane means you can induct more air and inject a bigger charge, but the composition of ethanol-laden gas would produce less energy and less exhaust for the same volume charge of pure high octane gasoline. So the only increases come from increased charge size, and that'll balance out with the energy decrease at some point.