FWIW and IMHO..... The above 2 posts are apples and oranges.
- Hotter iron chambers make more cylinder pressure inside (prior to the start of combustion)? Yes. But that value of cylinder pressure is not 'compression ratio'. It reflects both compression heating, AND the absorption of heat from the chambers, which are 2 independent contributors to the heating of the mixture.
- The effects of cylinder pressure on torque/HP all depends on the difference between the starting and ending pressure during the combustion and power stroke cycles, not by the absolute numbers themselves.
- More HP? Not just based on a different cylinder pressure (piror to combustion) by itself. I agree, I've looked and looked for definitive results showing a difference going from iron to AL and have found none published; the trick in such testing is to always make the other factors identical.
- BTW, RF, your positive experience with the head swap does not zero in on the exact cause of the difference you found unless you flowed both heads and found them identical. I'd guess you got some benefit from flow (but that is truly a guess); but you certainly got some benefit from more efficient combustion (partly from the higher CR, and partly from the smaller chambers), better cylinder clearing, and better extraction of power at the higher CR due to a bigger start-to-end pressure difference.)
I'd change AJ's statement to say that, rather than 'curing' anything, the cooler AL heads ALLOWS more CR without getting into detonation. AL heads widen up the operating range for CR and cam selection by reducing an undesired factor in the engine operation (extra mixture heating during compression).
