Suppose you had a set of identical cast iron heads and aluminum heads. Flow numbers etc. were identical. Cast Iron heads were installed with 9.5:1 compression and the aluminum heads were installed with 10.5:1 compression. All else being equal. Which would make more horsepower or would they make the same horsepower? Reason for the compression ratio's is that supposedly you can go 1 point higher with the aluminum heads. Allen

 

They would be about the same.

Not only is it possible to increase the CR with aluminum; it is required in order to obtain equal thermal efficiency, because the heat (energy) loss to the cooling system through the aluminum is greater than through cast iron.

So, although there are far more variables involved, it's usually a safe assumption that you need to raise the CR about ¾-1 full point with aluminum heads to make up for the increased losses. In the real world it isn't really an option, it's a design necessity.