Aluminum heads

Compression ratio and cylinder pressure are both results from mechanical features. Aluminum or cast iron has nothing to do with either, as long as the chambers are the same volume. The only positive attribute aluminum has on combustion is, the quicker heat dissipation, which does allow for more compression, but only through the more effective heat dissipation. The physical compression or cylinder pressure is unchanged from cast iron to aluminum all other things being equal. A compression test would surely be the proof there.