Aluminum vs. Cast Iron Heads - Many people don't pay attention to the compression ratio when going from cast iron to aluminum heads. Fact is that if everything was equal the cast iron head will make more power than an aluminum head. This is because the cast iron head doesn't absorb heat from the combustion process as fast at the aluminum head. Therefore more heat is available to push the piston down. The way to get back the lost horsepower is to up the compression on the aluminum head. Typically you can run about a half to full point more compression with an aluminum head over the cast iron.