indy "x" heads or edelbrock rpms?

They really don’t make more power-by themselves. Aluminum can dissipate heat better than cast iron heads, allowing the temperature in the chamber to be lower. This permits the compression ratio to be raised without raising the octane level in your fuel. The raised compression is what makes more power. Raising the compression still higher requires a switch to higher octane race fuels

http://www.worldcastings.com/customer_service/8/Why_do_aluminum_heads_make_more_power?.html