Higher compression is just about a necessity with an aluminum head, as more combustion heat is shunted away to the coolant, leaving less heat energy available to be applied to the pistons and ultimately to driving the vehicle. On average, 1/3 of the heat released by the burning fuel goes out the exhaust, a third goes to the radiator, and the rest drives the car. More efficient cooling provided by an aluminum head actually robs power from the pistons.
Jerry
