Guest

You're right, aluminum differs from iron when it comes down to a thermal issue. If you look at it in general, the purpose of an internal combustion engine is to convert stored energy (gas) to thermal energy, causing an expansion of the gas fluid, creating pressure that converts to mechanical force by turning the crank. Lots of energy loss. In no way is this related to compression, the compression stage is beforehand. The loss comes in with the aluminum transferring more of the heat (energy) out of the combustion chamber into other places, the block, the air, the coolant... results in a power loss when compared to the iron. Dynamic can be determined if you are really good at math (or have a program), the valve opening and closing times (and air under pressure) effect how much air you can actually get into the cylinder to compress. Static compression is based on the assumption of a full cylinder basically with no valves. Yea... like thats realistic With poor cylinder filling, and bleeding off air due to lots of overlap, you end up with less compression than static. Do it wrong, and you can really screw things up. Try to stuff too much air in, and you run out of octane in essence, and the engine will knock all the time. Thats why if you look at many of the larger cams, they recommend a higher compression ratio, and vice-versa.

Having a set of the vette heads, they are decent enough, but I wouldnt waste my time with them unless you got them reasonably priced (450 isnt IMO) or you wanted to shed some weight off the front of the car. You could probably port the stock heads to be close enough not to worry about. I think the SR's take the guesswork out of proper porting, thats about it... my .02

Ricktpi

AMEN
A .5 increase in compression is needed on an aluminum head to equal out to an iron head due to thermal loss.