Quote Originally Posted by t0oL View Post
Just delivered today a 68-72 455 Olds with 11:1 flat top pistons.
Figuring the 11:1 is based on 72 cc chambers and I add my 76 cc aluminum heads (Edelbrock) what would the compression be (approximatly).
If I add .040 thick Cometic head gaskets ( 4.125)
this adds 2.7 cc :
.04 X 4.125 X 16.39 cc/in3 = 2.7
taking the final chambers to 78.7 cc.
Would this allow regular gas with the aluminum heads?
First off, you don't want to add any gasket thickness to lower static compression ratio. When you do that, you increase the squish, which increases the tendency of the motor to detonate.

Normally, when a builder puts a motor together, he figures the stack of reciprocating assembly components, cuts the block decks to whatever he needs to work with the gasket and chooses the correct combustion chamber volume and piston volume to establish the static compression ratio he wants. Then he chooses a camshaft based on being compatible with the static compression ratio. After that, he would choose the torque converter, rear tire and rear gear that would be compatible with the cam timing.

If you'll list the following, we'll try to help you figure it all out.
1/2 crank stroke
rod length
piston compression height
block deck height

If you have to run on pump gas, you never know when the formulations will change, so you want to design the motor so that it will run well on the lowest grade fuel. Those of us who lived through the Arab Oil Embargo know that this can and did happen.

With aluminum heads, I would not try to go beyond 9.7-10.0 static compression ratio. And I would engineer the squish at somewhere between 0.035" and 0.045".

Here's an article explaining squish. They're talking about motorcycle engines here, but it's the same thing no matter the make of engine.
http://www.rbracing-rsr.com/squishcalc1.html