[DanEC]

Originally Posted by WardL

The online calculators will do it. Here are the equations: if you have a 427 Cubic Inches, multiply 427 by 16.387 to get 6,997 CC (your basic 7 Liter engine). Since you have 8 cylinders, divide by 8 to get each cylinder volume or 874.7 CC. So each cylinder is 874.7 CCs when the piston is at the bottom of the stoke. You say your combustion cylinder is 73 CC so 874.7 divided by 73 equals 12, so your compression ratio is 12:1. If your combustion cylinder size was 88 CC, then 874.7 divided by 88 would be 9.9 to 1. It's always good to check your calcs online somewhere.

I think to get very close with that you need to consider the head gasket thickness and definately the piston top popup/dish and where top of piston sits with respect to the engine block surface at TDC. These can make a pretty large difference in the final calculated compression ration.