[Excaliber]

Compression ratio is simple to calculate, but hard to get all the numbers to do the math!

FIRST: How many CC's is the size of the combustion chamber in question? If the the head has been "milled" or made "thinner" then the ORIGINAL CC of it's combustion chamber will be smaller. Small combustion chamber WILL equal higher compression than a larger combustion chamber assuming pistons stay the same.

SECOND: What is the shape/size of the TOP of the piston? Is it flat top? Does it have a bulge on the top? Is the top "scooped out" or "dished"? A dished piston will leave extra room for the compressed gases within the combustion chamber. A flat top will leave ONLY the room IN the combustion chamber head. A DOMED piston will make the combustion chamber area smaller, thus more compression.

THIRD: Compression height of the piston itself. Does that piston come to the top and stop exactly EVEN with the surface of the block? OR (usually) does it not quite come up far enough to be flush with the top of the block?

Assume a "zero deck height" piston, perfectly smooth with the top of the block. Assume a flat top piston, it will neither add nor subtract from the size of the combustion chamber. The last "variable" is the thickness of the head gasket! Thinner gasket, more compression. FAT gasket, less compression.

After you get all the above numbers then you calculate the bore size and the stroke to determine the compression ratio.