are you talking about the volume of the air/fuel mixture being drawn into the cylinder?

 

Swept Volume (volume at TDC to BDC) - same effect as decreasing the stroke of an engine, will decrease the compression, all else being the same.

 

it would be variable depending on how much carbon was on the piston and deposited in the combustion chamber itself, but we've done top engine cleanings for varying reasons, smog failure, detonation etc...and have seen compression numbers drop.

 

You are right; there would be a ton of variables involved.

 

gotta take these on a case-by-case basis sometimes