Say you have 8.5 to 1 compression ration with 72cc heads. What happens if you put 64cc heads on, does it raise or lower the compression ratio? Do you need to know more info and is there some sort of formula to figure this out?
Matt

 

Putting heads with smaller combustion chambers will increase compression. There is a formula, compression ratio=swept volume+clearance volume/clearance volume. Swept volume is per cylinder, so it is displacement divided by number of cylinders. clearance volume is chamber volume+deck volume+gasket volume+valve relief volume. Make sure you use all the same units, it is easiest to convert to ccs.