compression on a 318

Agree with treblig , it's is in the compression height of the piston and in the c.c.'s of the combustion chamber . speed pro makes a forged piston that is a flat top design which is a little help . IMO , milling the head or block to gain c.r. causes more issues than it resolves . Of course , custom pistons are an option . You cold order pistons for a 340 in 3.910" bore , spendy but doable .