Keith Black pistons VS Speed Pro for 318

Look at the piston specs for the compression distance. This is the distance from the center of the wrist pin to the top of the piston. A difference here would account for the difference in compression ratio. Although the top of the piston still travels the same distance (stroke), the size of the available space in the combustion chamber at TDC changes. I went to Summit's site & compared what I think are the 2 pistons you are talking about & the SP has a CD of 1.755 in. The KB has a CD of 1.810 in. The KB sticks up .055 in. higher.