Keith Black pistons VS Speed Pro for 318

The advertised compression is assuming that the block deck height is at it's nominal dimension. Since it hardly ever is the actual compression will be lower than advertised. The only way to know for sure what the compression is to measure the actual deck height of the block (or clearance from he top of the piston to the deck surface and actually measure the chamber size of the heads since they are rarely what they are advertised to be too.