I'd say if somebody bores the TDC (318 2bbl) out .030, they probably put in better compression pistons. So if the TDC engine went from, per say, 8.3 to 1 up to a true 10 to 1 compression, plus picked up 5 cubic inches, then yes, the 4 tenths you deduct is about right. I'd expect if you did that to a 318 2bbl, then add a 4bbl, you'd run 15.1 - 8 tenths down from the 15.9
