I do most of the machining myself. I personally prefer going 1.71" on the Intake and 1.56" ob the Exhaust. I have noticed better low end results, but it's a little PITA cause I have to get Pontiac Exh. and sometimes it's a little hard to get S/S 1.71". I only did a few of sets like these and the guys were very happy on the quick spooling.