Just to confirm what has been said too, we have tried them on stock port, ported , and radically ported strokers. They worked well on all but on the more "radical" ported bikes they felt alittle flat on top and the CPIs took care of that. They are made for stock or midly ported bikes. Dont be afraid to have some porting done with T6s as they will make you smile!!!! Great all around pipe if you keep port durations lower, just not a pipe for drag porting !