I have a 421 built by k&t performance.. Currently Runs absolutely fantastic.. I have upps on and it really likes them but some times boggs or fumbles around on the bottom(hauls ass on top) would a set of intakes with the built in cross over (k&ts) help the bottom end but not sacrifice the top end .. My bike is really known for the top end..currently I have gone 5.804@103.48 (500ft clay not a good solid hooking track)