I'm running a set of Promaxx BB heads. I upgraded the springs, locks and retainers to Comp Cams stuff that is matched to my .528 MP solid cam. I have only made a couple of passes since the head swap. With my big valve 906's my best e.t. was 12.55 @107.75. With the Promaxx's my best pass out of 2 was 12.64 @108.5 with no tuning. Also, that track was at 3,500 ft. Elevation vs my home track at 500 ft elevation. So, corrected it came out to like, a 12.28 pass. I still have a lot of tuning to do as well as practicing my launch technique. My 60' is pretty weak.. 1.84-1.90 range. But over all I'm happy with the Promaxx's.