I bought a set and used all top of the line stuff. Had them CNC ported and they just didn't compare to my B1's or my old Indys. If you buy them be forewarned that NOTHING is helicoiled on these heads and be sure to clean out all bolt holes as some are filled with chips. The valve job from the factory sucks and mine had to be cut .100 because they were not flat. I also had problems with leaking at the valve cover rail. You also have to use gasketed spark plugs with them. My advice, if you have a small budget buy the Stealths from 440 source. If you can buy them the B1 originals or B1-BS's are hard to beat. I would rather have a set of 915's than ever deal with another set of ProComp pieces of crap.