If this is the blue head 15, it should turn over 19,000 with an APC 6.5 x 5.0 on a 1/4" Sport Goodyear venturi on 10% nitro. And don't automatically assume the best performance will be with no shims. Try it with zero, one and two shims. However, the wrong amount of shims will NOT cost over 5000 rpm (unless the piston is actually hitting the head, maybe).