NDRRA, KOTS, RPM ( Dunn-Benson) and the PTRA all follow the same general set of rules. This is to allow the same cars to compete in different associations within our area.None of these groups allow anything over a stock valve angle. But in all honesty, I believe a MAX EFFORT 18 degree motor can also be 2 tenths quicker than a 23 degree motor with the same nitrous jet restriction. This is a rule that's already been argued within the class. IF we allow an 18 degree SBC, than why not a 13 Degree LS, etc.,etc. . IT's a can of worms just waiting to explode. Hope this helps in understanding. It's not just the PTRA.