Hell back in 1976 or 1977 NHRA pencilled my Z/SA 170 powered 65 Valiant out of competion. They said no 6 cyl cars in Y or Z stock. Only 4 cyls. I could run under the record/index in Z/sa, but was not competitive running two classes higher. Back then we couldn't adjust the weight. I still had to run the same power to weight ratio as a Z stocker, but had to run in X/sa.