the best mod you can do to a stock black cat or any stock muffler, cut it at the first weld and compleatly gut it out, then add 4 in length to the chamber, after cut out the old stinger and make one 4 in long and with at least a 5/8 inner diamiter. it will flow a hundred times more free than stock and have a good growling sound to it, not to mention that the big fat long hollow tube acts like an x chamber at higher rpms !