The big difference between small block pipes and big block pipes is mainly the overall size of the pipe. The more minute and unseen details are the shape of the pipe inside. The insides can differ greatly between any two pipes. The other small detail is the I.D of the stinger (exhaust outlet). Generally there are smaller pipes for .09-.12 engines (smaller stinger I.D), .15-.20 engines, and .21-.32 basically. The .09-.12 pipes and the .15-.20 pipes are usually close to the same size externally.