the pipe that extends from the cat to the axlebacks. OEM have a resonator just before the split where the canisters go to absorb the sound. The OBX B pipe is a straight pipe. It adds about 5 hp (according to Steve at Vertex Motorsports). The sound is great upon startup and over 70mph,but between 2-2.5K 000000000000000000000MMMMMMMMMMMMMMMMMM. The diameter is larger than OEM but it bolts right on with no alterations necessary.