Feature advanced acoustical tuning technologies for a deep rumbling sound. Come with 3" cores encased in proprietary Cobra sound tuning materials with specific acoustical dampening and thermal insulation properties. The frequency and location of the perforation patterns in the high-flo cores creates significant sound scattering in the mid frequencies, which helps to enhance the low frequency tones. 4" round muffler front expands just aft of the steel clam shell clamp to 4.5" diameter. PEM® fasteners in the wrap make mounting a simple task of threading in a bolt from the upper bracket into the Cobra strap. Like all Cobra exhausts the Race Pro slip-on mufflers are made in the U.S..