Mufflers Race Pro FlhRace Pro Slip On MufflersFeature 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