My 05 excursion has a 4" MBRP turbo back system. When I bought it, it had no muffler or cat. The jet sound was cool but the drone at highway speeds (especially while towing) was obnoxious. You could not hear the person in the passenger seat. I installed an FTE 4"x30" resonator/muffler. It still has a little bit of the jet sound at idle and a nice rumble while accelerating. It is quiet while cruising at highway speeds (I can hear the kids in the 3rd row without them having to scream) and the wife likes it.