Stock B-pipes have the Sensor Bungs in front of the Cat. Aftermarket B-pipes move the Sensor Bungs after the Cat, which is like moving your Primary cat rearward into the B-pipes. I don't know how much the noise is reduced by the stock resonated B-pipes. But, I know the stock '10 4x4 B-pipes reduce a lot of noise. When I switched mine out for the Walker's, my exhaust got so loud you could feel it. I sounded probably twice as loud.