The 12579 is a large case single inlet, single outlet muffler. Internally the exhaust is split running the length of the case rejoining before exiting. The muffler has a nice quiet rumble at idle with no drone at speed. It has a nice performance sound when the throttle blades are opened wide. Install consists of headers, 2 1/4" to 3" Y-pipe, 3" through remainder.
Gibson Metal Mulisha Exhaust Systems
The exhaust sounds and looks great. Beware, it is very loud on the highway when the truck downshifts to get up a hill when cruise control is on. But it sounds amazing. One of the pipes had a small dent that I had to beat out with a hammer in order for them to fit. Also, one of the hanger locations didn't match up with the factory location so I'm not using that hanger. Other than that, install went easy. You won't be disappointed.