BBK 2015-16 Mustang GT Varitune Complete Cat Back Exhaust System (Includes Resonator Delete X-Pipe)BBK Varitune cat back exhaust systems feature BBK Varitune mufflers that have a patented adjustable power valve inside that allows full adjustment of the exhaust flow through the mufflers chambers for a great sound and added performance. This adjustment is made at the rear of the muffler with a wrench by either moving the valve in or out. Once you are happy with the level of sound, it is locked in place with the Varitune locking nut. All of the