The COBB J-Pipe replaces what is traditionally referred to as the "downpipe" on previous Subaru vehicles. With the relocation of the turbocharger for the 2015+ WRX, the first section of exhaust no longer requires a downward turn but instead uses a sharp "j-turn" to route exhaust towards the back of the vehicle. During testing, we discovered when running a stock cat-back with an upgraded J-Pipe, there was a resonance issue when cruising at highway speeds. Thus, we have gone a step above and beyond to offer a resonated version to our customers that want to keep their stock cat-back. You can also use the resonated J-Pipe if you want to quiet down the turboback system!