The Borla Hush catback exhaust for the WRX & STI features 3" piping with a resonator and quiet twin-tip muffler. This is an exceptionally quiet exhaust for the WRX & STI. The section of the catback that bolts to the downpipe is slip-fit, and can be replaced with the 3" adapter for future upgradability. The downpipe section Borla includes will bolt perfectly to your stock downpipe, but the optional 3" adapter simply bolts on and removes the bottle neck if you plan to use a 3" downpipe. Backed by Borla's million mile warranty, and constructed of stainless steel -- this is an excellent choice for your WRX or STI.