The '11 GT's stock exhaust system's inconsistent layout certainly is a hindrance when it comes to horsepower. Starting at the catalytic converters, tubing progressively reduces from 3 to 23/4 inches, and down to 21/4 inches just before the H-pipe. It then steps back up to 23/4 inches after the H. The stock resonators are listed as 21/2-inch. However, we believe their actual inside diameter may be as small as 2-inch (to suppress noise). The pipe size once again jumps to 23/4-inch after the resonators and into the mufflers.