Glasspacks are GREAT for performance. Any type of muffler with a straight through design will give the best performance. The idea is to keep the exhaust pulse together so it can create a vacuum to help suck out the exhaust gas. Then again they are louder then chambered mufflers. Short packs are gonna be loud and crackly. Long packs help tone it down so there not loud but have a deep tone. Overtime they break in and get louder.