^^ Well you're going to hear 2 stories on that one.
Glasspack or straighthru's will increase exhaust gas velocity and a chambered like a Flowmaster will make backpressure, these are both 100% true
However you'll hear stories on how both will increase HP. Personally I think a flowmaster or other chambered muffler is your best bet. You want backpressure, you can reduce it to mkae HP but you don't wanna totally do away with it.
My Alero gained .3 in the 1/4 mile with a Flowmaster, before I had on one of those chrome ricey mufflers that you'd see on eBay. Granted it was a cheap muffler, but the Flowmaster is better, plus that sound
