i have a 2002 malibu and i took the stock muffler off and put on an APC racing muffler and i love the sound i was woundering if changing it added an extra horsepower?

if you just put a new muffler on stock pipes then you MAYBE gained 2 hp even if you do a cat back you will still only get like 5. new mufflers are mostly just noise

what kind of mods can i put on it to get more hp?

Do they have chips? Other then an air filter and CAI, nothing cheap.

yeah i have a CAI with a k&n filkter and it definately added some power and sounds great