i have a 1998 avenger...

There's nothing you can really do with this engine, I had the same one in my 1998 Chrysler Sebring. There really aren't any performance parts available, I had trouble even finding a K&N air filter that would fit.

If you are serious about horsepower and really like the Avenger, I would suggest getting a 2.0 liter version with a manual transmission. That engine accepts most Eclipse modifications (other than the cat-back exhaust), as it is essentially a long-wheelbase Eclipse, so you can find turbos, intakes, exhaust, etc. for it. So far I've given mine a short-ram intake (looking to buy the legit K&N intake for an Eclipse) and JVT Racing header (for Eclipse). Someday I'd like to buy the Hahn turbosystem and have nearly 300 horsepower. Not very much out there for the 2.5L v6, though... ebay has an intake but that is as much as I can find.

try disturbingmotorsports.com and andysautosport.com they have some add on out there although there is not near as many as the 2.0 also check out asog.net for any questions concerning your ride and sometimes they also have parts for sale