Second, there isn't a lot available for your car. You can do a high flow intake, cat-back exhaust, maybe headers, underdrive pulleys, and that's about it, short of nitrous. All together, those parts would probably cost you about $2000 and gain you...~10hp.
Any mods for performance will just shorten the life of your engine.
On the contrary, there are a lot of mods and upgrades for that car.
But for a Mitsubishi engine with that age and mileage, that is already over the hill, not like Honda or Toyota engines that last at least 300k.
Establish a budget.
Power isn't cheap, budget accordingly.
With added power comes added strain on the drivetrain, so budget for that as well.
Or, just enjoy your car and save thousands of dollars in engine and drivetrain work for a bit more power.
a tune is the cheapest to get max performance without any bolt ons.
change fluids
Just bought an 03 eclipse gts. I was wondering what kind of mods i could give it to add extra horse power. And also this has 138k miles on it, im curious what i could do to get it to its best preformance and keep it up to its best, for the longest ammount of time. I know keeping oil chainges are important with an engine. What else to keep, or bring it back up to its top preformance.