Re: trying to upgrade a 2.5, need some advice
These motors were not made for great amounts of power, and there's
not much you can modify externally to get impressive gains.
You are on the right track however. The exhaust is going to free
up the most power by allowing less restriction to exhaust gas flow.
The downside is that the exhaust volume may increase to an annoying
level if you install something too aggressive.
A cold air intake is a great addition in most cases, but you'll be
surprised to know that the stock intake system is not that bad.
The air is routed from outside the engine bay, and into the intake
manifold via composite/plastic tube which resists the conduction of
heat from under the hood.
You will find that replacing the filter element will yield the most
gains. Get rid of the stock paper filter, and drop in a high flow
K&N, or Accel filter.
There's not much else you can to that will improve gains without
spending a mint. Ignition upgrades are a waste. Just make sure
the coils, wires and plugs are in good condition. Fuel injectors
should be left alone. They will handle the 10-20HP increase you
will achieve with intake and exhaust mods.
Computer tuning may help as well. Timing and air/fuel changes
can always improve the situtation. Don't spend too much time
and money in this area because the trade off is not worth the
expense (unless you plan an entire internal engine rebuild).