by Desert Dog » September 2nd, 2010, 9:49 am
The first thing to do is bring the engine up to good running order (plugs, wires, cap/rotor, seafoam treatment, etc). Check the compression to make sure that the engine is performing well and you don't have other issues like a head gasket or bad cylinder. Engine mods are a waste of money if this stuff isn't in order.
GEARS: make a huge difference in being able to maintain speed without shifting all over the place. My son has a 2.5L automatic on 33s with 4.88 gears. Before the gears, we had to go up the pass in 2nd gear at 25 MPH - now it does a whopping 45 MPH up the pass!
One of the best things you can do for a good running 4-banger is let it breath better.
Cold air intake will help it inhale. Header and exhaust will help it exhale.
COLD AIR INTAKE: Be mindful that adding a CAI will increase the possibility of hydro-lock during water crossings. So if you add one, stay out of deep water.
HEADER: Don't get the cheap Pacesetter header for it, those are junk and crack faster than the stock manifolds; run the Dynomax or Gibson made for the 2.5L (they are Cali legal). Be mindful that a header adds a lot more heat to the engine bay, so you may need to install a header blanket and wrap the fuel rail if heat-sink or vapor lock issues arise (I had to do this on my son's 2.5L). The header will be useless without adding a good aftermarket exhaust too. The headers for the 2.5L will make a big difference in performance at highway speeds, but may lag a little while taking off.
You did not state the year of your YJ? If it is a 91 or newer with MPI fuel injection, you can swap out the throttle body for a 4.0 throttle body. This will give a little more umph and better throttle response off the line.
Even with all of this done, and the ability to actually do normal highway speeds and stay in gear is achieved, these engines will still only do 45MPH up the pass. Just the nature of the beast, and one of the reasons why the 2.5 Jeeps has a very low resale value. But, these little engines get good MPG, make for a light jeep, don't blow axles like the 4.0 does, and are reliable as hell.
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