Yeah, yeah, after trashing the four-cylinder Jeep at every corner and telling you guys to to just go buy a six-cylinder we are going to show you some ways to make power.
The 2.5L four-cylinder put in the Wranglers from ’87-’03 was a beast. Not a beast in the power department, but a beast in that it can take a beating and it lasts forever. We run out of fingers and toes when we try to count the number of Jeepers who are just waiting for the four-cylinder to die so they can swap in a more powerful engine. This author swapped one out at 350,000 thousand miles. They can go further than the sixes, but because of their lack of power, they don’t get near the attention. We are going to cover the multiport-injected engines because there just isn’t much support for the ’87-’90 TBI-injected models.
In stock trim, the little engine that can is rated at about 120hp/140lb-ft for the 2.5L and 147hp/165lb-ft for the 2.4L. When compared to the four-cylinders that powered the earliest Jeeps, that sounds like a lot. Although when you consider the curb weight it had to push around was 3,200 pounds, it sounds like a bit less. Jeep typically put 4.10 gears in the differentials of the four-cylinder models to offset the lack of power.
Like any other engine, if you let it breathe better and ignite more fuel, it will make more power. But beware, bigger isn’t always better with the mighty four-cylinder.
Letting the engine breathe...
Letting the engine breathe better is never a bad thing. Shown is an AEM Brute Force air intake system. The dry element filter is good for us dirt-hounds because it won’t attract dirt like some oiled-element filters do. In fact, if it gets too dirty, we just smack it around a bit to clean it. It is good for about 5hp. The company makes kits for both the 2.5L and 2.4L engines.
The factory throttle body...
The factory throttle body of the 2.5L is a paltry 52mm in diameter. By swapping to a 4.0L throttle body you bump up to 60mm for even more flow. Beware though; the opening in your factory intake plenum is only 52mm. There is no point in running a bigger throttle body without also hogging out the plenum. Also, the idle air control solenoid (IAC) housings have a locating tab. The 4.0L IAC housing is different than the 2.5L IAC housing. You need to install your 2.5L IAC housing on the bigger throttle body before bolting it on. There is a tab on the intake plenum that will interfere with the tab on the IAC housing from the 4.0L. This swap will open up an approximate 5-10hp.
Once you get more air in,...
Once you get more air in, you’ll need to get more air out. There are many aftermarket headers available for these engines. Pictured is a Borla header. Get one that interfaces with the factory exhaust system or you’ll end up spending way more money or time than it is really worth (about 5hp).
From there, a high-flow catalytic...
From there, a high-flow catalytic converter and after-cat exhaust will get the gasses gone that much faster. You can run a 2½-inch system, but if you’ve already upgraded your intake side, you might find the engine has a big flat spot right off idle. A 2¼-inch system will avoid the flat spot problem, but won’t flow as well when cruising down the highway and won’t give as much power as the 2½-inch system either. Again, it is worth about 5-10hp.
Ditch your parasitic drag....
Ditch your parasitic drag. While we’ve not dyno’d before and after, we have noticed about a 1mpg improvement from just tossing the factory engine-driven fan. This Ford Taurus fan is only needed on the hottest days or when stopped in traffic. Most of the time while moving the fan isn’t needed to keep the little engine cool. Also, if you have A/C or an automatic those are two big power-sucks there too… even worse than the engine-driven fan. No one likes guys dressed as girls. Get rid of drag and gain some power and mileage.
Hopping up the ignition is...
Hopping up the ignition is never a bad thing. The OEs use parts that will last or are inexpensive to make. You aren’t constrained by these things, so when you find yourself needing ignition stuff, look to Davis Unified Ignition. The company has kits for both the 2.5L and 2.4L (shown). Open up your plugs to .065-inch and you can burn more fuel. While it is true that the factory sets these things lean, at wide open throttle, they go rich. The four-cylinder spends a lot of time at WOT, and a hotter spark really helps get more power out of all that fuel. Good for 5-10hp, depending on how trashed your existing parts are.
There are roller rocker kits...
There are roller rocker kits on the market that will act like an after-cat or intake kit, but inside the engine. The factory stamped-steel rockers have a 1.5:1 ratio. The rollers rockers feature 1.6:1 which will magnify the cam movement. It is like putting in a cam with more lift without opening the engine. The roller fulcrums and tips also cut down on drag, and we already talked about that. Good for about 10hp. Yella Terra makes a great set for the 2.5L under part number YT6626A
An adjustable MAP sensor will...
An adjustable MAP sensor will make your Jeep think it needs more fuel and it will open the injectors a bit longer. Make sure to juice up the ignition system for the most power possible out of this modification. Good for up to 10hp.
It might seem silly to talk...
It might seem silly to talk about gears in a power article, but it’s true. While the 2.5L isn’t built for high revs, it will spin in the 3,000 RPM range all day long. If you are running 30-inch tires, your 4.10s will eke you by. If you have 31s and live in Kansas or somewhere else flat, you’ll manage. Once you go bigger, don’t even think about pussy-footing around gears. We ran 5.13s with 33s and it was about as good as the little engine can get. If you have the 2.4L engine, it will rev even higher. Don’t be afraid to gear it down. Good for 5-10 hp on the dyno and will save countless gray hairs in real life.
If you have done all of the...
If you have done all of the above and you are still underwhelmed by the power output of your Jeep, try a 505 Performance turbo kit. We have a few thousand miles on our ’94 since we put the turbo on it and it is by far the most fun we’ve had with those squirrels. We lost 1-2 mpg, depending on how much fun we are having, but we gained 40hp at the rear wheels which makes the Jeep a joy to drive.