Power Improvements for the
Jeep 2.5L 4-Cyl Engine
I receive a lot of question from people who, like me, are unhappy with the power (or lack thereof) that their Wrangler 2.5L 4-banger has. The 4-cyl, in my opinion, has plenty of low-end torque for low-speed wheeling, and it's adequate around town. Where it really comes up lacking is on the highway, where the poor aerodynamics of the Wrangler make it that much harder for the little engine to maintain the speed limit, especially when climbing steep hills.
My family of 4 drove my '95 YJ to Ouray, Colorado recently. I was able to maintain only 35 mph climbing Monarch Pass (11K feet, 7% grade). Not that I would have wanted to go much faster on that narrow, winding road, but it would have been nice to be able to, and to accelerate sometime this century if I'd had to stop for some reason.
Fortunately, the aftermarket has not ignored our plight, and there are several things that we can do to improve the upper-rpm horsepower of our mills. Here's a few of them. If you have other suggestions, I'd be happy to hear them.
The first thing to do is make sure your factory ignition is up to snuff. Check the inside of your distributor cap & rotor for cracks, carbon tracking, and terminal wear. When replacing them, get good quality components that have brass or copper terminals rather than the cheaper aluminum terminals.
Make sure your spark plugs are gapped properly (0.035") and that they're not fouled. Check out the second page of this tutorial for how to read your spark plugs. With a hot ignition in an MPI engine like the late-model 2.5L's, there really isn't much reason to waste money on high-dollar plugs, or to replace them before they look worn (which could be 50-100K miles). I've tried everything from $.99 Champions to $4 Bosch Platinums, and have seen no difference.
The spark plug wires in a 2.5L are pretty short, so you won't see much improvement by replacing the stock 7mm wires with super high end 10.5mm wires. However, after 100K miles, any kind of wires will be worn out and could stand to be replaced. A decent set of 8mm wires wouldn't hurt anything. I imagine they'd last longer if you avoided the hairpin turns that existed in the original routing configuration.
The factory coil in newer EFI engines is usually plenty hot to completely burn all the fuel. They're certainly much hotter than the old can-type coils from the 1970's. You probably won't see any gain from upgrading to an aftermarket coil (Accell / MSD / etc). However, if your factory coil is showing its age, it should be replaced with a new one. Most large auto parts chains can test a coil for free if you bring it in.
Next, get a high-flow air intake. A K&N filter in the factory air box will help, but you'll see better results from an entirely new air intake tube and conical filter. Cost: $40-200 (parts only)
Next, get a high-flow cat-back exhaust system like those from Borla, Gibson, or others. After having to disassemble my rusted exhaust system, I'm now quite fond of stainless steel exhaust components, so I'd recommend Borla or Banks. That recommendation not withstanding, I took a cheaper route and installed an aluminized Pacesetter TFX Kat-Back system on my own YJ. If your catalytic converter is damaged (by a wayward hammer, for instance), you can legally replace it with a high-flow cat from Random Technology. Doing this at the same time you install the cat-back exhaust will make the installation much easier. Cost: $250-750 for cat-back; $150-200 for cat (parts only)
The combination of these first two will allow your engine to breath better at higher rpm, and they're a prerequisite for getting anything else to work better.
I'm told that replacing your belt-driven radiator fan with an electric fan will give back 5-10 hp, especially at higher rpm's where the old fan would have been spinning fastest, but where it's not really needed for cooling due to the wind speed through the grille (assuming you don't have an 8274 winch). An electric fan gives you the added benefit of being able to turn off the fan before you make a deep water crossing so that the fan doesn't get sucked into your radiator. Cost: $200-300 (parts only)
Although it may sacrifice low-end torque, installing a high-flow exhaust header and front pipe will help at higher rpm's. Cost: $300 (parts only)
It's rumored that the throttle body from a 4.0L is larger than the one for a 2.5L, and that you can gain some power by swapping them. I'm not sure if that will work on a pre-'91 2.5L, though. Don't they just have throttle-body injection? Cost: $50-200 (parts only)
I'm not aware of any power chips for the lowly 2.5L, so that's probably not an option.
If you have larger tires, you can regain quite a bit of your originally performance back if you change your axle gears to bring your highway rpm's in 5th gear back into the 3000-rpm neighborhood. There's a conversion chart half way down this page. I've noticed that my engine doesn't start making good power until it hits at least 2500 rpm. These little engines can run at 3500 rpm all day long, but I wouldn't sustain much more than that. Gear installs require special knowledge and tools to do correctly, so unless you really know what you're doing, these should be left to professionals. Cost: $400 (parts only, both ends); $600-1200 (labor only, both ends)
Of course, there's always a supercharger. I'm told it'll add 40% more horsepower at higher rpm's, but some people have said it provides an unpredictable boost off-road that's hard to control. Maybe that's just inexperienced drivers talking. You certainly need the high-flow intake & exhaust before doing this. Rimmer Engineering, the original maker of Jeep superchargers, seems to have dropped off the radar. See this installation writeup by Jp Magazine. Avenger Superchargers now offers one for about $3700 (parts only).
Of course, there's always an engine swap. The ubiquitous small block Chevy (SBC) 350 is the most common swap, and adapters to perform this conversion are readily available. So, if you're a lemming with no creativity, this is probably your best option. Other popular options are the 4.3L Chevy V6 (basically just a 350 that's missing 2 cylinders), the Ford 5.0L (302ci), the AMC 360 (the most common Jeep V8, usually found in full-size Jeeps and large AMC cars), and the Jeep 4.0L I6 (the factory alternative to the 2.5L). There are some problems with V8 swaps that make me shy away from them, though. First, the additional torque requires that you upgrade the rest of the light-duty that came behind your 2.5L. The Dana 35 rear axle and the AX5 manual tranny will be the first things to die spectacular deaths if you leave them in place behind a larger powerplant. Second, larger engines are heavy. A fully-dressed 360 or 4.0L add about 300 lbs to the front end weight compared to a 2.5L. This hurts your center of gravity, and it requires stiffer front springs which won't flex as well. Third, larger engines require more cooling capacity, and that's hard to come by if you've got a Warn 8274 winch blocking most of your grille. Even my 2.5L runs about 10 degrees hotter on the highway in the summer since I installed it. Cost: $1000 and up
The final, and probably the most expensive, option is to get a tow rig. I first bought a 1977 Jeep Wagoneer with a 401 V8, which was fun, but a little rough for my wife's tastes. It was cool to be able to tow a Jeep with a Jeep, though. I later added a 2001 Chevy Tahoe with a 5.3L Vortec V8. The Tahoe has more power, better brakes, more weight, a longer wheelbase, and more creature comforts than the Wag ever will. It also gets about the same gas mileage as my Wrangler does. The Tahoe allows us to pack as much stuff as we want and travel in style to the trail head. Of course, then we've got to do something with the Tahoe when we get there, and getting everything ready to flat tow the Wrangler is more of a production than simply hopping in the YJ and driving off. Cost: $1000 and up
Of course, the best online support group for Jeepers with 4-cyl engines is the 4Bangers United mailing list. Think of it as "4-Cylinders Anonymous."
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Originally written 18 Aug 2004 Last updated 5 July 2012 Obi-Wan (obiwan@jedi.com)
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