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Aftermarket Jeep Engine Parts - Power Parts

Posted in Product Reviews on January 23, 2007
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How many times have you read the ad by the company claiming you'll get 25 extra horsepower by bolting on their widget? You can't always believe the hype. While it's true that most power-adders really do add power, the reality is that the results often fall short of the claims. Here are some of the hottest power mods and what you can expect from them in the real world.

High-Flow Air Intake
It's one of the first modifications many Jeep owners make, whether it's an injected 4.0L or a 327 Vigilante V-8. A good, high-flow filter will really free up some power across the rpm range. Replacing the convoluted factory ducting and keeping the filter away from engine heat usually translates into noticeable power and increased mileage numbers.

Look For: Drop-in replacement filters, whether flat or round, look for a tight fit that doesn't leave any gaps between the sides of the filter and its housing. Also make sure the cotton, gauze, or other filter material is sturdily bonded to the rubber in which it is encased. Cheaper filters can come apart over time and allow dirt and particulates to enter the engine. For entire replacement air intake systems, look for free-flowing bends, good silicone or high-quality rubber couplings, and some form of air dam that isolates the filter element from engine heat.

They Claim: Anywhere from 8-18 hp.We Say: Expect about 5-10 hp at the rear tires with a noticeable improvement in driveability.

Large-Bore Throttle Body
While a larger-bore throttle body allows more air to enter the engine, in reality the factory part offers decent airflow to accommodate most stock engines. Unless you've added a bigger cam, a free-flowing exhaust, or other aftermarket power-adders (like a supercharger or turbo) you're not really going to notice much, if any, difference from the driver's seat. It's a complementary component that works best in conjunction with other parts. In fact, we noticed a substantial decrease in power when we installed a 58mm 4.0L throttle body on our 2.5L Wrangler in place of the stock 52mm unit. This is one item that should really only be added once you've hit a brick wall and nothing else you're doing adds power.

Look For: Aftermarket units machined from billet aluminum are less likely to have throttle shaft leaks than some older, bored stock units. Most 4.0L units are punched out to 62mm and don't feature a tapered bore. This will give you some whistling at certain rpms, so be prepared for it. Quality units will come with new Torx fasteners and an anti-tamper Torx driver to remove the factory sensors.

They Claim: As much as 15 hp.

We Say: On a stock engine, don't expect to notice any difference and only 3-5 hp with air intake and exhaust mods.

Free-Flow Exhaust
Just about any V-8 and most six-cylinder engines will respond favorably to a free-flowing after-cat muffler and mandrel-bent tailpipe assembly. Headers will also go a long way in waking up these engines, letting the engine scavenge spent gasses from the cylinders and rev more quickly.

For the four-cylinder crowd, a free-flowing exhaust is usually going to do more harm than good to everyday driveability. A four-popper will gain a little on the upper end, but the loss in torque below 3,500-4,000 rpm is going to make itself known.

Look For: When selecting headers, Jeep four-cylinder and six-cylinder flange thicknesses are dictated by the intake manifold, so the flanges are going to be thick by design. V-8 applications should look for flanges at least 31/48-inch thick for a good seal. Look for a good, high quality stainless steel construction or a coated 16- or 14-gauge mild steel. Mandrel-bent tubes that are fully welded inside and out, tapped O2 bungs (when applicable), and collector flanges that use a factory-type, ball-style flange gasket rather than a flat paper gasket are preferred. For after-cat systems, look for a fully welded muffler rather than a crimped case design and high-quality stainless steel or aluminized steel tubing with mandrel bends and a diameter of about 2.5 inches. Healthy V-8s can go for a 3-inch or a dual 2.5-inch.

They Claim: 10-15 hp for the header, 10-20 hp for the after-cat system.

We Say: Expect 5-10 hp with the after-cat system and an additional 5-10 hp with the header. However, in some cases we have seen horsepower numbers drop after installation of an after-cat system.

Throttle-Body/Carburetor Spacers
If ever there was a camp vehemently enthusiastic about their modification, it's the throttle-body spacer guys. While many people claim huge and noticeable differences, we've just never seen it on an injected rig. There are several on the market with spiral or helix bores designed to create turbulence in the air charge to better atomize the air/fuel mixture. The theory is sound, but so is the whistle it can create.

On the other hand, adding a 1-inch phelonic or other heat-resistant spacer under your carburetor can deliver a noticeable increase in seat-of-the-pants operation in a vehicle with a free-flowing exhaust and a good ignition system.

Look For: A quality spacer kit is going to include all the gaskets, longer bolts, and any linkage or adapter spacers that may be needed for installation. Some are taller and some are shorter. For a more noticeable gain, go for the taller spacer, although realize that it may cause fitment problems in your aftermarket air intake systems.

They Claim: 5-11 hp.We Say: Probably 1-2 hp for the throttle body, 5-8 hp for the carb spacer.

Ignition Hop-Ups
While newer 4.0L engines are calibrated lean and don't really need much help igniting a stock air/fuel charge, the 2.5L engines often run on the rich side and can benefit from a good set of aftermarket low-resistance plug wires and a hotter coil. There are hand-held programmers for these engines as well as all sorts of chips and methods to advance the ignition curve. The 4.0L engines should have the fuel bumped up, whether by an adjustable fuel regulator or larger injectors, to really take advantage of a more aggressive spark advance and increased ignition timing.

Older V-8 and V-6 engines will really wake up with a custom-tuned or recurved distributor that tailors the spark advance to come in sooner in the rpm range. The result is almost always more power and mileage. Likewise, a quality set of spark plug wires and a hotter coil will help ignite the mixture from the carburetor.

Look For: For 2.5L and 4.0L coils, make sure the manufacturer offers an adapter harness to plug the coil into the factory wiring on '98-'99 vehicles. Plug wires should be a low-resistance type, with no more then 500 ohms per foot of resistance, thick silicone or other high-quality shielding, and vulcanized boots that won't separate when pulled off. There are lots of programmers out there, and we tend to favor the ones that plug into the OBDII port (for those vehicles) and that allow changes to the speedo, rpm limit, governor, and other engine management systems in addition to timing changes.

They Claim: 5-25 hp.

We Say: We've seen gains of 5 hp with power programmers at the rear tires. A recurved distributor and high-performance ignition on a mild V-8 can net 15-25 hp at the tires.

Nitrous Oxide
You've heard the expression "It's not the fall that kills you, it's the sudden stop." Likewise, it's not the nitrous that makes horsepower, it's all the extra fuel the nitrous allows your engine to burn. Nitrous oxide, or N2O is an oxidizer that introduces massive quantities of oxygen molecules into your engine, allowing a proportionate amount of fuel to be introduced and burned. Nitrous systems for injected engines are commonly of a wet or dry type. In a wet system, the nitrous and extra fuel are injected before the throttle body through a nozzle, and the mixture is then drawn into the engine through the throttle body and manifold. In a dry system, only nitrous is injected into the engine, and the vehicle's fuel rail pressure is manipulated to add the extra fuel necessary to support the nitrous load. For carburetors, a wet plate system is most common, in which the nitrous and extra fuel are sprayed into the intake via a plate mounted in between the carburetor and manifold.

Since the increased combustion volume greatly increases cylinder pressures it's important to pull back the total ignition timing when running nitrous as well as making sure your pistons, rods, crankshaft, and head gaskets can survive the added pressures. As a general rule of thumb, most stock V-8s can ingest a 75hp shot easily, while six-cylinders can take 50hp levels.

Look For: You want to find a complete kit for your engine type, whether carbureted or injected. Piecing together a system is frustrating and may result in a greater possibility of engine damage. Injected engines would do best to use a wet system. It's a good idea to run an electronic bottle warmer to keep the nitrous at the proper pressure.

They Claim: 75 hp, 100 hp, 125 hp, and so on, with huge torque increases.

We Say: Deduct about 15 percent for normal parasitic drivetrain losses and the figures are right on the money.

Whether it's a V-8, six-, or four-cylinder, a supercharger is a great way of making big power and torque across the entire rpm range. The supercharger compresses the intake charge, usually to about 5-8 psi in a street application. This forces the mixture into the cylinders, hence the term "forced induction." The upside is a great increase in torque and power. The downside is that the intake charge heats as its compressed, which can result in detonation and lower power levels than with a cooler charge. To combat this, some supercharger systems use an intercooler to chill the charge before it's introduced to the cylinders.

For carbureted V-8 centrifugal blow-through applications, in which the compressed air is introduced into the carburetor via an air horn or enclosed box, the atomization of the fuel as it enters the intake manifold chills the charge back down and helps prevent detonation. This allows for a little more boost and higher power numbers to be realized than with a regular-roots or twin-screw supercharger.

Look For: Once again, complete systems designed for your particular application will be less likely to cause engine damage and will be easier to install than a scraped-together system using whatever you find laying around. Injected applications will make better power and have higher tolerance to pump gas at larger boost levels with an intercooler. A centrifugal system allows for more hood clearance at the cost of front engine accessory drive real estate.

They Claim: 40-75 percent power increase.

We Say: Expect a true 45 percent increase at the rear tires with an intercooled system running 5-8 psi.

Aluminum Cylinder Head(s)
If the camshaft is the brains of your engine, then your cylinder heads are the lungs. Adding a high-flow cylinder head to an otherwise stock engine isn't going to net you that much gain. However, when used to complement other power producers-like a more aggressive camshaft, a better intake manifold, increased fuel, and a good flowing exhaust-huge power gains can be realized. Larger valves, smaller combustion chambers for higher compression, and better flowing ports are all hallmarks of aftermarket performance cylinder heads.

Look For: Aluminum heads have better heat-shedding capabilities and will usually allow pump gas use with a full point of compression more than iron heads. Engines with a cylinder bore size of 3.95 or bigger can usually go with a 2.02-inch intake valve. Smaller engines and lower performance engines can use 1.88- or 1.94-inch valves to maintain good cylinder-to-valve clearance and low-speed port velocity.

They Claim: Up to 70 hp at the crank.

We Say: With the right match of components, up to 70 hp at the crank is possible. On a stock short-block Jeep with the regular intake and exhaust mods, expect about 20 hp at the wheels.

For the stock crowd
If you're dealing with a Poindexter of an engine to start with, know when to say when. While many early Jeep L- and F-head engines offer better driveability with an electronic ignition conversion from points and maybe a 2bbl injection system or rebuilt carburetor, it's really not worth chasing fairies trying to eke out any real power.

Sadly, we've begun putting the ubiquitous 2.5L Jeep four-cylinder in this category. While they do respond well to turbo or supercharging, it's often an effort in futility trying to squeeze performance out of these engines with regular bolt-on parts.

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