Ultimate Jeep 4.0 Mods Guide

For 20 years, the AMC/Jeep 4.0 inline-six engine powered various off-road machines and was an extremely reliable engine. They are known for going well over 150,000-250,000 miles without breaking a sweat, and they are damn dependable when out on the trail or on the rocks. Yet, if there is one knock to them, it’s the lack of power. For their time, 170-190 horsepower and 215-225 lb-ft of torque was actually pretty good. But by today’s standards, where the 2023 Gladiator is pumping out 285 horsepower, it’s underpowered. That’s why today we’re looking at the best Jeep 4.0 mods for horsepower and performance. 

Read on to find out the best ways to increase performance through the top Jeep 4.0 mods. Also, make sure to check out our other Jeep content, including our Jeep 4.0 engine guide, Jeep XJ Lift Kit guide, Jeep Grand Cherokee vs Toyota 4Runner comparison, and our Jeep Wrangler reliability and common problems guide. 

Jeep 4.0 Engine Upgrade Basics

AMC/Jeep produced their 4.0 liter straight-six engine from 1987–2006. It appeared in the Jeep Cherokee, Grand Cherokee, Wagoneer, Comanche, and Jeep Wrangler. It was an iron block and head engine that was naturally aspirated. The 4.0 used a single in-block camshaft with an overhead valve train. Compression ranged from 8.8:1 to 9.2:1, which is pretty standard for a naturally aspirated engine.

Over the years, Jeep upgraded the 4.0 straight-six a few times. The original engine used a RENIX multiport fuel injection system, but it was prone to issues and struggled to produce solid low-end torque. In 1991, Jeep upgraded to a better fuel injection system that allowed for better low-end torque. The engine also has higher flowing injectors, a larger throttle body, and a different camshaft. With the improvement, which added around 20 horsepower, Jeep started calling it the “high output” 4.0 inline-six. 

Further improvements improved the cylinder head and strengthened the engine block, but power output remained the same. From the beginning, the Jeep 4.0 made a name for itself for being extremely reliable and capable of some serious mileage. It has garnered an especially solid reputation among off-road and outdoor enthusiasts, as it can take on any conditions and will usually prevail no worse for the wear. 

Jeep Mods that Don’t Work

Before we go into the top Jeep 4.0 mods, first let’s talk about the common Jeep 4.0 mods that just simply don’t work. When researching Jeep 4.0 engine upgrades, you’ll notice a lot of people suggesting things like cold air intakes, chip tuners, 4 or 12 hole injectors, throttle bodies, and much, much more. Unfortunately, most of these upgrades do absolutely nothing except cost money and effort. They add very little in terms of actual measurable performance. In some cases, these mods can even reduce horsepower and torque and make the engine perform worse. If any of the following mods are on your 4.0 Jeep mod list, take a second look and decide whether it’s really worth it.

Upgrades to Avoid

First up are cold air intakes. These are incredibly popular for the 4.0 Jeep, but they are also a complete waste of money. Max gains are around 3-5 horsepower under the best conditions, as the stock airbox is capable of flowing the 190 horsepower the engine produces. The only time to put on a new intake is if you plan on adding a turbocharger or supercharger. The stock intake likely won’t fit or connect to the new blower, and you’ll want something capable of flowing 300+ horsepower.

Next up are throttle bodies and throttle body spacers. People commonly recommend a 62 mm throttle body (stock is 60 mm) and a throttle body spacer to add 10 horsepower each. However, once again, they add maybe 3-5 horsepower combined, and that would be with a full intake manifold and exhaust manifold upgrade, too. They just aren’t worth it.

You’ll also see some people recommend going with a lumpier camshaft. While we’re not debating whether or not a camshaft will add power, typically, for a 4.0 Jeep, it’s in all the wrong places. The stock camshaft does a great job of providing low-end grunt, which is exactly what you want with off-roading and crawling. A lumper cam will add power but sacrifice low-end torque, which will actually hurt you on the trails. 

The final things we’ll mention are ignition and 4/12-hole fuel injector upgrades. Importantly, if you plan on going forced induction or stroking the engine, you will need larger injectors. However, using the 4 or 12-hole injectors will do absolutely nothing to improve power on their own, and are another myth. In addition, ignition boosters that run hotter spark will again do nothing for power on the jeep, unless combined with forced induction or a stroker kit. 

Top AMC and Jeep 4.0 Upgrades Mods

Our top Jeep 4.0 mods are:

• Long-tube headers
• ECU Tuning with HP Tuners
• Forced Induction
• Stroker Kits

1) Jeep Long-tube Headers

For the first of our Jeep 4.0 mods, we recommend going with long-tube headers. Headers replace the stock exhaust manifold, which is not only restrictive but also prone to cracking. AMC/Jeep used cast iron for the OEM exhaust manifold, which is pretty standard within the industry, especially for a car of this value and time period. Unfortunately, cast iron is incredibly heavy. It can also crack after repeated heat cycling, such as occurs over 150,000+ miles and many years. 

Upgrading to a set of long-tube headers is one of the few mods that will actually show some noticeable benefits. With a good set of long-tubes, you can see 30+ horsepower and torque easily. This largely comes from removing or replacing the OEM catalytic converter. That creates a reduction in back pressure and improvement in exhaust flow. That allows air to exit the engine faster, which means clean air can enter quicker, producing more power. Long-tubes will also weigh a lot less than cast iron headers, which also helps performance.

Some people will also suggest upgrading the second half of the exhaust or the cat-back. However, without also upgrading the exhaust manifold to long-tube headers and either removing the catalytic converter or using a high-flow cat, a cat-back will show next to no real gains. They will however increase sound.

Best Jeep 4.0 Headers

There are not a ton of options for 4.0 Jeep headers, but some of the top options come from American Racing Headers (ARH). There are both ARH XJ 4.0 Jeep headers and ARH TJ 4.0 Jeep headers, and they perform about the same as each other. ARH is one of the top exhaust manufacturers in the business, and their products are very highly reviewed across the spectrum.

The Jeep 4.0 headers have a unique 6-to-1 merge collector and are made from 304 stainless steel. ARH offers both ⅝” and ¾” primaries, and either high flow cat or catless versions. Keep in mind, catless exhausts are only legal for off-road situations, and not the street.

2) Jeep Tuning with HP Tuners

Our next recommended mod is ECU tuning. We’re not talking about chip tuners or gimmicky throttle manipulators that don’t actually do anything. We mean actual flash ECU tuning to do things the real way. Unfortunately, this is only an option for 1996+ vehicles, as the ECUs on 1995 and earlier models are not tunable. 

Still, if you have a 1996+ 4.0 Jeep, tuning is one of the quickest ways to increase horsepower and torque without doing any bolt-on modifications. By adjusting parameters like fuel pressure, air-to-fuel ratios, ignition timing, and more, you can increase the performance of the 4.0 inline-six fairly easily.

In addition, if you plan on installing any kind of forced induction, like a supercharger or a turbocharger, or if you plan on going with a stroker kit, tuning is a must. Without tuning, you won’t reap near the full benefits of your mods, and might even end up damaging the engine. Mods are beneficial for a stock engine, but necessary for a boosted one.

Best Jeep Tuners

There is really only one option for ECU tuning the 4.0 engine: HP Tuners Jeep 4.0 supported vehicles. HP Tuners is an ECU tuning company that provides support for an incredibly wide range of Chrysler/Stellantis vehicles, including several Jeeps. HP Tuners has their own flash tuning device and software, which are used to tune the car. You can either do the tuning yourself, or preferably find a local dyno tuner who is knowledgeable with Jeeps. If you do plan on tuning, you’ll want to also look into upgraded headers at a minimum to get the most horsepower and torque. 

3) Forced Induction the 4.0 Jeep

For our third Jeep 4.0 mods recommendation, we suggest going with forced induction. Besides headers and tuning, the next best way to bump up power on the 4.0 Jeep is to add a blower of some kind. Superchargers and turbochargers are both viable options on this platform, but usually superchargers perform a bit better.

The purpose of forced induction is to add extra air into the engine, resulting in more horsepower and torque. Superchargers work by running off a belt connected to the crankshaft, while turbochargers run off the exhaust gasses coming off the engine. Since the engine is normally naturally aspirated, if you plan on installing a turbocharger you will also need a new turbo exhaust manifold. This will route the exhaust gasses into the turbocharger instead of out the tailpipe, which will allow the turbo to work.

For this reason alone, it’s usually easier to go with a supercharger for the 4.0 Jeep. However, both are still viable options, as you’ll see below.

Best Jeep 4.0 Supercharger and Turbocharger Kits

Starting with superchargers, you have a couple of different options. The top of the line is the Magnum Powers Wrangler 4.0 supercharger kit for the 2000–2006 Wrangler. The Magnum Power blower adds anywhere from 100-250+ horsepower and torque depending on the blower size. The blower is a MPX90 roots style supercharger, with different pulleys for different boost levels. It can be tuned with HP tuners, and it uses an air-to-water intercooler for optimum cooling.

The other option is the Boosted Tech XJ Cherokee 4.0 supercharger kit and the Boosted Tech TJ Wrangler 4.0 supercharger kit. These kits use the Eaton M90 supercharger, running 4-7 PSI of boost. They won’t add as much as the extreme version of the Magnum Powers, but are still solid kits nonetheless.

As for turbochargers, your two options are the Banks Power Jeep 4.0 turbo kit or the 505 Performance Jeep 4.0 turbo kits. However, the Banks Power kit is no longer being manufactured as of 2020, so they are much harder to find from vendors. The 505 Performance turbos are available for every year of the Jeep, and add around 100 horsepower and torque through 7 PSI of boost. Both kits are highly recommended, though 505 Performance requires an in-person install for their kits, so it does require a trip to Arizona.

A custom sourced turbo kit is the only other option.

4) Jeep 4.0 Stroker Kits

For our final Jeep 4.0 upgrade, we’re looking at stroker kits. To be clear this is a pretty big move and should not be taken lightly, but it’s the next best way to add power. By using factory 4.0 rods but a 4.2 liter crank, you make the jeep into a 4.6 stroker. There are also kits to make anywhere from a 4.5 up to a 5.0 liter stroker motor from the 4.0.

A stroked motor or stroker motor, is accomplished by lengthening the piston stroke of the engine. This allows for the volume of the combustion chamber to increase, meaning more air can enter. Without adding another cylinder, increasing the stroke or bore of an engine is the only way to add displacement. Stroking is a better option than boring the Jeep 4.0

As you can imagine, a larger motor will be capable of producing more power than a smaller one. By increasing the displacement by 0.5–1.0 liters, you can see some substantial gains in horsepower and torque. In addition, most of the time stroking also means replacing the internals with forged versions. This means you can add forced induction and really crank up the boost with a 4.6 stroker, and make gobs of horsepower and torque on-demand. 

Top Jeep 4.0 Stroker Kits

There are quite a few stroker kits on the market for the 4.0 Jeep, and we suggest either the Clegg Jeep 4.0 to 4.6 stroker kit or the 505 Performance Jeep stroker kit. Both of these kits are very highly reviewed and capable of some serious performance. The 505 Performance kits will offer more flexibility and a larger number of displacements, from 4.5–5.0 liters, while the Clegg is only a 4.6 kit. Either way, you can’t go wrong, and both kits will be solid upgrades. 

Top Jeep 4.0 Mods FAQ