The ubiquitous Chrysler 4.0L inline-six was (and is) perhaps one of the best engines ever offered in a 4x4. Depending on chassis and model year, these engines carried a net power rating of 173-195hp and 220-235lb-ft. As delivered in bone-stock trim the ’91-’06 High-Output 4.0L engine found in Jeep Wrangler, Cherokee, Grand Cherokee, and Comanche vehicles generally lays down 130-140hp and 175-185 lb-ft at the rear tires. That’s right in keeping with a standard 25-percent parasitic drivetrain loss, or the power required to spin all the rotating components inside the transmission, T-case, driveshaft, axles, and so on.
So, if the 4.0L is so good, why bother trying to make it better? Why not! With a few simple modifications that can be performed all at once or one at a time as funds permit, you can up your HO 4.0L’s power by 20hp and 20lb-ft at the rear tires. And even the low-po ’87-’90 Renix-injected engines respond well to some of these mods.
Don’t think 20hp and 20lb-ft sounds like a lot? Keep in mind the difference between a “pedestrian” 5.2L Grand Cherokee and the nearly legendary 5.9L Grand Cherokee is only 25hp/35lb-ft. And the difference between a 5.0L Camaro and a 5.7L IROC-Z is 15hp/45lb-ft. So dust off that credit card and upsize your power without upsizing your engine.
Cold Air Intake
The factory filter and air box offers a slight restriction in intake air flow. However, the convoluted ducting is something else. The manufacturers are often more concerned with maintaining low NVH levels than maximum power, so shaped and chambered air ducting that cancels or muffles the intake air charge noise is what’s under your stock Jeep’s hood. Replacing the entire shebang with a high-flow cold-air intake system and filter will not only give you a noticeable bump in seat-of-the-pants acceleration and power, but it often results in a 1-2 mpg improvement in fuel economy. Just be prepared; you will hear some more whistling and whooshing when you mat the gas pedal.
62mm Throttle Body
The factory throttle body on a 4.0L HO engine is 60mm in diameter. There are plenty of 62mm (and even 68mm) aftermarket options available to allow your 4.0L to ingest more air. Generally, the bored throttle body will only make a noticeable difference from the driver’s seat if you’re running an air intake, high-flow exhaust, and maybe a header.
We don’t really think going larger than a 62mm throttle body is advisable on a naturally aspirated 4.0L for a couple reasons. First, the plenum bore on the intake needs to be opened up or there will be a misstep that can actually hinder flow. Second, the big throttle body can actually reduce the intake velocity, hurting performance at slower engine speeds. The 62mm size offers a good balance between increased airflow and intake velocity in non-supercharged or turbocharged applications.
High Performance Ignition
The 4.0L engine is calibrated to run on the lean side and the factory ignition system is actually pretty good. However, that’s not to say there’s no room for improvement. Once your ignition parts are due for replacement, take the opportunity to upgrade to a high-performance cap, rotor, plug wires, and coil such as the Firepower Ignition Kit from Performance Distributors. The increased ignition energy will allow you to open up your plug gaps for a hotter, denser spark that will more-easily light off the air/fuel mixture at lower engine speeds for improved low-end torque. Likewise, at higher engine rpms you can realize a more complete burn inside the combustion chamber for improved top-end power and slightly better fuel economy.
The factory 4.0L engines were equipped with a tubular exhaust manifold that actually doesn’t flow that bad. However, nearly every stock 4.0L manifold we’ve examined has cracked at or above the collector gasket area. A combination of a long downpipe imparting leverage on the header collector and a lean-running injection system that creates high exhaust gas temperatures are two big causes for the high failure rate. Adding an aftermarket header made of thicker stainless or mild steel will not only improve the survivability of your exhaust system, but will offer increased exhaust flow for better scavenging of the cylinders. In other words, the exhaust flowing out of the cylinders will be able to pull the incoming air/fuel intake charge into the cylinders more efficiently.
After-Cat Exhaust/High-Flow Cat
Like the air filter and intake ducting, the factory muffler and tailpipe were not designed primarily with high-performance in mind. Rather, the factory was concerned with quiet operation and cheap manufacturing methods. That means meandering internal muffler passages and crimped exhaust tubing. Replacing these parts with a high-flowing aftermarket muffler and tailpipe will really uncork your 4.0L. It’s a misnomer that less backpressure means less torque. In reality, less backpressure means higher exhaust flow as long as the exhaust velocity isn’t reduced.
For most 4.0L Jeep systems, the industry-standard muffler and tailpipe diameter of 2¼-2½ inches reduces backpressure without any detrimental effect to exhaust velocity. The result, as with the header, is improved exhaust flow and cylinder scavenging for more power and torque. While you’re in the exhaust, check your factory catalytic converter to make sure the brick inside isn’t dislodged, plugged, or disintegrating. Any of these issues aren’t uncommon in modern Jeeps and an aftermarket high-flow catalytic converter can often be purchased for the same or even less money than a factory replacement.
Once you add the amount of air going in and out of your 4.0L, you can increase your fueling. Adding fuel to a ’96-up OBDII returnless Jeep injection system can be a tricky undertaking. For the earlier Renix and HO OBDI systems, you can simply install an adjustable pressure regulator or swap injectors to higher-flowing units to get a little more squirt into your cylinders. However, OBDII ECUs revert back to pre-mapped fuel tables, so even if you bump up your injector size to 21-, 23-, or 24-lb/hr units, the computer will simply fire the injector for a shorter burst so it delivers the same amount of fuel your stock 19-lb/hr units would. In lieu of (or in addition to) an injector upgrade, you can try an adjustable MAP sensor so the ECU thinks the engine is under more load than it really is and will give correspondingly more fuel. Turbo City has an adjustable MAP sensor, or Edge Products offers an electronic controller as part of its Trail Jammer package that does essentially the same thing. Hesco is a good source for adjustable fuel-pressure regulators and larger injectors, as well as other 4.0L fueling components.
Electronic Tuners & Tricks
There aren’t a ton of hand-held, dash-mounted, or plug-in tuners available for older Jeeps, but beginning a few years into the ’96-up OBDII applications, many companies like Hypertech, Superchips, and Unichip offer electronics tuners that either piggyback through your OBDII port or are uploaded and rewrite your ECU programming to tailor spark and fueling parameters to increase power and mileage. Claims and actual results may vary, so it pays to do your homework and shop with some due diligence.