Gearing: Most agree that regearing a TJ with a 4-cylinder will provide the most noticeable improvement to performance compared to other modifications. Adding larger tires will drastically degrade engine performance because of the larger tires’ effect on overall ratio. Most 4-cylinder–equipped TJs came with 4.10:1 ratios; often, regearing to 4.88 or lower will bring life back to the small engine. Gearing and its impact will be explored further in chapter 5.
Stroker, Turbo Charger, Supercharger, and More: In recent years, availability of performance parts for both the 2.5L and 2.4L has been diminishing due to less interest in aftermarket modification of these engines. As stated earlier, there is a money line to cross for a swap to a larger engine that is more cost effective and that will produce better results. Some of the fanciest and most expensive supercharger kits and turbo kits for the 4-cylinder engines produce 170–190 hp in the stock 4.0L range.
For those willing to spend the money for the challenge of squeezing more out of their small engines, stroker engines are available that can push the engine size of a 2.5L up to 2.7L. The 2.4L engines have a bit of a larger aftermarket following due to their use in a variety of Chrysler vehicles. Once again, being conscious of cost versus benefit will save you money that may be used better elsewhere.
The stock 4.0L is almost the perfect engine for the TJ, its modern fuel injection and engine technology only adds to the reliability of this engine. Putting out approximately 190 hp and 235 ft-lbs torque, it is well matched to the size and weight of the TJ. It’s not uncommon for a well-cared-for 4.0L to last more than 250,000 miles.
4.0L MPFI I-6
The 4.0L remained rather consistent through its final nine-year run. It experienced some slight design changes and a small bump in horsepower and torque. The 1997 4.0L produced 181 hp at 4,600 rpm and 222 ft-lbs of torque. The 2001 and later produced 190 hp and 235 ft-lbs of torque.
The 4.0L was so well matched to the Jeep both on- and off-road, providing excellent torque at low engine speeds; it was a definite advantage in slow off-road driving conditions. While the 4.0 isn’t a high-horsepower engine, it is capable of providing a pleasant driving experience on the street. Combine the low-end torque with excellent reliability and you can debate the merits of an engine swap away from the 4.0.
Slight variations in the run generally brought improvements as the years ticked by. The tubular exhaust manifold, which was prone to cracking, was replaced with an improved cast version in 2000. The distributor also disappeared in 2000 and was replaced with a one-piece three-coil rail system. The Jeep’s PCM controlled the spark timing after the distributor was removed, which meant that manual adjustments were no longer possible or needed. These improvements just added to the 4.0L’s reliability.
Cold air induction systems come in many forms from several different companies. All use a similar concept of a free-flowing reusable air cleaner that is protected from engine heat by an enclosure. This enclosure brings in fresh, cooler air from the grille area or cowl. These systems usually result in a gain of a few horsepower and a little extra fuel economy.
4.0L Performance Upgrades
The trouble with modern computer-controlled engines is that the computer attempts to maintain predetermined performance characteristics even after modifications. Rewriting the programming is not technically possible but programmers and performance modules attempt to piggyback on the Jeep’s computer and alter programming along the way. Even then, gains are minimal and the cost often exceeds the reward. It is a truth that is common to many performance upgrades, so a good piece of advice is be cautious of engine performance modifications; it’s often money better spent elsewhere. That being said, I list many of the common add-ons that can make some improvement, whether it is in fuel mileage, throttle response, or power.
Clean, Less Restricted Air: A new or, even better, reusable-style air filter can improve engine performance alone. These filters allow increased airflow and improve throttle response as well as fuel mileage. K&N filters are possibly the most popular reusable, high-flow filters available. A cold air intake system (CAI) can improve engine performance further by replacing the entire air cleaner assembly with a less-restrictive system that isolates the air cleaner to allow it to receive cooler outside air rather than hot underhood air. Airaid, Volant, and K&N are some of the more popular makers of CAI systems. Most of these systems are installed with simple tools in a short amount of time.
Replacing the CAI System
1 The stock air intake system on a TJ isn’t particularly bad but improvements in the style of the filter and keeping the air pulled into the intake as cool as possible will add some horsepower and fuel efficiency. Most cold air intake (CAI) systems replace the filter box with an open element housed in an isolated box to separate the air intake from underhood heat.
2 Loosen the clamp attaching the intake tube to the box and pull the tube from the box. Remove the airbox lid and air cleaner. Remove the three internal bolts to allow removal of the factory airbox. Then remove the bolt on the passenger-side radiator bracket.
3 Pull out the PCV tube and air temperature sensor (if equipped) from the intake tube and remove the factory intake tube. With all the components removed, installation of the new components can proceed.
4 Unbolt the radiator support and install the heat shield, passing the radiator support through the rear wall of the shield. Tighten the bolts.
5 Install the intake tube to the heat shield and throttle body. Install the PCV tubing and the aFe air cleaner.
6 Make sure that all of the bolts are tightened and install the trim seal on the heat shield. The installation is now complete. Test-run the engine and check for vacuum leaks or loose connections.
Exhaust and Headers: The TJ 4.0L exhaust options are plentiful from cat-back systems to entire replacement systems that include performance pipes and low-restriction emissions-legal catalytic converters. Some of these systems give the Jeep a nicer exhaust sound and tone, as well as improve performance through less restriction.
The factory exhaust manifold on the TJ can be either a tubular-steel or a cast-iron design, depending on the model year. Some model years have manifolds that are prone to cracking; typically, these manifolds can be replaced with a direct fit high-flow header that will mate up with the stock Y-pipe tubes or aftermarket tubes. For maximum exhaust performance, replacing the entire system from the header to the catalytic converter to the muffler and the exit is recommended.
A TJ exhaust system can be broken down into three sections, from front to back: the manifold, catalytic converter section, and cat-back section. The exhaust system configuration changed a few times in the TJ era. The 1997–1999 (earlier) versions used a manifold that converged from two sets of three tubes merging into one then those two sets into