Throttle Bodies and Spacers: The 4.0 throttle body simply serves as an air regulator, unlike a traditional carburetor that regulates air and fuel. The throttle position sensor on the throttle body provides feedback to the computer to indicate the position of the throttle so that other engine systems, such as the fuel injection, are matching the engine needs of the driver. Performance throttle bodies that use a larger opening are available; they allow an increased volume of air to pass through the throttle opening, allowing more fuel to be burned, thereby increasing power. Painless makes bolt-on throttle bodies that feature a 70- and 72-mm bore as opposed to the factory 60- to 62-mm bore.
Several companies make throttle body spacers that are designed to alter the flow of the air moving into the engine, allowing increased efficiency. Airaid, Hesco, and aFe make throttle spacers that can be installed in less than one hour.
Superchargers: For those dying for more horsepower, and have money to burn, a supercharger will add a 40- to 50-percent horsepower increase and a 30- to 40-percent increase in torque. At the time of this writing, a supercharger kit for a 4.0L costs in the range of $3,000 to $5,000. That’s a lot of money to go from 190 to 270 hp. It’s likely still less expensive than a V-8 swap with much less fabrication and hassle, but the numbers aren’t as impressive as a V-8 on its own. In addition, a supercharger can be hard on an older, high-mileage engine.
Sprintex continues to make a bolt-on supercharger system for the TJ while many other companies have discontinued their systems due to the age of the TJ. The Sprintex system mounts directly to the top of the intake and relocates the throttle body, unlike many older or homemade systems that pressurize the entire intake system with a separate mounted supercharger feeding air through a tube to the throttle body.
Jeep owners have used Eaton M90 superchargers from late 1990s to early 2000s GM vehicles to create custom installations. Boosted Technologies took this practice a step further by making a true bolt-on system that uses an M90 twin-rotor design supercharger mounted to a custom adapter on the intake manifold and relocating the throttle body. The supercharger pulley is located near the power steering pump and runs off the engine’s serpentine belt.
Programmers and Performance Computer Modules: Like the 4-cylinder engines, Jeep owners can add a performance computer module or programmer to their Jeeps. These components work similarly in that they alter the original programming to allow the engine to increase power and efficiency. Modifying fuel ratio, spark timing, and other points often accomplishes this. Some modules are fully adjustable and some require high-octane fuel as well as changing engine thermostat to allow the engine to run cooler. Most modules attach to the PCM directly and can be installed in less than an hour.
Stroker: A stroker is an engine that uses a crankshaft from a different engine or a custom crankshaft to increase the stroke of the engine, resulting in increased horsepower and, more significantly, torque. For many Jeep owners, this is an attractive way of gaining some increased performance without the expense and complexity of an engine swap. A stroked engine fits in the same space and requires no adapters, fabrication, or electrical modifications, and it shouldn’t create any emissions nightmares.
A 4.0L can be commonly (safely) stroked from 4.6L to 4.7L; some have pushed the engine to a 5.0L. A common trick is installing a crankshaft and connecting rods from a 4.2L (258 ci) into a 4.0L and increasing the bore up to .060 inch. This method almost “squares” the engine, meaning that the bore and stroke are almost identical. Some builders will use the 4.0L connecting rods and special pistons to further increase low-end torque, but this method is often more costly and uses non-stock pistons. The horsepower and torque differences between using the long or short rods is likely trivial. An off-road Jeep will benefit from the added cubic inches in either way.
It is common to include an array of performance parts in a stroker engine. Keith Black pistons are a popular high-performance piston, as are performance camshafts from Elgin or Crane. Several camshaft duration/lift options are available to suit the need of the application. Edelbrock makes an emissions-legal performance aluminum head for the 4.0 that increases and improves flow in both the intake and exhaust ports. It should be noted that the Edelbrock Performer head will work with the 1997–2006 TJ 4.0L, but a 1997–1998 exhaust manifold or headers must be used. In addition to the internals, higher-output fuel injectors are often included.
The 4.0L can be stroked to increase displacement up to 4.8, often using a crankshaft from a 4.2L along with bored cylinders. Several companies, including Hesco, ATK, and Clegg, make stroker kits as well as fully assembled engines that are ready to install. (Photo Courtesy Titan Engines)
A stroked 4.0L combined with a free-flowing exhaust, larger throttle body, higher-output injectors, and cold air intake may be the ultimate I-6 combination without the hassle or expense that comes from an engine swap. It is common to see gains of 40 to 50 hp and 60 to 80 ft-lbs of torque. Building a stroker at home is no small task, and it is probably best left to those with engine-building experience. Golen Engines, Titan, and Hesco make full and partial 4.0L stroker kits for the TJ.
Cooling: Keeping the engine cool, especially on the trail where the Jeep is likely maintaining a slow speed for extended times, can put stress on the system. In general, a 4.0L, even with the stock cooling system, is very capable of maintaining proper engine temperature even on the hottest days on the trail. Over time, if this system is neglected its capacity is diminished and overheating problems, which are bad in general but worse on the trail, may appear. Performance upgrades, primarily internal engine mods, will increase the amount of heat that the engine generates, which increases the load on the cooling system. In short, this is not a common condition in a Jeep with a 4.0L. Regular flushing, inspection, and (especially) keeping the radiator fins clean will make this system perform at its optimum level.
Direct-fit aluminum radiators can do a better job of displacing engine heat than the factory radiator. These radiators usually use more cores and full-aluminum tanks. Many aluminum conversion radiators are used for swaps that are direct fit to the Jeep to match the engine.
Hood vents are extremely useful for reducing underhood temperatures. Some Jeep owners install vent kits or purchase aftermarket hoods with integrated vents. A Jeep that experiences slow off-road conditions in hot environments will benefit from a vented hood.
Gearing: If you don’t go to a full-on engine build, such as a stroker, changing gears can have a dramatic impact on the Jeep’s performance by getting the engine back to its designed power range. Adding larger, heavier tires will effectively alter gear ratios, causing a reduction in engine RPM at the same speed. Performance and fuel mileage will suffer if the engine falls too far below its designed power range.
The 4.0L is a well-known low-RPM-torque engine that will operate in the 2,000-rpm range at 65 mph. Finding the proper gearing to keep the engine at peak RPM will enhance both performance and fuel mileage and that will make the driving experience better. Gearing and ratio choices are discussed in chapter 5.
Swapping Engines
Swapping engines in Jeeps is common and has been performed countless times, stretching all the way back to the earliest CJs. For many Jeep people, it seems impossible to satisfy the hunger for more power. As time moved on, engines became increasingly sophisticated and complex, mostly to improve vehicle emissions and improve fuel mileage. There was a time in the mid- to late 1980s when swapping engines became almost impossible, especially when the Jeep needed to pass an emissions test.
Luckily, the aftermarket caught up to the times and began offering components to make swapping a modern engine into a TJ easier than ever. We are in good times now. Modern engines