Because the 4.0L engine performs so well in a TJ right out of the box, spending extra money for improving performance should be considered after items that make a bigger difference off-road are installed first. A quality CAI system and a performance exhaust system will give the largest improvement in the TJ 4.0L; these items may be the first of the modifications a Jeep owner may do.
The 4.0L is a well-matched engine for the TJ and serves the Jeep well in almost all driving situations. There was much fear in the Jeep community when Chrysler announced it was being discontinued and replaced with a new engine in the new JK model. Adding a few performance modifications, watching gearing, and performing proper maintenance will allow this engine to power a Jeep for years.
Swapping a 4.0L in place of a 4-cylinder engine is not terribly complicated and can be done at a reasonable cost for a significant improvement. Having access to the donor Jeep will reduce the expense and complexity of the swap.
Retaining an already-installed 4.0L is more cost effective. It also eliminates the almost endless potential for problems, including adapters, fitment, exhaust, emissions, and electronics compatibility.
V-8 and other non-factory swaps should be performed in accordance with your local emissions laws. A modern, fuel-injected engine with all of its necessary components will perform best in a TJ. The GM V-8 is one of the simplest and most-developed engines swapped for a TJ engine. Planning and preparation are the keys to a successful swap.
CHAPTER 3
TRANSMISSIONS AND TRANSMISSION SWAPS
The engine has much to do with the “personality” of a Jeep, but the transmission may play a bigger role in the Jeep’s overall capability. A Jeep and its intended use are quite different from most other vehicles that spend their lifetime on the street. A properly matched transmission will only make the Jeep’s use in both on- and off-road conditions better. The transmission is the first part of the three components that are part of the crawl ratio calculation. Crawl ratio is defined and explored in chapter 5.
Factory Transmission Offerings
The Wrangler TJ was equipped with eight different transmissions through its production run. When Jeep changed a transmission model, it was often an improvement over the previous version. Of the five manual transmissions and three automatics, the 42RLE automatic and the NSG370 manual are standouts. These models were installed in Jeeps after 2003. However, the reality is that all of the transmission models are a good fit for the TJ; in fact, the NV3550 is a commonly swapped transmission. Transmission choice among Jeep owners is a matter of personal preference and, like many things, the debate on manual versus automatic continues.
The factory automatic is easily spotted by the shifter sitting in the center console or the presence of a transmission skid plate in front of the main center skid plate. The 4-speed automatic uses the same shifter as the 3-speed. The fourth gear, also known as overdrive, is engaged and disengaged by a switch on the dash.
Manual Transmissions
A manual is the more common type of transmission found in a TJ, with the exception of the TJ Unlimited, which was more commonly fitted with an automatic. Jeep owners often prefer a manual transmission for added control off-road, increased fuel economy, and the fun. Manual transmissions are much simpler in design and create less heat, which, in general, increases life and makes them more reliable. All of the five manual transmissions featured an overdrive gear that kept engine RPM speeds lower, increasing fuel efficiency.
Determining the particular manual shift in a Jeep is difficult by looking only at the shifter. The exception is the NSG370, which has six speeds. The manual transmission is much more common in the TJ series except for the Unlimited, for which the automatic tends to be more popular.
When building and modifying a Jeep, it is helpful to know the first gear and last gear ratio; this is used to calculate overall crawl ratio and highway cruise RPM. Both of these concepts are explored in chapter 4.
Aisin-Warner AX-5
This fully synchronized light-duty 5-speed transmission was used until 2002 in 2.5L-equipped TJ Jeeps. Its light-duty design makes this transmission a poor candidate for a larger, more powerful engine and/or hard off-road abuse. The 21-spline output shaft makes a Jeep equipped with this transmission have a NP231 transfer case equipped with a 21-spline input. Many transmissions and adapters use the 23-spline input.
First gear: 3.93:1
Fifth gear: 0.85:1
Aisin-Warner AX-15
This fully synchronized medium-duty 5-speed transmission was used from 1997 until 1999 in 4.0L-equipped TJs. The AX-15 had wide use in Jeep vehicles, including the YJ Wrangler and XJ Cherokee, and is capable of handling the 4.0L easily along with swapped mild V-8 power. The split cast-aluminum case transmission uses a 23-spline output shaft and an internal hydraulic clutch release bearing. The numbers on the lower part of the case identify the AX-15. This requires a chart listing the many part numbers. An easier way is to check the intermediate plate that is aluminum on the AX-15 and steel in the AX-5.
First gear: 3.93:1
Fifth gear: 0.79:1
NV1500
This fully synchronized light-duty transmission was only found in the 2003 and 2004 2.4L engine–equipped TJ. It is likely the least desired or sought after transmission of the TJ line. This transmission is up to the task of handling the 4-cylinder engine but not much more.
First gear: 3.85:1
Fifth gear: 0.80:1 or 0.83:1
NV3550
This fully synchronized medium-duty 5-speed transmission was used from 2000 until 2004 in 4.0L-equipped TJ Jeeps. This popular transmission is capable of handling the 4.0L power as well as mild swapped V-8 power. It is more than capable of withstanding off-road abuse and its low first gear is a good contributor to overall crawl ratio. There are a couple of things to note. First, the NV3550 is a rather noisy transmission that is even noisier in a Jeep with no carpet. Second, it doesn’t use standard gear oil; it uses the more expensive and more difficult to find Synchromax oil.
First gear: 4.01:1
Fifth gear: 0.79:1
This NV3550 sits ready to go into the Jeep’s frame, freshly mated to the Magnum 5.8L V-8 from a Grand Cherokee. This popular 5-speed transmission, found in Jeep vehicles as well as Dodge trucks, has a respectable low first gear and will handle mild V-8 power. (Photo Courtesy Ben Mann)
NSG370
This fully synchronized medium-duty 6-speed transmission was used in 2005 and 2006 TJ Jeeps with the 4.0L and 2.4L engines. The final two years of the TJ saw the best manual transmission in the run. For the first time, the same transmission was used with both engines, reducing production costs for Jeep and providing an excellent transmission for a V-8 swap for either engine. The NSG370 incorporated many modern improvements that reduced transmission noise and improved shift quality. The 6-speed offered a wider gear