THIS TIME WE GIVE YOU THE BASIC XJ SKINNY ON GETTING THE POWER TO THE GROUND. THIS INCLUDES THE TRANSMISSION, TRANSFER CASE AND AXLES.
TRANSMISSION
Overall, the best factory manual transmissions are the Aisin AX-15 and NV-3500. Both have reputations for strength and performance in the stock realm. These manuals can easily take a 20 percent increase in torque if they are in good shape so they have some potential for being retained in the case of an engine swap. The AX-5 is reliable, but not up to lots of power or much hard work. The "87-90 BA-10 Peugeot is a toss away in a swap situation. If you are running one behind a stock engine and it fails, do yourself a favor and swap in an AX-15 or NV-3550 rather than attempting to rebuild it.
In the automatic world, there were only two choices and the AW-4 holds the edge. This automatic isn"t the best choice for a swap because its electronics are difficult to integrate with a swapped engine. With V8 or big V6 swaps, it"s better to use a trans that was originally mated to the donor engine and simply adapt to the transfer case. The 3-speed automatics that came with the fours and V6s, the TF-904 and 30RH/32RH (basically the same trans with different names) are decent units and capable of handling an upgrade in power, but neither feature an overdrive. Buildup tricks for the factory trannies include the usual for automatics; install a trans temp gauge and usually a cooler. Install the temp sender into the pan or in the cooler return line. Install an auxiliary cooler as needed if you see consistent temps over 225 degrees or regularly spike over 250. Plumb the cooler after the radiator cooler in warm climates and before the radiator cooler in cold ones. The AW-4 is known for running hot, and almost always needs a good cooler. Not much is available to beef the AW-4 internally but the 3- speed Mopars have lots of options.
Beyond running a good synthetic oil, manual trans buildup tips usually involve the clutches. The kiss of death for any clutch is too-tall gears behind a torquey engine on rough terrain with sticky tires. Step one is to have your gearing situation right! Without going into clutch theory too much, you need holding power at least equivalent to your engine torque. The late model ("91-up) stock units are built to handle up to about 270 lbs-ft when new. They go downhill quickly with age and don"t handle slipping very well. Aftermarket clutches, like the Centerforce II, use better lining materials (composites, including Kevlar, and ceramics) so they can handle heat better. On a "wheeling machine, look for a clutch that has an increased torque capacity over stock. Increased capacity comes from a combination of larger amounts of an improved friction material and higher static clamping pressure from the pressure plate. Expect pedal effort to increase.
TRANSFER CASE
The NP-207 and NP-229 from the first generation XJs are not worth many words because they are virtually unsupported by the aftermarket. The "88-up NP-231 Command-Trac is the most common t-case and is a direct swap for the older, weaker NP207 or NP-228/229, as long as the trans. output/ t-case input spline counts match. If your NP-207 or NP-229 is intact now and you"re a mild "wheeler, use it "till it"s used up and then replace it with a "231 (or "242). If you plan extensive mods or hard use, replace them in your shop rather than on the trail.
Next most common is the full-time "87-up NP-242 Selec-Trac. It has a fulltime mode, with a center diff lock and a 2wd mode. It"s great for a daily driver, especially in snow country. It"s moderately stout but the added internal complexity makes it a bit more troublesome in hard core situations.
There is lots of support for the NP- 231. Internal upgrades including big chain kits (1-inch to 1.5 inch chains) and 2-low kits are available from many retailers. Most importantly are the short tailshaft, fixed yoke kits for correcting ujoint angularity. These kits are available from many sources, including 4 Wheel Parts. Many companies also have fixed yoke kits for NP-242 full-time units as well. The better tailshaft kits use new 32 spline output shafts as opposed to remachined shafts. If you run a 4:1 low range, a splitter or a much more powerful engine, use the bigger output shaft. Tailshaft kits will be needed with lifts over 3 inches and with full-time t-cases lifted over an inch. Likewise, new CV driveshafts will be needed. Older XJs used a GKN CV joint on the front axle driveshaft that has limited angularity and should be replaced in most cases.
A lower-than-normal low range is sometimes useful and there are kits to install 4:1 planetary gears into the NP- 231. These allow you to use axle gears that are more street friendly and still have a good gear reduction for the trail. You can also install the NV-242OR "Rock-Trac" used in the Jeep Rubicon. It"s available used or new aftermarket. You could also go the full gonzo Advance Adapters Atlas route, either the 2 or 4-speed model and choose a low range from 2:1 to 10:1.
AXLES
The most common XJ axle combo is a Dana 30 front and a Dana 35 rear. In 1987, possibly into 1988, a Dana 44 was offered as part of a trailer towing package, but is very hard to find. The 27-spline Chrysler 8.25 Corporate axle came next and started appearing in the mid "90s. It"s stronger than the D35 in all ways but was only used occasionally. An even stronger 29-spline version of the 8.25 started appearing in 1997 XJs, gradually becoming more common as time passed. The 29-spline version is far better than the D35 and about 8/10ths of the way to a D44.
Most rigs built for the trail will mount lockers or limited slips and there are some cautions particular to XJs. If you install an automatic locker or a limited slip in the front diff, your driveshaft will turn, even on part-time 4wd rigs with a disconnect. If your front driveshaft angles are a fraction out of sync due to a lift, you will have vibration, even in 2wd. The first cure involves setting pinion angles perfectly, using a CV driveshaft like those from Tom Wood"s Custom Driveshafts, or going to a set of disconnect style hubs so the axle can be uncoupled. Warn Industries builds a kit to install locking hubs.
If you have an XJ with a full-time tcase that you want to use in full-time mode, locker selection narrows considerably. The full-time features begin to fight the lockers or limited slips and you end up with clanks and bangs, odd locker engagement, vibration and steering difficulties. You can usually get by with a rear automatic traction aid alone, but not front and rear. The cure is an on-demand locker like the ARB Air Locker, Ox Locker, Detroit Electrac, Eaton E-Locker or Auburn ECTED. With the exception of the Electrac and the ECTED, which both have limited slip operation when not locked, the diffs operates as normal open units when unlocked. On the trail, where manners are less important, you can lock or unlock as needed.
Front Dana 30 Tips- If your XJ has the Dana 30 disconnect axle, there are two cures for the common vacuum problems. The first is to install a Posi-Lok. This replaces the cranky vacuum motor with a driver controlled cable actuator. Option two is to replace the two piece right axle with a one-piece unit. Warn makes a kit to do this. The front driveshaft will then be turning, as mentioned above and the same cures apply.
Strength becomes an issue for many D30s with tires above 31 inches. The high-pinion D30 center section can be built to exceed stock Dana 44 strength overall using the following blueprint: Start with an aftermarket locker that includes a new carrier, add 4340 alloy inner and outer shafts (one piece inner on the right) and a set of Spicer cold forged 760 series u-joints, or the superstrength types. This will give you the ability to run 35s safely. Not everyone will have the need or the bucks to go full-boat on their D30 all at once. If your XJ is older than "95, it has the small axle u-joints (5-260x) and these should be the first to go. From "95, the 5-297x size was used, which is some 35 percent stronger than the 260. The first level of that upgrade uses OE replacement shafts that just bolt in. Premium replacement shafts may be of a stouter 1050, 1540h or 1341h alloy. The strongest are of the aforementioned 4340 chrome-moly steel that is some 50 percent stronger than stock. Several companies, including Warn and Superior, offer 4340 alloy shafts. Ox, CTM, Yukon, and others offer super strength u-joints for these applications. Superior Axle makes a 30-spline upgrade for the front Dana 30 also
Rear Dana 35 Tips- This axle can be a major issue. In light use with tires up to 31 inches in diameter it"s adequate. Just. Beyond that, it needs help. That help comes in the form of alloy shafts or better yet, 30-spline alloy shafts. The 30-spline shafts are limited to only a few locker or limited slip types. Warn and Superior both offer full-float kits. The housing is one of the weakest parts and the D35 is one of the few axles out there that really needs a truss, which Superior supplies. The bottom line on the Dana 35 is that if you plan on running tires above 31 inches and working the Jeep hard, your money is better spent on a swap.
Rear 8.25 Tips- The pre-"97 27- spline 8.25 is uncommon, but by replacing the carrier and the axle shafts with 29-spline OE or aftermarket parts, you will have upgraded its strength by 25 percent. The 29-spline 8.25 is a prize. While not quite up to the D44, it"s lots cheaper and more easily found by swappers. The downside is limited aftermarket support. The selection of gear ratios is narrowed to a low of only 4.56:1. The locker and limited slip lineup is also pretty small. If what you need fits in the parameters of what"s available, it"s a fine axle.
Rear Dana 44 Tips- The thing to remember about the XJ D44 is that it was a light duty housing. In stock form, this axle is capable of handling 35s on an XJ easily and 37s with less of a strength cushion. If you need more beef, consider a custom housing with thicker tubes and a stronger center section. Improvements start with alloy shafts from the many manufacturers offering them and Warn has a full-float kit. Superior, Yukon and others offer a 33 spline conversion for the D44. Take your pick, just about any locker or limited slip is available as are gear ratios which go as low as 5.89:1.
Axle Swaps- Two desirable bolt-in upgrades would be the XJ version of the Chrysler 8.25, which is fairly common, or the rare "87 Dana 44. The MJ pickup Dana 44, which requires only new spring perches mounted above the axle, is also getting scarce but is a relatively easy swap. The Ford Explorer 8.8 requires a bit more adaptation than the MJ D44 but Mountain Off-Road Enterprises offers kits for most Jeeps with everything you need. The Explorer 8.8 from "95 on comes with rear discs and the same wheel pattern as the Jeep. It"s about 3/4 inch narrower, so wheel spacers are often used. Though a cclip axle, the disc brake equipped 8.8 is relatively safe from losing an axle shaft in the event of a break because the calipers will hold it in place. Its weak links are its relatively thin tubes (.180-in. wall vs .250 on the D44) and the fact that it hangs lower than the other axles, reducing ground clearance.
Up front, a Dana 44 is the axle of choice, but because of the XJ"s limits regarding tire size, itis not often needed. The Rubicon D44 will bolt up in the front of an XJ, but it"s hardly worth the effort as there is little difference in strength between a high pinion D30 and a low pinion D44. The low pinion Rubicon D44 is really a D44 center section with D30 front tubes and ends plugged in. Also, because the center section is larger, and a low pinion, you end up with some front driveline angle problems that are almost impossible to correct for street driving thus a locking hub conversion may be required. Overall, your money goes farther building up a D30.
CAPTIONS
The Dana 35 began life as an AMC axle called the AMC-15 that dated back to 1962... the AMC-20"s little bro. When the XJ debuted in "84, it was still listed as the AMC-15. Dana began manufacturing them for AMC in the late "70s. By "85, Dana had acquired the rights to produce it and renamed it the Model 35. Its bad reputation mostly revolves around big tires and "wheeling. It"s usually more cost effective to swap it out if major upgrades are needed for tires above 31 inches. How well or poorly it serves you depends on how hard you work it.
There are three versions of the Dana 30 front axle in the XJ with stock ratios of 3.07, 3.55, 3.73 and 4.11s. Shown is the mid-90s non-disconnect version, which was used on full-time units through "91 and all XJs from "92. Through "99, the high-pinion style was used, but in the last two years of the XJ, a TJ style low-pinion was substituted. The high pinion is definitely preferable. The disconnect apparatus is a pain and to save costs, Jeep eliminated it even on part-time Cherokees. The difference in mileage was negligible, but because the driveshaft was spinning, there were vibration issues.
The NP/NV 231 is the best overall t-case for a trail rig. It"s simple, stout and reliable. This one has a short tailshaft kit and a CV rear driveshaft installed. In all but the mildest buildups, you may as well put these items on your budget list now. If you will be running a splitter ahead of the t-case, consider a short tailshaft kit that uses a stronger 32-spline rear output.
The Ford Explorer 8.8 is one of the more popular and least expensive swaps for the XJ. Mountain Off-Road Enterprises (M.O.R.E.) offers a kit to install this axle into the XJ. The 8.8 is a bit ahead of the XJ Dana 44 in beef overall.
