Top 25 Most Asked Jeep Tech Questions

A The on-road ride comparison between a shackle-reversal suspension and the standard factory shackle-forward design is like night and day. But that's not to say that either is without its own set of advantages and disadvantages.

Although positioning the shackles at the rear of the spring pack in a shackle reversal allows for a more natural compression of the spring as the front axle travels upward and back, it can be argued that it doesn't offer as much traction to the front tires when off-road. In the standard shackle-forward configuration as the tire starts to climb an obstacle and the springs compress, the tire is forced forward into the obstacle, increasing bite. However, having driven both suspension types off-road, we'd say we haven't really noticed any measurable difference in off-road performance. But the on-road ride is way better with the reversal.

A For the umpteenth time, if you want a 4.0L Wrangler it's cheaper and easier to sell your 2.5L and just buy the Jeep you want. Even if you've loaded your 2.5L with a bunch of modifications, it still makes sense. Regardless, let's run through the mechanics of it all.

For starters, you can't just bolt in a 4.0L to the AX-5 that originally backed the 2.5L. Aside from the fact that the two engines have different bellhousing bolt patterns, the AX-5 doesn't have a prayer of lasting more than a couple hundred miles with the power and torque of the 4.0L. So, you can automatically add a transmission swap into the mix as well. Oh, and you'll need to take the NP231 T-case apart to install a 23-spline input gear since the 2.5L Jeeps have a 21-spline input gear. Add that to the pile.

Then, there's the sundries that everybody seems to forget about. Since the 4.0L is longer than the 2.5L even if you put the 2.5L's accessories onto the 4.0L's brackets you'll need new power steering hoses, radiator hoses, heater hoses, and a new fan shroud. Don't forget that you'll need to lengthen the fuel lines to reach the 4.0L manifold, do a new exhaust forward of the catalytic converter, and install new engine brackets on the frame. Then you'll need to lengthen pretty much every sensor wire as well as the charging system wires and then plop in a new 4.0L factory computer and add the harness connectors for the extra two injectors. Man! Now, consider that you can pick up a decent 4.0L TJ for well under $5,000 or a clean 4.0L YJ for $3,000 or less and it doesn't make a whole lot of sense to go through all of the above. Does it?

A Don't you think if the swoopier U-shape of the '99-up manifold added that much power over the older square-shaped 4.0L manifold (shown) the factory would've claimed that in its marketing? We've done a fair amount of dyno testing with 4.0L Jeeps and, aside from a 10hp difference either way, there's usually not that much variation between the baseline numbers generated by stock H.O. 4.0L Jeeps from '91-'06. They all put down right around 135-150hp at the tires.

However, if you still want to swap to the later manifold, you'll need the '96-up power steering bracket and will need to lengthen some of the sensor wires. The fuel rail should work. Be warned, however, Trasborg did the swap and actually lost power on the dyno with his '97 XJ, so unless you're adding a turbocharger or other big power adders, the log-style manifold seems to be a better match to the '98-earlier head and camshaft profile.