Too Slow-to Four-Oh
Q I've got a 2.5L Wrangler and want to swap in a 4.0L. Is this a bolt-in swap?
A Aaargh! If I had any hair it's this question that would make me pull it out! 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. The 4.0L requires a bigger radiator. Also, 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. 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 $3,500 or less and it doesn't make a whole lot of sense to go through all of the above. Does it?
Man Oh Manifold
Q I read that swapping to a '99-up intake manifold will get me 20hp. Is this a bolt on deal?
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? I've done a fair amount of dyno testing with 4.0L Jeeps and, aside from a 10 hp difference either way, there's usually not that much of a difference between the baseline numbers generated by stock H.O. 4.0L Jeeps from '91-'06. They all put down right around 143-153hp at the tires. And I attribute most of that to differences in drivetrain losses and exhaust restrictions of different chassis.
However, if you still want to swap to the later manifold, I've been told you'll need the '96-up power steering bracket and will need to lengthen some of the sensor wires. The fuel rail should work.
H.O. 4.2L
Q What parts do I need to put a 4.0L head on the 4.2L in my CJ/early YJ?
A The H.O. 4.0L head found on '91-'06 TJs and XJs is drastically better than the factory head found on 232 and 258 engines.
Hesco (hescosc.com) offers a kit under PN HES4240CHC that includes a modified H.O. 4.0L head for use with the 4.2L block, gaskets, and a valve cover. You'll still need to obtain a 4.0L exhaust manifold or header.
If you choose to go the low-buck junkyard route, you'll need to fill in some of the water passages on the 4.0L head with epoxy or JB Weld. Most machine shops I've spoken with recommend using water soluble packing peanuts. You shove the peanuts into the hole, leaving roughly 1/2- to 1/4-inch of room for the epoxy. Then fill it up and grind it smooth. The peanuts will dissolve into the coolant and won't affect flow within the engine.
The coolant passages that need filling will be evident, when you compare the 4.0L head to the 4.2L block, but they're on the intake side and flank the head bolt holes.