Jeep 4.0L
The candidate: The Jeep 4.0L was first put into XJs in '87 and upgraded to H.O. status with a better head and injection in '91. Used in Jeep Wrangler, Cherokee, Comanche, and Grand Cherokees for over 20 years and as durable as an anvil, there's no shortage of good swap candidates.
The perception: I'll take my 2.5L Jeep and just slap in a 4.0L.
The reality: Yeah, you can. Buy why would you? You've got to cut off the 2.5L engine brackets from the frame, weld on new 4.0L brackets, change out the front accessory package, modify or completely replace the wiring harness, swap out the transmission, clutch, and T-case input gear, change the tranny mount, rework the complete exhaust system, radiator hoses, ECU, and more. The only way it makes sense is if you have a complete donor sitting next to your Jeep and then, isn't it easier to just put your lift on the donor?
The exception: We can see doing the swap if you've got extensive chassis modifications and/or extensive cage and tubework done to your vehicle. But then, wouldn't you just go with a V-8 instead?
We think: You're still better off selling your 2.5L Jeep and buying a 4.0L Jeep if you're stuck on having the inline-six.
3.9L Cummins 4BT
The candidate: The four-cylinder 3.9L Cummins 4BT engine can be had in normally-aspirated, turbocharged, or turbo/intercooled versions and puts out 105-135hp and 275-350lb-ft depending on year, model, and what's been done to the fueling.
The perception: The more-compact 4BT will make any Jeep a real econobox commuter without giving up insane off-road crawling grunt and offers little-to-no packaging issues.
The reality: The 4BT is easier to fit into most Jeep engine bays and will deliver great mileage numbers on the road if gearing is carefully selected to keep the rpms down. But again, the noise, vibration, and fumes from one of these little paint shakers will be enough to push most Jeep owners over the edge of the nearest cliff. Plus, they weigh a little more than an all-iron big-block. They sound good on paper, but when you've lived with one of these conversions for a little while the buzzing and belching can really test the temperament of some drivers.
The exception: In the May '07 issue we featured Dave Smith's '80 J-20 pickup with a 4BT and Ford ZF five-speed tranny, and we've run into New Mexico's Jeff Wood's 3.9L-powered CJ-5 on the trail for more than a decade in one variation or another. Conversions like this do work, but in the end the success of such a swap is really more about the temperament of the owner/driver than the engine itself. It's only the right engine for the right driver.
We think: They're great, durable little mills that make good torque, but not a lot of power unless modified. For those with more refined tastes, a CRD from a Liberty would be easier to live with on a day-to-day basis, but there's no denying the dirt-simple engine management of the 4BT that will surely continue to hook prospective swappers. Trasborg sharply disagrees, but we have noted a difference in drivability between the 4BT and the 2.8L CRD in the small diesel Jeeps we've driven and ridden in.