Introduction   Choosing and Prepping the Engine   NP435 Transmission and Clutch   TCase #1: NP203   TCase #2: Dana 20   Wiring Harnesses   Miscellaneous Bits   Doing the Swap!   The Butcher's Bill   Acknowledgements

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Powertrain Swap du Jour:
Turning My YJ into a Bronco in Jeep's Clothing

For many years, first AMC and then Chrysler used the venerable 4.2L (258) straight-six engine in CJs and YJs.  It's a good engine really, lots of low-end torque, and quite reliable if well-maintained.  My YJ had one from the day it was built back in '89 up until September '99.  So why'd I get rid of it?  Well, nobody ever accused the 258 of being a powerhouse.  As people who went on road trips with me will attest, that engine just didn't have the poop to turn 35" tires at any kind of highway speed.  On long, gradual hills it was a real dog.  The mileage was pretty bad too, and it had the annoying and occasionally dangerous habit of stalling on steep inclines when off-road, thanks to the notorious Carter 2-bbl carbeuretor.

Fortunately, nothing disastrously wrong ever happened to force me into doing something right away about the power problem, so there was lots of time to contemplate the options:

1. Soup up the 258

There's plenty of aftermarket stuff available to add a few hp here and a few there, but I'd still have been stuck with that carbeuretor, and the feeling that I'd have been throwing good money after bad in the long run.
2. Add fuel injection to the 258

The creative folks at Mopar have a bolt-on EFI solution for this engine, which basically uses factory parts from a '94 4.0L YJ engine.  Last I checked, this MPI kit cost roughly $2k-$2.5k CDN.  That's a lot of cash!  I hummed and hawed over this option for a long time, but couldn't shake the feeling that there had to be a better answer for that kind of money.  After all, $2500 is a lot more than the price of, say, a complete '94 4.0L-HO engine...
3. Okay, so swap in a used 4.0L and be done with it!

That'd be pretty easy, but the 4.0L puts out less torque than the 258, and while it's got the HP to push those 35" tires, there wouldn't be a whole lot left over.  To be honest, this option just wasn't interesting enough!
4. Swap in... something else

This option involved the most time, effort, and frustration, so naturally it appealed to me!  After a LOT of back-and-forthing with both local friends and those on the Jeep mailing lists, I finally decided on the Ford 302.

What's that?  You didn't think there was enough wheelbase in a YJ to do a dual-transfercase setup?  I was surprised, too.  But as it happens, the length from the bellhousing to the first driveshaft u-joint in the AX15/NP231/Currie SYE setup is within half an inch of my NP435/NP203/D20 setup!  Figure in the reduction in block length with the V8 over the I6 engine, and I ended up with a longer rear driveshaft than before!

To make a much longer story even shorter, the project began one day in late 1998, when I bought a complete used 1990 Mustang 5.0L engine from Craig at National Mustang in Langley, and it ended (sort of) about ten months later, when I took the Jeep for a very cautious maiden voyage around the block after three weeks of downtime doing the swap.  Here's what I ended up with:
 

Engine	1990 Mustang 5.0L, 302 cid, completely stock Mileage: 100k kms
Transmission	Ford NP435 4-speed of unknown vintage (early 70s?) 6.69:1 unsynchronized first gear
Transfer case #1	Gear reduction box from a Dodge NP203 transfer case 2.0:1 low range
Transfer case #2	1972 Early Bronco Dana 20 2.46:1 low range

At the time of this writing, I've been driving the Jeep for four or five weeks.  The clunky NP435 took some getting used to, but otherwise my rejuvenated Jeep has been a ton of fun!  I don't think the mileage is as good as I was hoping, but until my speedo is calibrated again I won't know for sure.  There's still a few things to take care of before this swap is truly finished, like shifters for the transfer cases, improved motor mounts, and a new tach, but it should be okay through the winter as it is.

It's been offroad once already, for a couple hours of playing in some local sand dunes.  The 5.0L's power was nice to have, getting up those sandy slopes!  Didn't stall either, except when I got off the clutch too fast.  Soon I hope to have a chance to explore the super-low gearing, though until I make up some shifters I've got to get under the truck to switch the t-cases in and out of low range!

On-road, the extra power is really, really, really nice.  I can actually accelerate up hills!!!  Pushing the accelerator pedal down actually does something besides burning more gas!  That alone was worth the price of admission.

Wondering how much this project cost me? Here's an itemized list of all my expenses, not counting my time of course, and all the skinned knuckles!  Yes, I know it wasn't cheap, but it was a lot of fun (in hindsight, of course).  I'm very glad I did it, and I'm very thankful that it came together so well.

Acknowledgements

...and thanks in particular to a local Jeeping friend (who happens to be a damned good CNC machinist), whose assistance in making my custom adapters was the only thing that made the dual-transfer-case idea even remotely practical!

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