Powertrain Swap du Jour:
Making my YJ a Pony 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

  2. 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.
  3. Add fuel injection to the 258

  4. 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...
  5. Okay, so swap in a used 4.0L and be done with it!

  6. 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!
  7. Swap in... something else

  8. 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.
As if you hadn't guessed, the 302 (aka the 5.0L) doesn't just bolt right into a YJ.  One of the first questions that comes up is what to do about connecting it to the Jeep's transmission.  To make a long story short, answering that question resulted in a decision to turn this engine swap into a complete powertrain swap, since it's much easier to use a tranny that's made to connect to the 302 (say, the NP435) than it would've been to adapt it to the Jeep's tranny.  And so on with the transfer case, although that somehow turned into the idea of stacking the reduction box from an NP203 in front of an EB D20 t-case.

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 front of 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 longerrear 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

Brag List

Power 200+ HP, 300+ ft-lb torque!
Crawl ratio 135:1!
Rear driveshaft 4" LONGER than before!


Rather than going through the project from start to finish, I've broken it down into sections, listed here:

Choosing and prepping the engine
NP435 transmission and clutch
Transfer case #1: NP203
Transfer case #2: D20
Wiring harnesses
Miscellaneous little bits and pieces
Doing the swap!

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 isn't 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.

Acknowledgments

There's a number of people I've got to thank for their help through this project.  No way would this have been done anywhere near as quickly (or as cheaply) as it was without their assistance, both in person and via e-mail:
 
Thomas Sternberg
Larry Soo
Chris Bradley
Glen Hogenson
Gordon Pritchard
Ash Hamidi
Chad Koehn
 Ben Olsen
Darryn Alegretto
Gil Meacham
Ed Tapanes
Jason Lockwood
Mike Garner
 Brad Nelson

...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-transfercase idea even remotely practical!

References

Thinking about doing a 5.0L conversion yourself?  Here's some links with more useful information on the topic:


Questions?  Email me!

Or, go back to my home page!