Chris Waterman's Jeep/Bronco
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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:
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. The same strategy also applied to the transfer case,
though that somehow turned into the idea of stacking the reduction box
from an NP203 with an EB D20.
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
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...
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
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
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:
1990 Mustang 5.0L, 302 cid, completely stock
Mileage: 100k kms
||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
||200+ HP, 300+ ft-lb torque!
||4" LONGER than before!
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.
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
...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
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