Ahead in traffic,
you spot a fellow Jeeper one lane over in a built-to-the-hilt,
all out, pushing-the-limits-of-all-credit-cards-known-to-man rig
that you have got to get a closer look at. You jam the burrito
in your mouth, grab a handful of gears, and floor it, all while
balancing the nachos in your lap. Slowly, but surely, as you bounce
back and forth between lanes, you begin to lose your grip on the
burrito, but you’re gaining. As you near your goal, you
see decals on it bearing the logo of a sorority, and sure enough,
it is carrying what appears to be the cast of Charlie’s
Angels. You successfully save the burrito.
Then the Angels
begin to pull away, you stick the burrito back in your mouth,
grab more gears and gas it. Now you’ve got `em. You are
pulling up on their right side and take a quick look in the mirror
to check your hair just in time to see the burrito fall from its
inadequate perch and spread ultra high octane hot sauce all over
your shirt. Fortunately, you timed it just right so you roll right
up next to the Angels, look over, and give them a good laugh as
they pull away. Not that this has ever happened to me, but an
automatic transmission could have saved our hero in this case.
My Jeep is
my daily driver and I accumulate 350-500 miles per week in it.
This includes a lot of rush hour traffic and city streets. Anyone
who has driven a manual transmission in traffic knows how much
fun this can be. My Wrangler (YJ) is the first vehicle that I
have driven on a daily basis that was a manual and I was beginning
to miss the automatic more and more. It didn’t bother me
on the trail. I was just sick of the work, although the smoothness
that an automatic offers on the trail was certainly very appealing.
I was also attracted to the idea that with this smoothness comes
a reduced chance of trail breakage due to the cushioning that
the torque converter provides.
this in mind, I began researching the automatic swap. Initially,
I thought about using the factory Jeep automatic so I could use
all factory parts. There are some advantages to this, not the
least of which being that I am only using stock parts that can
be found at any dealership. As my Jeep grew and I researched this
more, I found that with the addition of big tires and lower gears,
I didn’t want to give up the overdrive gear that I had with
my 5 speed manual. I also had the additional horsepower of a 4.7L
stroker being sent aft through the gearbox that I didn’t
think I wanted to send through the stock Jeep automatic. With
these considerations in mind, I gave up on the factory unit and
began researching other options.
not a lot of other options to be found for putting an automatic
behind the Jeep 4.0L. The only others that I thought may be feasible
were the AW4 found in Cherokees (XJ) or a Chevy 700R4. The AW4
is a good candidate for an automatic swap for several reasons.
For one, it was designed to run behind the 4.0L. The NP231 transfer
case will bolt right up to it. Factory parts, although from an
XJ, could be used throughout. It is about one inch longer than
the stock five speed so driveshaft length won’t get ridiculously
good points are not without a catch, though. The AW4 is a computer-controlled
unit and to graft that into a vehicle that it was never designed
to be in could present gremlins that I wasn’t real interested
in dealing with. With this in mind, I began to look more serious
at the Chevy 700R4. The more I looked at this transmission, the
more I liked it for several reasons.
available to fit the 700R4 to a 4.0L and to the NP231 so it is
actually quite easy to bolt the whole drivetrain together. The
700R4 has a first gear ratio of 3.06:1 so it will crawl pretty
well on the trail. The overdrive gear ratio is .70:1 so I can
run a lower axle ratio for crawling ability and still turn a reasonable
RPM on the highway. It is hydraulically controlled so no splicing
of wiring harnesses would be needed. It has a lockup torque converter
so gas mileage shouldn’t be affected. It is among the most
common Chevy automatics ever produced, so finding one is pretty
easy. I have also rebuilt my share in my years in a transmission
shop so I am very familiar with them.
that a 700R4 was the way to go for my rig. My goal then became
how to install one into my 1991 YJ as seamlessly as possible with-off
the-shelf parts and as little custom fabrication as possible.
Once I had
decided that I wanted to install a 700R4 in place of my stock
AX-15, I turned to the swap experts at Advance Adapters. A little
perusing of their catalog and I was able to put together a list
of what I thought would cover the swap. I called them up to place
the order and they pointed out about five things I had forgotten.
These guys know their stuff! I have grown up working in a transmission
shop so I thought I would be able to spot a majority of the gremlins
that I would encounter. Little did I know what I had gotten in
to. Fortunately, they were able to guide me through most of the
700R4 was available from GM in many different applications. It
was put behind everything from a 2.8L V6 all the way up to a 6.2L
V8 diesel. It was used in compact and full-size trucks, station
wagons, sedans, and sports cars. With all of these to choose from,
what makes a good donor for the swap that I want to make? I was
looking for something that was originally built to hold up to
a high-performance engine since I was getting around 240 horsepower
from my 4.7L stroker. This ruled out a lot of the potential donors
since even most V8’s are relatively mild from the factory.
I decided that the best donor was a sports car that had a fairly
healthy V8 in front of it. I did some hunting and found a core
from an `85 Pontiac Firebird for $125 that I thought was a perfect
candidate. It was originally behind a high-performance V8 so the
valve body was already tuned for a performance engine.
item I needed to consider as far as the transmission was concerned
was the torque converter. Each of the applications that the 700R4
was used for had a specific torque converter based on the engine
that it was behind. The difference in the converters is most easily
explained using the term stall speed. The stall speed of a torque
converter is the peak engine RPM with the transmission in gear
and the vehicle being held at a stop. The same torque converter
will stall at a higher RPM if it is installed behind an engine
with more power.
advantages to both a high stall and a low stall converter. A high
stall allows the engine to get deeper into its powerband before
it stalls but it also has to be at a higher RPM in order to multiply
torque. A low stall will multiply torque at lower RPM but the
engine may not be able to get into the meat of the powerband before
the converter stalls.
The low stall
will also generate less heat because there is less turbulating
of the fluid than in a high stall. I elected to go with a low
stall V8 converter. This converter was originally behind V8’s
in full-size trucks and certain cars. I went with this converter
because my engine is putting out more power than these V8’s,
so the low stall speed that is has stock will be slightly higher
behind my engine. I also wanted a low stall converter because
my engine was built to produce torque at low RPM for wheeling.
I thought that this in conjunction with the low RPM torque multiplication
of the converter would work very well together.