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Jeep YJ Manual to Auto Tranny Swap

By Tony Lopez

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Tony Lopez - MoabRush Hour
You just pulled out of the drive-thru and onto the street with a bucket of soda too big for any cup holder in one hand, a two-pound burrito in the other hand, and a plate of nachos in your lap. Traffic speed is varying between .5 and 85 MPH. You are negotiating your lunch without the help of a co-pilot or spotter.

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.

Tony Lopez - MoabWith 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.

There are 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 short.

All these 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.

Adapters are 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.

I decided 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 problems.

Choosing a Donor
Tony Lopez - MoabThe 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.

The other 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.

There are 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.

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