Mopar Overdrive Transmission Swap

This was how the GTX finished HOT ROD’s first Power Tour.

You can see how similar the A518 (rear) and the 727 Torqueflite (front) look.

With about 1.5 inches removed around the crossmember housing, the overdrive tailshaft cleared the body.

Using the original crossmember’s end pieces, Art Carr fabricated a new crossmember to secure the A518 in place.

Because of the GTX’s floor shifter, a small bracket (arrow) was fabricated to hook up the shift linkage.

Art Carr installed a 10-inch torque converter (part No. 36011) with a 2500-rpm stall. While not a lockup converter, its internal design greatly reduces slippage at highway speeds.

The JVX adapter kit includes an adapter plate, a steel crank hub and a flywheel.

The factory throttle linkage and tranny cooler lines bolted up perfectly. Did Chrysler plan on people doing this conversion?

If you took part in our first Power Tour or read our coverage in the September ’95 issue, you may remember this ’70 Plymouth GTX. It was one of the HOT ROD Tour vehicles that couldn’t complete the 2900-mile trip. While the Ford and Chevy guys might get a kick out of that, it should be noted that the car’s demise was caused by a cracked aftermarket torque converter—the only non-Mopar part of its drivetrain.

While the obvious fix would be to simply replace the lame torque converter with a Mopar unit, 1500 miles of high-rpm Power Tour driving before the breakdown suggested there might be a better alternative. Why not install one of Chrysler’s new four-speed overdrive automatics in place of the original 727 Torqueflite? The reduced engine noise and improved fuel economy would make it even better for Power Tour ’96.

First introduced for the Dakota trucks in 1988, the A500 and A518 automatic overdrive transmissions are based on Chrysler’s 904 and 727 Torqueflites, respectively. Either transmission can be found in the Mopar Performance catalog, (part No. P5249165 for the A500 and P5249166 for the A518), and the gear ratios are identical to the older trannies, with the exception of the overdrive. However, the catalog listing included the ominous warning, “requires major modifications on A-B-C-E body.” After a quick call to Art Carr Transmissions in Fountain Valley, California, to confirm that he would help us perform the “major modifications,” we had Mopar Performance ship one out.

As mentioned before, either tranny is available, but because it would be dealing with a 400-plus-hp 440, we went with the beefier A518 and sent it directly to JVX, Inc., in Memphis, Tennessee. An adapter kit from JVX allows these new overdrive transmissions to bolt directly to Mopar B/RB engines. Besides supplying the adapter kit, JVX’s John Vinson went through the A518 to give it “crisper” shifts and increased durability (see the accompanying sidebar, “JVX For the GTX”).

The first thing we noticed when the new unit arrived at Art Carr was how closely it resembled an old 727. The tranny’s front half is identical to the old Torqueflite, and the A518 is only a few inches longer, with a thicker tailshaft housing for the overdrive mechanism. How tough could this install really be?

Well, we can’t say we weren’t warned. As the Mopar catalog stated, a fairly “major” chunk of the driveshaft tunnel had to be “modified” before the tailshaft housing would clear the body. About 1.5 inches from each side of the crossmember housing was shaved off, but even more challenging was fabricating a new crossmember to hold the overdrive unit securely in place. Rather than creating a new piece from scratch, Art Carr’s technicians used the original crossmember end pieces and fabricated a center section designed to match up with the A518’s lower rear mount. Finally, the A518’s increased length required cutting the driveshaft down from 51.5 inches to 48 inches.

On the plus side, the original starter, tranny cooler lines and throttle linkage worked with the new transmission. So while not exactly a “bolt-in” upgrade, getting the A518 to physically fit into an old B-body (or E-body) Mopar can be done, and much of the original hardware will bolt directly to the new unit.

But getting it to fit was only half the battle. Now we had to figure out how to make the GTX shift into a gear it was never meant to have. As with the physical installation, it was almost as if Chrysler planned on people doing this conversion. First, the A518 continues to use only six shifter-selector positions rather than having the “OD” position that GM and Ford use. This means that with a column-shift musclecar, the shift linkage will completely bolt up.

The GTX’s floor shift, however, required some additional custom fabrication to get the shifter to operate the A518. Once bolted up, the linkage banged into an unused ear on the tranny’s case, so it was ground off, giving the linkage plenty of clearance to operate smoothly from Park to First gear.

While Chrysler employs a number of sensors, switches and engine controllers to operate its overdrive transmissions, all you really need is one dash/console-mounted switch to energize the overdrive solenoid. When switched on, the solenoid closes and the tranny will immediately shift into overdrive whenever it’s in Third gear. Once in overdrive, the transmission will not come out until the solenoid is switched open again, or vehicle speed is slow enough to cause a First gear downshift (less than 5 mph), or the vehicle is floored at a speed low enough to catch First gear. JVX and Mopar Performance are working on a system that uses engine vacuum to operate the solenoid. Until that’s available, just keep the switch off during stop-and-go driving.

Of course, the real benefits of overdrive can’t be experienced in the city anyway. It takes a cross-country road trip like Power Tour to fully appreciate the overdrive tranny. Since that’s still a few months away, we ran some numbers to give you an idea of what highway travel is like in this “old” musclecar. With the GTX’s 3.55 gears and 27-inch-tall rear tires, it used to turn 2654 rpm at 60 mph. With the A518’s 0.69 overdrive, that number drops to 1832! Now that’s assuming a factory stall torque converter, which we did not use. But even with an additional 200-rpm worth of converter slippage, that still figures out to around 2032.

Here’s another way to look at it: If we installed a set of 4.56 gears, the car would get all the increased benefits of monster low-speed launches and turn only 2603 rpm at 60 mph with a factory stall converter. That’s still less rpm than it used to turn at the same speed with 3.55s. Finally, let’s not forget about the positive effects on gas mileage and engine life. If you figure a 30-percent across-the-board decrease in highway rpm, engine wear and noise, plus the increased gas mileage that goes with it, you start to see why the benefits of overdrive can’t be overstated.

Before you start planning this summer’s family vacation in your overdrive Hemi Charger, remember that this is not a project for the faint of heart—or wallet. The retail price for an A518 is around $1600 (not including the torque converter), and the JVX adapter kit adds another $495. That’s more than two grand, and you haven’t even started grinding metal or fabricating linkage pieces, which can cost in the neighborhood of $800, according to Art Carr. But if you’re willing to spend the time and money, this is a great way to give an old Mopar new shifts.