Steve Baur
Former Editor, Modified Mustangs & Fords
January 1, 2007
Photos By: Bob Watson, Darrin Burch

While fast cars with manual transmissions can be loads of fun, not everyone has the desire and/or coordination to swap gears on their own. There are also instances, such as the one we have here, where the car is just too fast for its own good. MM&FF shootout alumnus Bob Watson found himself in just a situation with his '01 Bullitt Mustang.

Watson's special-edition Pony has steadily progressed from a blower and exhaust to a more potent Vortech T-Trim-fed, poked, and stroked VT Engines' Darton MID 5.0 modular powerhouse, which led its owner to drop in a Tremec TKO five-speed to keep up with the 600-plus rear-wheel horsepower. A switch to a more aggressive clutch took away some of the Bullitt's driveability and fun factor, which are extremely important to Watson as this car gets quite a bit of street use. An aggressive clutch combined with multitudes of horsepower and street tires can make for an eventful driving experience-and a weak left leg should you get caught in rush-hour traffic.

Another problem is the lack of appropriate elapsed times. While the Bullitt's driver can wheel pretty well, the elapsed times just weren't as deep as they could be with a race-style stick. That, combined with the aforementioned driveability issue, pushed Watson to look into the option of installing an automatic in his '01 Bullitt Mustang, a car that was available only with a manual transmission.

Our subject Bullitt Mustang already had its stock Tremec 3650 five-speed swapped for a heavy-duty TKO unit prior to this story. But the aggressive aftermarket clutch took some of the fun factor out of the ride during street driving. The 4R70W/converter combo is marginally heavier than the TKO and its clutch/flywheel setup.

Transmission swaps are nothing new in the Mustang world, though you hear about auto-to-five-speed swaps far more often than the opposite. That said, we decided to follow along with the Tremec-to-4R70W swap and see how it turned out both on the track and off.

The key players in this tech story are Watson; BC Automotive of Carmel, Indiana; and HP Performance of Orange Park, Florida. Darrin Burch of BC Automotive has gained a good rep in the modular arena with regard to stout auto transmissions, and he hauled a fully customized 4R70W down from the shop's Indiana digs. Alan Brown of Dirtydog Performance accompanied Burch and also lent his services on this install. Brown handles all of BC Automotive's torque converters and offered us a custom unit for this application. The converter is a lock-up unit with a stall speed of approximately 3,500-3,800 rpm.

While Burch and Brown handled the majority of the installation, the HP Performance staff supplied its lift, Dynojet, mechanical exper-tise, used-parts finder, and tuning prowess to help get the job done. Also making this happen was Jerry Wroblewski of SCT who supplied an NUX2 automatic ECM, along with upgrading the SCT Pro Racer Package so we could program the new NUX2 computer code.

Some of the swap parts include the torque converter, the dipstick and tube, the flexplate, the driveshaft yoke, the transmission crossmember, and the auto shifter.

Prior to the transmission swap, we researched the fitment of the present BBK long-tube headers with the automatic gearbox. All of our sources pointed to them not fitting, so we put in a call to Kooks Custom Headers in Bayshore, New York, for a set of its 131/44-inch stainless steel long-tube headers and a 3-inch X midpipe. We'd heard Kooks was the only game in town, and talking to George Kook Jr. confirmed that these headers would indeed fit the auto despite the larger-diameter primary tubes.

Watson had the headers ceramic-coated at a local shop prior to installation. We also had planned to dyno test the Bullitt before and after the transmission swap, and after borrowing an external fuel pump to replace the one that quit on us on the dyno, we were able to get a baseline figure of 622 rwhp and 536 lb-ft of torque. Post transmission swap, we saw the numbers dip to 577 rwhp and 494 lb-ft. That comes out to a little less than 8 percent loss, or about what the BC Automotive guys had predicted.

Dyno numbers don't tell everything, though, as we expected the automatic to pick up in the quarter-mile over the stick-shift box. The best run on this motor and TKO tranny combination (Watson had a standard block 5.0 modular previously) was an 11.076 at 127.80 mph with a 1.551 60-foot time in cool weather. This was backed up with a tire-spinning 11.30 pass.

On the control side of things are the shifter and shift-lock cables, the ECM and transmission harness.

After installing BC Automotive's 4R70W, the first pass out was an 11.15 at 118 mph. This was an easy, get-acquainted pass (1.79 60-foot) that we followed with a more aggressive launch the second time around. On pass two, we saw the short time drop to 1.50 seconds, and shortly thereafter the Traction-Lok went and the car veered sideways. That was the end of the day for us, but things were looking hopeful, as a three-tenth reduction in the 60-foot time is usually good for six-tenths at the other end-and in our case, possibly a 10.50 elapsed time.

Not one to give up easily, Watson had the HP Performance crew install another 31-spline limited-slip unit. This time, a 1.68-second 60-foot time netted an 11.17 at 121 mph. This was followed by several more low-11-second runs-something unobtainable before the swap. The car's ultimate track potential was never realized in this test, and we are looking into a possible tuning issue and the need for a higher stall speed converter. Aside from that, Watson gushed about the automatic's much improved street manners.

"Unfortunately, the first aftermarket clutch started failing at the higher power levels," Watson says. "I decided to upgrade it, and it worked well at the track but was not as street friendly as I would have liked. You would have to leave a stop light at higher rpm and slip the clutch more to keep it from chattering. It took a lot of fun out of driving my car around town, and being able to enjoy the car on the street is important to me.

"The automatic eliminated this 'compromise' I made between strip and street performance with the heavy-duty clutch. It has totally changed the personality of the car for the better. I've yet to find a downside to this conversion."

We did find out later that SLP Performance Parts in Toms River, New Jersey, also sells long-tube headers that fit the automatic application, though they are available only with a 151/48-inch primary tube. Both companies utilize stainless steel for header construction, and Kooks Headers offers both 151/48- and 131/44-inch tubes, the latter of which was far more applicable for our horsepower level here.

The direct clutch piston (center) is milled down to allocate room for the two extra clutches that Burch installs. A dial indicator (right) is used to verify the allotted clearance.

A job like this is not an easy task, by any means, and should be attempted only by a professional as it requires quite a bit of labor, wiring knowledge, and ECM programming to make it all work. The results may be worth it, however, if your Bullitt (Mustang GT or Cobra) has trouble getting those ponies to the pavement.

Parts List
Dipstick TubeXR3Z-7A228-BA
Driveshaft Yoke1R3Z-4841-EA
Shifter Assembly1R3Z-7210-AA
Trans Shift CableXR3Z-7E395-AA
Transmission Crossmember and Isolator2R3Z-6068-FJ
Trans Kickdown Cable BracketXR3Z-7B229-AA
Shift Interlock CableF6ZZ-3F719-AC
Shift Lever StudE43Z-7B415-A
Plug for the Mechanical Speed SensorF2UZ-7H183-A
Speed Sensor1L3Z-7H103-AB
Digital Range Transmission SensorF7LZ-7F293-AB. Also called shift position sensor-
make sure you get the kit with the spacers and bolts.
'99 GT 4R70W Transmission Wiring Harness 1R3Z-7C078-BA
Mach 1 Brake Pedal Assembly3R3Z-2455-AA
Mach 1 Shifter Bezel3R3Z-6304567-AAB

    Ford Mustang Automatic Transmission Swap Parts And Steps

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