Muscle Mustangs & Fast Fords
Ford Mustang Cobra Automatic Transmission Swap - When Automatics Attack - Tech
Tired Of Shifting Your Supercharged Cobra? Put It In Drive And Stomp On The Gas With A 4R75W Automatic Swap.
If you've been reading Muscle Mustangs & Fast Fords for some time, you've no doubt seen all of the press that the supercharged '03-'04 SVT Cobras have received. Many people knew beforehand that modular engines liked forced induction, but things really took off once Ford equipped its top-tier Mustang with a bulletproof short-block and a supercharger.
Starting out with 390 hp from the factory, induction and exhaust bolt-ons made 450-500 rwhp seem like child's play. Soon, there were 550-, 600-, 700-rwhp cars out there on the street.
Transmission-wise, the Terminator ('03-'04) Cobras' T56 gearbox is a stout piece and handles most of what Cobra owners throw at them. Clutches, on the other hand, don't last as long, and most Cobra owners know they have to sacrifice clutch longevity for quick elapsed times. The weight of the supercharged Cobras is also an issue.
The good thing is, the aftermarket has you covered on clutch and T56 upgrades, but what if your car is too fast for your driving-skill level? Or maybe you just don't want to drive a stick anymore? There's an answer, and it comes in the form of a 4R75W swap.
Craig Smith of Jacksonville, Florida, is a retired electrician who regularly enjoys driving the wheels off of his '04 Cobra. His venomous snake is a little more potent these days thanks to the Kenne Bell supercharger and a 100-shot of nitrous oxide, in addition to a full complement of bolt-ons. At 62 years of age and troubled by an occasional bad back, Smith found it increasingly difficult to properly powershift the T56 like it needs to be and sought a remedy for the situation.
In the Jan. '07 issue of MM&FF, we followed along as Orange Park, Florida's HP Performance installed a 4R70W in a Bullitt Mustang. Since that time, HP did another auto swap in a bone-stock Terminator. While we didn't get to cover that one, we grabbed our cameras when we learned the company had another one lined up, this time on Smith's Cobra. What made this deal even better was that Smith had baseline track numbers and was more than willing to blast down the 1,320 for some post-swap testing.
The T56-to-4R75W swap is much like the one we performed on the Bullitt Mustang. Having a great experience with the company before, HP went back to BC Automotive, which built the Bullitt's 4R75W, and ordered a 4R75W automatic transmission and lock-up torque converter.
BC modifies the trans with upgraded bands, a stainless steel forward sun shell, and gears. They also fit extra clutches into the internals to spread the torque load across more area.
"We tightened up the torque converter for this application because of the power characteristics of the engine," said BC's Darrin Burch. The extra low-end grunt of the twin-screw supercharger and nitrous combination will work the converter a little harder, and after modification, an approxi-mate stall speed of 3,200 rpm was achieved. The torque converter is a lock-up unit that features furnace-brazed fins, a billet lockup piston, Torrington bearings, and a large-diameter clutch for added surface area.
The total power handling capability for this transmission is tested continually throughout BC Automotive's customer base. Burch says he has had a 4R75W behind a 700-rwhp twin-turbo car with no problems, and that he wouldn't be surprised to see it handle up to 800. "At that point, something will probably break, and we'll just fix that until the next weak link appears," he says. At that kind of power level, most people go for a C4, but if you like your Overdrive, BC Automotive has you covered.
That being said, you may want to think about where your supercharged Cobra is headed in terms of modifications and final power output. The BC trans and converter package retails for about $3,400 by itself. HP Performance's Tony Gonyon noted that having it installed, along with the additional parts involved, can bring the total price near $5,000.