Vinnie The Hitman
November 1, 2008
Photos By: Michael Galimi

In 2003, we were blessed to finally get a T56 in a Mustang from the factory. This six-speed gearbox was considered bulletproof with its stout gearset and triple-cone synchronizers in First and Second gears. The 390-horse Cobra certainly needed this robust gearbox to handle all of its torque, but after a few track outings and dyno days, enthusiasts began to realize that even this ultimate Mustang still had some shortcomings in the driveline department. Over time, these issues have grown apparent and are now considered recurring problems on the '03 and '04 Cobras. Today, we will address the two most common-the clutch and the input shaft.

Built To Spec
The Cobra's factory 11-inch clutch has never been a model for long life, especially when slicks are part of the equation. It's also saddled with heavy pedal effort and vague action, making it almost unbearable for many to live with. Many Cobra owners relied on the aftermarket shortly after taking delivery of their cars, and the results were mixed, as finding the right combination of pedal effort, feel, and clamping power has been an elusive combination for a conventional single-disc design. However, with the advent of twin-disc technology being available to the Mustang masses (that would be you and I), we can finally get everything we want with an over-the-counter twin-disc clutch setup.

Although there are a few clutch companies out there that offer a twin-disc system for the 4.6L engine, few offer as many different versions as Spec. Its Super Twin clutch assemblies are constructed of billet aluminum and are a direct fit for any Cobra. They allow you to reuse the existing clutch release fork and require no machining or special fabrication. As a matter of fact, the assemblies are a direct replacement that replace your clutch and flywheel assembly in one fell swoop.

For our test mule, we decided to go with Spec's most street-friendly setup, the SS-Trim (PN SF87SST), which uses one spring-hub disc and one solid-hub disc. Both use a combination of Kevlar and metallic facings for smooth engagement and, of course, massive torque-holding capacity. Speaking of torque capacity, Spec tells us that this SS-Trim can handle an incredible 900 lb-ft of torque without a whimper. For our Cobra, which currently belts out 453 rwtq (about 525 lb-ft at the crank), it won't be an issue.

Torque capacity is certainly great to have, but we also looked forward to taking advantage of the reduced pedal pressure. Because of the large amount of clutch surface area that a twin-disc clutch inherently has, there's less need for a radical spring rate, and this ultimately translates into a lighter pedal for a given torque load. This means we can finally cancel that knee replacement surgery!

Getting Shafted
The T56 is a darn-strong gearbox, but its weakest link is the 10-spline input shaft. When the folks at Tremec were tasked to fit the T56 into the Terminator, they equipped it with this 10-spline shaft to match up with the Ford clutch disc. Although it seemed like a good idea at the time, the problem was that in designing this piece, it was not as thick or robust as the 26-spline shaft that typically comes with a T56 when it is used in a Viper or Corvette. Of course, companies like G-Force Racing Transmissions can sell you the parts or build you a killer T56, but we went with this simple upgrade to keep down the cost.

Side by side, we noticed that the thinnest part is within the splines where the inner diameter of the splined area is much smaller on the 10-spline unit. This is where many people have snapped shafts, even at lower power levels like ours (gee, who'da thunk that 453 rwhp would ever be considered a lower power level?). Going to a 26-spline input shaft is a highly recommended upgrade no matter how modified or unmodified your Cobra may be. We recommend having a qualified transmission shop swap the shaft for you, as getting into the inner workings of a transmission may be a bit daunting for the first-timer. If you're a hands-on kinda guy, it would be ideal to have a buddy that is familiar with T56s get involved, as he'll probably get dirty for a beer and a sandwich (much like us). Keep in mind that you'll need to order a clutch disc to match the 26-spline shaft.

With our Cobra put back together, we added four quarts of Royal Purple Synchromax synthetic transmission fluid and were rewarded with silky-smooth clutch action and tenacious bite. The best part of this upgrade was the increase in driveability and comfort. Pedal effort with the Spec Super Twin is literally half of what it used to be with the stock Cobra clutch. It makes the car a true pleasure to drive, and being stuck in traffic no longer makes us feel like we're training for a powerlifting championship.

Follow along as we swap out the clutch and input shaft in our own garage. With some free time and savvy wrench turning, you can also get some more bite from your Cobra's clutch.