5.0 Mustang & Super FordsHow To Drivetrain
Super Twin Clutch - Twice As Nice
Spec's New Super Twin Clutch Gives Double The Clamping Force With Half The Pedal Effort
Horse Sense: SPEC isn't only about high-end twin-disc clutches; the company has a full line of single-disc units covering the range from mild street cars to full-on race cars (Stages 1-5). SPEC also manufactures flywheels and lightweight clutches.
Times have never been headier for horsepower junkies. Factory power levels are at all-time highs, and the aftermarket is ready and willing to take power to the stratospheric level. The only question is, how high can we go and still drive it on the street?
We're fairly sure there will never be a limit to what people try to drive on the street. But having such huge power on tap begins to call into question the durability of some other parts. In manual-transmission Mustangs, the clutch has long been the fuse that kept the rest of the drivetrain from snapping like a twig. These days it's not uncommon for transmissions, driveshafts, and rearends to be fortified for big power. As such, the clutch has to hold up its end of the bargain.
If you've driven any big-power, manual-transmission Mustangs, you know the Herculean effort often associated with pushing in the clutch pedal. It's a trade-off hardcore 'Stang owners often make so they can still bang gears and have a blast. At some point, there was always the inevitable swap to an automatic transmission to make such cars livable. Thankfully, technology is keeping pace with horsepower these days, so transmission choices can be a matter of personal preference rather than necessity.
For the gear-banging crowd, the latest evolution in clutch technology is the twin-disc clutch. These arrangements allow increased clamping force without doubling your pedal effort, as they use-you got it-two clutch discs. One of the latest in this genre is the Super Twin from SPEC. Depending on what discs are supplied, it can handle up to 1,795 lb-ft of torque. That's right-17 hundie! If you have an '05-'07 Mustang GT, an '01-'04 Mustang GT, or a '99-'04 Mustang Cobra, SPEC has Super Twins designed to survive 900; 1,395; 1,595; and 1,795 lb-ft of torque depending on the disc material. Since we're in the infancy of 1,000-horse street 'Stangs, the Super Twin might give future King of the Street winners room to grow.
After installing the Super Twin in Jim Gifford's '01 Cobra, we were fortunate enough to take it for a test spin. The car sports a D-1SC blowing into a bolt-on-laden but stock Four-Valve engine. At the time, it put down 460 to the tire. That doesn't necessitate the Super Twin, but it gives Jim room to climb the horsepower ladder. In practice behind the stock 3650 five-speed transmission, the Super Twin seemed to be a lighter version of the factory twin-disc in a GT 500. As such, the pedal was light with only a hint of chatter when the rpm was kept too low. When you consider the GT 500 has a hydraulic clutch and Jim's uses a cable, you know the Super Twin felt great.
To check out how a Super Twin goes in, we ordered one for Jim's car. We then headed to Modular Madness in Sarasota, Florida, for the nuts and bolts.
An impressive-looking unit, the SPEC Super Twin clutch (PN SF87ST; $1,899) features an all-billet construction. Some of that billet is aircraft-grade aluminum and some is high-carbon steel, but it's all good stuff. Aside from the construction, the Super Twin offers a bolt-in assembly with no setup or shimming necessary, a welcome change from twin-disc clutches with plenty of adjustment. Moreover, the clutch is SFI-approved and fully rebuildable.
Before installing the included eight-bolt flywheel, Don applied blue Loctite threadlocker to the fasteners. Then he lined up the flywheel and crank and hand-threaded the bolts before grabbing the torque wrench and applying 65 lb-ft to each bolt.
These marks and painted lines allow easy lineup of all the parts for the twin-disc clutch, which is critical to proper operation. The good news is that's all the adjustment necessary-line everything up and bolt it in.
With the flywheel secured to the crank, Don sprayed it with brake cleaner to ensure no slippery stuff would foul the clutch's engagement.
To ensure that everything was properly lined up, Don slid the assembled clutch on the transmission's input shaft. The Super Twin assembly is made up of the pressure plate, two discs, and an intermediate plate that rides between the two discs. According to SPEC's mechanical engineer, Matt Rocksvold, our Super Twin featured full-face carbon semi-metallic discs designed to corral up to 1,595 lb-ft of torque. We're fairly sure Jim's stock five-speed will give up first.
With everything in line, Don slid in the clutch installation tool and lined the pressure plate upon the flywheel's dowel pins.
Next, Don torqued the pressure plate to the flywheel. After applying blue Loctite to the the bolts, he spun the fasteners to 20 lb-ft, then stepped up to the final 25 lb-ft rating.
While there's a lot more going on with a twin-disc clutch, the assembled unit doesn't seem to take up much more room than a single-disc SPEC.
The Super Twin clutch includes a new throw-out bearing. Replacing the old one is as simple as popping off the clutch fork, sliding out the old bearing, and sliding in the new one.
The toughest part of the clutch swap is reinstalling the tranny. Having a fully outfitted shop meant Don had the assistance of a tranny jack. He says this can be a blessing and a curse; sometimes it's easier to get a friend and muscle it in. Of course, he was trying to sneak it in with the shifter still in place, but after jockeying, the bellhousing was up against the block.
Getting the input shaft in is half the battle. The top bellhousing bolts can be tricky. Don's solution was to use tape to hold the socket on the bolt. Once it was tightened, he reached up and pulled the socket and tape off. It beat fighting with the socket slipping off the bolt.
Then it was time to reinstall the driveshaft and exhaust system. We made sure to give Jim a hard time about his rusty stock driveshaft.
With all the undercar work finished, Don moved inside to bolt on the shifter handle and returned the shift boot and knob to their rightful place. Then we could take Jim's car for a spin. The clutch felt good, with light pedal effort and just a hint of chatter if the rpm wasn't high enough. SPEC assured us this minimal chatter would disappear with some break-in mileage. Beyond the clutch, Jim's car sounded great-its bypass dumped to atmosphere like a turbo.