Installing SPEC’s New, Organic Twin-Disc Clutch Assembly


Finding yourself on the receiving end of an “I told you so” is never any fun, especially when it is delivered by the CEO of your company. That’s exactly what happened after Vortech Superchargers installed our Paxton Novi 2200SL blower on our 2005 Mustang GT. We immediately found the weak-link in the factory drivetrain, and our friends at SPEC Clutches were there to help us.

It only took one pull on the dyno for us to immediately shake our heads and laugh. The clutch was slipping, and skewing the post-install results. We should have known better. In fact, in that first dyno pull, the clutch began slipping half way through generating boost under the curve.

Safe to say, we were in dire need of a new clutch assembly, but there were a few variables in need of consideration before we stuffed a new clutch assembly into our newly supercharged Three-Valve Mustang.

After finding ourselves with all of the boxes checked on our list, we hit up SPEC for a formidable clutch assembly. Our top priority was selecting a clutch assembly we could grow into, as we have some additional upgrades planned for our Mustang project. So, while one clutch assembly might work for us today, it might not work for us tomorrow, or down the road when we add more power.

Choosing Our Clutch


By now you’re probably aware that our supercharged project car has plans to eventually make a 10-second pass at the drag strip, while still retaining daily driver manners. This necessitates a clutch assembly that can wear many hats. For example, our new clutch must have a high torque capacity, stock-like pedal feel, and interchangeability. Fortunately, our friends at SPEC were able to hook us up with a sort of one-and-done clutch that delivered all of the above.

SPEC Super Twin Clutch Features

  • Twin-disc clutch assembly
  • Endure organic clutch discs
  • Billet aluminum flywheel
  • Rated for 800 lb-ft of torque
  • Infinitely interchangeable assembly
  • Smooth engagement, light pedal feel
  • SFI Approved
  • For 2005-2010 Ford Mustang GT 4.6-liter six-bolt
  • PN SF46SST-O
“We recommended the Super Twin for its versatility,” David Norton, founder of SPEC, told us. “Being an infinitely rebuildable multi-disc configuration with a street material matrix (made to handle extreme track duty), this Super Twin offers excellent daily drivability in traffic, while also offering a capacity well beyond the car’s output. Furthermore, the unit is upgradeable (and downgradeable), so any increases in output that take it beyond the current capacity can be cured by a simple disc pack.”

It’s important to keep in mind that our Mustang is currently on a stock-bottom-end, which means we’re currently still using the factory six-bolt cast crank. Moreover, we’re still on the stock Tremec TR-3650 five-speed manual transmission as well – which, if you know these transmissions, will tell you that they utilize a 10-spline input shaft from the factory. The beauty of our clutch assembly is that we can rebuild it to the correct specifications – eliminating the need to purchase an entirely new clutch assembly again.

“The mod-motor flywheels can only be fit with one crankshaft pattern, so the flywheel will have to change with the crankshaft,” David elaborated. “But a simple disc pack will transform the unit to fit the 26-spline transmission. Also, all components of the unit are available – all the way down to a fastener. So when the time comes, the Super Twin is fully serviceable with simply what it needs, offering a cost-effective replacement of parts that does not require the purchase of a new assembly. A rebuild can be executed by a shop or end-user; until the cover shell needs a new pressure ring, for which it has to come into our facility.”

The Twin-Disc Advantage


David also explained why a twin-disc unit isn’t overkill in virtually any high-performance application, no matter the power output. There are an abundance of advantages to this style of clutch assembly both from a performance standpoint and a street driven one.

“A multi-disc can drastically increase effective surface area, which in turn adds capacity and progressiveness to the engagement,” he said. “The Super Twin can achieve a higher capacity with equal or better drivability. All other specifications equal, a twin-disc will double torque capacity.”

A multi-disc can drastically increase effective surface area, which in turn adds capacity and progressiveness to the engagement. — David Norton, SPEC

Adding to that equation of street-friendly drivability is the clutch disc material choice. We’ve highlighted in the past that there are several material choices that manufactures can use when it comes to a vehicle’s clutch assembly – and David recommended SPEC’s latest advancement in clutch disc material, dubbed endure organic.

“Our latest organic matrix is an advanced mix of polymers and metals that boost the dynamic friction coefficient and reduce wear rate, while offering an even smoother initial engagement,” David explained. “The nature of the material, in conjunction with the increased surface area from the multi-disc design, makes this unit extremely smooth in all environments. There are no disadvantages to this material is this application, and we use it in several classes of racing, including NMRA Pure Street.”

Commencing The Install

Yes, that is our Paxton supercharged Mustang in a home garage, on jack stands for this install.

Yes, that is our Paxton-supercharged Mustang on jack stands in a home garage, ready for this install.

After a few weekends of spirited driving in our supercharged Three-Valve Mustang, we could start to feel the clutch slowly slipping away with every boosted gear change. Fortunately, our home garage is filled with more than enough tools to handle the job – but with no lift. Nevertheless, we headed this endeavor straight on and tackled the job with nothing more than four jack stands, a two-ton hydraulic jack, and a trans jack.

Once we disconnected the battery and got the car lifted off the ground and on jack stands, the installation began by removing the essentials, which included the factory H-pipe and cat-back, as well as the factory driveshaft and the transmission crossmember. (We used the trans jack to support the transmission during the removal.)

While we were replacing our clutch assembly, we decided to install a few additional components which were not included from SPEC (although they were recommended, as they are wear items). We decided to purchase a LUK clutch slave cylinder (PN LMC485) from RockAuto to replace our worn-out stocker. We also purchased a brand new pilot bearing from Ford Performance Parts (PN M-7600-B), as well as a McLeod Racing stainless steel hydraulic clutch line (PN 139251) from AmericanMuscle.

Removing the worn-out, factory clutch assembly.

Next up was to remove the starter and drop the transmission so that we could remove the worn-out factory clutch assembly.

After removing the factory clutch assembly, we began by analyzing how the clutch looked overall. When we removed it, the assembly had just over 72,000 miles on the clock. Albeit after more than a dozen drag strip passes on DOT slicks and a handful of burnouts, it actually didn’t look quite as bad as we had thought.

As you can see, the clutch discs still had a hint of life left in them. However, the hotspots on the factory flywheel tell a different story.

Removing the factory pilot bearing required us to rent a pilot bearing removal tool from our local parts store. Plan on putting down a deposit for $50 or more, but 9 times out of 10, you'll get that deposit back free and clear.


Make sure you hit everything with the proper torque specs!

Make sure you hit everything with the proper torque specs!

With everything buttoned back up, we took our blown Mustang out to the streets to ensure everything was in order. As we’re sure you already know, with any twin-disc assembly, there’s going to be some added noise. However, in the case of our new setup from SPEC, we were pleasantly surprised that the additional noise was inconsequential. In fact, the only time you really hear anything more is when the clutch pedal is depressed. This is due to the floater discs in the assembly freely spinning, causing a slight chattering sound.

We’ve put about 1,000 miles on the clutch since the installation. In the beginning of the break-in period for the first 200 miles or so, you’ll notice the clutch pedal is extremely consistent – but it’s important not to confuse this with stiffness. At first, the clutch pedal does appear to be more noticeably stiff than the factory components, but after reaching the break-in point (SPEC recommends at least 500 miles), you’ll feel the pedal-feel transition into a softer, yet still consistent throw.

Consider this our "clutch killer."

Consider this our “clutch killer.”

“Our race customers have gone with the lightest options to take advantage of the power and the rate of rev gains,” David said. “When considering the moment of inertia (MOI), you should consider mass from both the clutch and flywheel. The fuller diameter twin-disc assembly can weigh as much as (or more) than a single-disc assembly – so a lighter flywheel can offset the mass and offer a nice reduction that does not adversely affect the drivability or momentum. We make the recommendation based on how the car is built and prepared, but there are few instances that we would recommend the heavier parts as the ‘best’ option for a Mustang application.”

If you’re in need of a new clutch assembly, like we were for our supercharged Three-Valve Mustang, checkout SPEC Clutches’ website for more information on its clutch choices for your application.

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About the author

Harrison Noble

Living in San Diego for most of his life, Harrison was exposed to a variety of cars at an early age. His passion for anything that is fast, or has a V8, brought him to Power Automedia.
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