SPEC Shares How To Choose The Best Possible Clutch — Part 2

In Part 1 of this story, we introduced you to SPEC Clutches and the company’s founder, David Norton. He told us how he began SPEC to fulfill the need for very high-tech performance and competition-level clutch assemblies that could still be driven on the street. In Part 2, we dig deep and continue to quiz him about selecting the best-possible clutches for our cars, and we hear about SPEC’s matching flywheels as well.


TURNology: What are the benefits and downsides of lighter-weight clutch assemblies? Why do you offer so many lightweight clutches and flywheel options?

Norton’s personal time-attack car, a 240SX.

Norton: There’s no need for a sales pitch, as there never is when the benefits are infallible. We simply cite dyno results and refer to those who make their driving impressions available. There are very few builds and environments — street or track — in which the benefits of lightweight parts don’t heavily outweigh the downsides if any exist.

It’s true that every car is different, and based on an owner’s unique situation, we may recommend a median mass reduction at times, mostly for feel. So please call us for direction on lightweight components before you purchase. For a track rat like a modified Miata, 99.9% of the time, we will recommend the lightest setup.


TO: What differentiates a street clutch from a drag racing clutch, or one optimized for something like autocrossing or road racing?

The SPEC Stage 1 single-disc clutch is built for the street as it has a more forgiving engagement, but can still be used on the track, such as endurance or drag racing. This clutch is the SF48. It is a 10.5-inch clutch for small-block Ford applications such as the ’86-01 Mustang, 5.0-liter, and 5.8-liter Windsor.

Norton: It depends on the application, as every car is different in regards to bellhousing size, weight, powerband, and gearing. Drivers also differ in style and preference. In the SPEC product line, you can often choose a street-friendly unit for racing, as long as the capacity covers your output.

We have Stage 1 single-disc clutches that will support 9-second quarter-mile passes in some cases. We also use our Stage 1 material quite a bit in endurance racing. Our race-clutch technology supports daily street use and vice versa. It is not necessary to use aggressive materials for race applications, just durable materials.

A road race setup would be catered to the driver’s skill level, as an inexperienced rev-matcher will need a slightly more forgiving engagement. For road racing, lighter is better 99.9-percent of the time, so mass reduction is a goal. Mass reduction is suitable for a lot of drag racing setups as well. The amount of reduction depends on weight, gearing, and the vehicle’s powerband. For autocross enthusiasts, manageability is paramount so drivers can modulate the clutch.

This is the SF87 Stage 5 single-disc clutch. It is an 11-inch clutch for Mustang mod motors — so ’99 to ’04 — but it can also fit the Coyote through ’17 with the correct spline. Note the absence of springs on the clutch plate, signaling a more aggressive track clutch.


TO: What about the benefits of a multi-disc clutch assembly? For what applications are they best suited?

Stepping up to a multi-disc clutch set up like this SF18ST Super Twin for ‘18-and-newer Mustang GT isn’t as harsh as one might think. Given the right configuration, a multi-disc can have good road manners and still handle the track.

Norton: A multi-disc is recommended when an end-user nears (or eclipses) the capacity, compromises the reliability, or exceeds the tolerable drivability, of a single-disc. This power level will differ from car to car and driver to driver.

Multiple-disc systems can hold more torque in the same amount of bellhousing space than their original equipment single- or twin-disc counterparts. Additionally, more discs can mean more surface area. More surface area not only provides a higher capacity but also adds progressiveness to the engagement and, thus, a more street-friendly unit. Multi-disc units can be configured to hold more power with less load, decreasing thrust wear, and even reducing pedal requirement.

A multi-disc setup can drastically increase the effective surface area, which in turn adds capacity and progressiveness to the engagement. If all other specifications are equal, a twin-disc will double the torque capacity of the clutch!

The SPEC Super Twin is a multi-disc clutch for progressive engagement on the street or track and provides several benefits over a single-disc setup. This clutch is available for a wide array of applications.

Multi-disc clutch assemblies can be a benefit in three areas. Firstly, you can optimize torque capacity with a multi-disc by utilizing all the depth available in an application. Obviously, the same diameter multi-disc can exponentially increase torque capacity. But even a smaller diameter multi-disc unit can, on average, offer a 20- to 40-percent increase in capacity over a large single-disc unit.

Secondly, a multi-disc can offer a weight and inertia advantage, while still maintaining torque capacity. A smaller diameter unit will save power and provide quicker shifting, as well as enhance the reliability of major driveline components.

Thirdly, a multi-disc can offer a drivability advantage. End users beware: not all multi-discs will provide better drivability. It depends on the configuration. But, a multi-disc can allow the use of a lower-load pressure plate [softer pedal feel], less aggressive discs [smoother engagement], and can hold equal or more torque than a more aggressive single-disc.


TO: Multi-disc clutches sound like the best of both worlds, but you mention sometimes they can be the wrong choice? How could they be a disadvantage?

Though it resembles Rome’s coliseum, the SL54MT3C is a multi-disc setup for the Lamborghini Gallardo and Audi R8. SPEC manufacturers clutch assemblies for a ton of performance cars.

Norton: Multi-disc units can only be a disadvantage if chosen in error. Most of the time, a multi-disc will hold more power and drive better than its single-disc counterpart. But sometimes, the configuration can have some downsides. Some applications with shallow bellhousings can dictate a thin disc and surface, reducing wear life and increasing the possibility of warpage. Multi-disc setups using thin disc assemblies can also be more prone to chatter than a thicker, beefier single-disc. Also, a very lightweight twin can cause bad drivability in an application that is not suitable for lightweight parts.


TO: How do we choose a SPEC multi-disc clutch/flywheel assembly for our specific application?

Norton: A proper recommendation is the most crucial aspect of any major purchase decision. At SPEC, we welcome recommendation inquiries all day, every business day, and even find time to field questions over the weekend when we’re not racing or attending events. However, you can be equipped to make the right choice of clutch configuration by knowing the differences in the Stages and using the information provided on our website. We categorize the various performance capabilities, driving characteristics, suggested driving environments, and inertia properties of our product line.

Choosing a multi-disc clutch is a more specialized process. End-users in search of a multi-disc would be those either making big power or planning for it in the future. They could also choose a multi-disc for lower inertia and quicker shifts, or a higher rate of rev.

This exploded view of the SPEC Super Twin shows how two discs are used with two pressure plates for progressive clamping force.

Most realize that multiple disc systems can hold more torque in the same amount of bellhousing space than their original equipment single- or twin-disc counterparts. Most also know that more discs mean more surface area. More surface area not only provides a higher capacity, but also adds an element of progressiveness to the engagement and, thus, a more streetable unit. But not all multi-disc systems shift extremely fast, provide extremely low-inertia properties, or drive smoothly. You must know what you need and how to ‘SPEC’ it out.

All SPEC multi-disc units are all-billet, designed for each application, and infinitely rebuildable. They are not merely single-disc units modified to be multi-discs.

This is the SC66 Carbon Mini Twin for road-course use. SPEC makes the Carbon Mini Twin for all GM LS/LT applications, although the SC66 number is used specifically for the LS2, LS3, and LS7.

SPEC manufactures small-diameter, and large-diameter, multi-disc clutches termed ‘Mini Twins’ and ‘Super Twins.’ Some applications have both units as an option. But, some just have Mini-Twins due to shallow or small bellhousing limits (the Honda D- and B-series are a good example of this).

Mini Twins, sized at 140, 184, and 215mm, are configured for road racing, drag racing, or drifting. All Mini Twins shift like lightning. However, they are generally not as smooth from a stop and come with a ‘Limited Street’ (LS) rating on our site. This means they can be streetable in many applications with the correct flywheel inertia properties and diaphragm spring. These units will be more street-friendly in smaller, lower-weight vehicles and well-geared applications.

The Super Twins are larger diameter units, sized at 215, 228, 240, 254, and 305mm. These assemblies are designed to hold a lot of power, wear exceptionally well, and feature more stock-like drivability. A Super Twin with two 6-puck discs will drive much more like stock than a 6-puck single-disc, allowing a more aggressive setup to be comfortably driven daily. They can often be configured for road racing, drag racing, drifting, and normal daily street usage — in the same unit.

The SB60PT is a Super Twin for street and track for E60 M5.

The Super Twins target 900 to 2,000 horsepower Supras, Skylines, 350/370Z, BMW M3/M5/M6, Vipers, Corvettes, and Mustangs. These are all cars that have sufficiently-large bellhousings to accommodate a multi-disc clutch without reducing the diameter so much. The Super Twins can also be light and shift very fast, and often can be bought with a choice of lightweight aluminum, steel, or heavy steel flywheels. The heavier flywheel option caters to some dual-mass flywheel cars that have gear noise when the original flywheel is not used. It greatly reduces the amount of noise heard at idle with the pedal out.

So, the considerations are large-diameter versus small-diameter, lightweight versus not-as-lightweight, and which material stage to choose. The SPEC website gives torque capacities and recommended driving environments, in addition to material and flywheel options to equip the end-user with information and flexibility to choose the best possible multi-disc for their car or truck.


TURNology: Regarding clutch diameters – is bigger always better, or is there a scenario where the smaller sizes would be more desirable?

Here is an exploded view of the Super Mini Twin clutch. It works the same as the Super Twin, only it has a smaller diameter.

Norton: It depends on what you are trying to achieve. If the highest-possible torque capacity is desired, then bigger is better if the clamp load provided by the larger pressure plate is high enough to support the gain. In other words, some pressure plate designs, regardless of diameter, support higher clamp loads than others. If pressure plate A is 240mm diameter and 2,800 psi, then a 254mm pressure plate B must have at least 2,640 psi to match the capacity of the 240mm unit, and obviously more load to exceed its capacity and justify the increase in diameter and rotating mass.


TURNology: Can you define the various “Stages” of SPEC clutches, and tell us about the friction materials used?

Norton: We go into detail about that on our website. It’s easiest to just go to www.specclutch.com/products to get all the details.


TURNology: Finally, we wanted to ask about the broad line of SPEC flywheels. Obviously, they are engineered for use with SPEC clutches, and we’d like to hear how the two designs are maximized for use together.

The SP90A is an aluminum flywheel for Porsche 996 and 997. It’s so pretty, you wish you could see it in the car.

Norton: Not all of our flywheels require our clutch and vice versa. Our flywheel line is categorized as non-proprietary (direct replacement) and proprietary. We offer a full range of flywheels that can be used with any clutch made as a direct replacement for the stock parts. We also manufacture a line of flywheels that accommodate a line of proprietary clutches we develop for select applications. These clutches are an extreme redesign — from cover shell parameters to bolt pattern, to release characteristics, etc. — that require a specialized flywheel to maximize the gains of the unit and allow for a direct-bolt-in installation.

Obviously, we recommend our flywheel whenever a person’s budget allows because we believe we manufacture the highest-quality flywheels in the industry. When considering material grade, machine tolerances, assembly hardware, installation hardware, and catalog of offerings, we are the pinnacle.

We use forgings where most use bar-stock. We use machined billet triggers when most use stampings and a waterjet. We include the correct high-grade clutch fasteners with every flywheel, we diamond-cut the friction surfaces, use high-carbon steel, etc. So, we carry a high level of quality that all of our products benefit from, but we offer clutches that can be used with other brand flywheels, as well.

SPEC’s Shelby Mustang GT350 flywheel is not only lightweight, but it is rebuildable as well. It comes with all the high-quality hardware you expect for a high-performance car.


We want to thank David for sharing all of this information with us, and hopefully, you learned a bit more about the options available to you in the crowded aftermarket clutch scene. We didn’t know that so many multi-disc options were so streetable, as they always seemed pretty exotic and racy to us. Obviously, this is no longer the case!

We all want a clutch that can put up with the kind of enthusiastic abuse we enjoy when pushing our cars out on the track, but we also want them to last long-term on the street. It’s encouraging to hear straight from a premium manufacturer like SPEC that we really can have the best of both worlds now. It’s also encouraging to know we can call and discuss our wants and needs directly with the engineers who create these fantastic parts. Thanks David!

Make sure to check out part 1 of this article if you haven’t already, HERE!

At the 2018 SEMA Show, SPEC had Lucky 7 Racing’s Global Time Attack car owned by Jonathan Grunwald on display.

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About the author

Scott Parkhurst

Beginning his career in the U.S. Air Force, Scott transitioned from hands-on positions assembling aircraft to racing engines when he became Tech Editor of Popular Hot Rodding. He was later Editor of Engine Masters and is a published author.
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