Today’s crop of softer Homo sapiens really make things difficult for driveline engineers. As if getting a clutch to hold up behind an 800hp LS small-block isn’t already challenging enough, today’s enthusiasts demand a soft clutch pedal and smooth engagement to go along with immense grabbing power. Oh yeah, they also prefer the luxury of A/C, GPS, heated seats, kickin’ stereos, and every power accessory in the book. This brutal combination of big horsepower, big weight, fragile drivers, and the stickiest tires mankind has ever produced should all but guarantee clutch failure. Miraculously, clutches haven’t just survived—they’ve flourished.
Just 15 years ago, when Camaros still had 5.7L LS1s, smoking a clutch at just 500 hp wasn’t that uncommon. These days, however, aftermarket clutch assemblies routinely endure twice that hp output despite the fact that fifth- and sixth-gen Camaros pack an extra 300-600 pounds of fat. Equally impressive are the stock-like levels of pedal pressure and smooth engagement that modern aftermarket clutches deliver. Largely responsible for this revolutionary jump in performance is the growing shift toward multi-disc clutch assemblies.
Doubling the surface area of friction material has a nice way of significantly increasing clutch bite. “The key to today’s clutches is making them friendly enough for everyday driving while still being able to hold a ton of power. We do not sell ‘heavy-pedal’ clutches,” Bob Scheid of McLeod Racing explains. “For our twin-disc clutches, we use a pressure plate that actually gives a softer pedal feel than stock. That is combined with very friendly disc materials and a strapped floater resulting in a clutch that holds up to 1,200 hp yet drives like a stock Honda.”
Nevertheless, attributing all the progress in clutch performance to multi-disc technology is a bit shortsighted, as pressure plates, disc designs, and friction materials have evolved dramatically as well. “Initially, higher pressure plate loads and more advanced disc lining materials were sufficient solutions, as they increased friction coefficient while maintaining driveability. However, modern torque and weight factors, along with the streamlining of actuation components toward shorter-throw systems, have pushed us to expand multi-disc applications, increase clutch diameters as bellhousing dimensions allow, as well as develop new bolt-in hydraulics to improve clutch control and feel,” David Norton of SPEC clutches explains.
Additionally, lesser-known variables such as gearing and tire diameter also affect the loads a clutch must endure. Determining which type of clutch works best in any given application requires making sense of all the available options, so we consulted with the leading clutch manufacturers in the industry to get the inside scoop.
The relationship between pressure plate clamping force and a clutch’s torque capacity is very simple to grasp. The more force the pressure plate exerts upon the disc, the better it will hold. What isn’t quite as intuitive is how manufacturers can manipulate clamping force, and how that force affects the rest of the clutch system. “Clamping force is simply the amount of pressure squeezing the disc between the pressure plate and the flywheel. Clamp load can be increased by changing the diaphragm spring or moving the fulcrum point,” Will Baty of Centerforce explains.
“The problem with a heavy diaphragm is that it decreases modulation control and it adds pressure throughout the entire system, not just on your left foot. When you depress the clutch pedal, the diaphragm pressure is directly applied to the flywheel, which pushes against the thrust bearing in the engine,” Baty adds. “You are basically trying to push the crank out of the front of the block every time you depress the clutch pedal. The average OE 10.5-inch clutch takes about 400 pounds of pressure to operate, and changing the diaphragm in aftermarket applications can increase that number to 600-700 pounds. The other way to increase clamp load is to move the fulcrum point internally in the clutch. This increases the pressure but decreases the life of the clutch and will slow the release rate of the clutch as well.”
Like most aspects of clutch design, finding the right balance between friction and slippage is a compromise that comes down to friction material and disc design. “Materials like metallics that have a higher coefficient of friction increase the holding capacity of the clutch as well as its ability to withstand heat under punishing conditions, such as at the dragstrip. We can keep the clamp load and pedal effort lower using these materials, but the trade-off comes in driveability, as they tend to engage more aggressively and chatter on takeoff,” Mike Norcia of RAM Clutches reports.
Although high-friction discs increase torque-holding capacity, they also decrease clutch longevity. “It’s a lot like comparing 60-grit sandpaper to 220-grit sandpaper. The 60-grit paper is aggressive and has excellent bite,” says Will Baty of Centerforce. “It’s going to work awesome for a short period of time, but the trade-off is something is going to wear out. The 220-grit paper is less aggressive so it won’t wear out as fast, but since it doesn’t have as much bite as the 60-grit paper it will require more clamp load to do the same job.”
While terminology can vary from one manufacturer to the next, organic linings offer the least torque capacity and the smoothest engagement, while metallic linings offer the greatest torque capacity and the most aggressive engagement. In between those two extremes are materials like Kevlar and carbon, as well as ceramics that are a blended together from multiple compounds. “Organic clutches are common in OE applications. They have great driveability but do not like heat when used in extreme conditions,” Jeff Neal of Quarter Master advises. “A sintered metallic friction material works very well in high-horsepower road racing applications thanks to its high heat capacity and coefficient of friction. Sintered iron is great for drag racing, as it can hold a ton of heat while the clutch is slipping and it won’t weld to the floaters or pressure plate. Finally, carbon is the best of all worlds. It’s very expensive, but it holds a lot of torque, works extremely well when hot, and it’s also very driver friendly.”
The numbers speak for themselves. “At the same pressure plate clamping load, going from our high-performance Stage 1 organic lining to a Stage 2 Kevlar represents a 15 percent increase in torque capacity,” David Norton of SPEC reports. “Stepping up to a Stage 3 carbon-graphite semi-metallic represents a 36.5 percent increase in torque capacity. Our iron ST5 lining isn’t suitable for street cars, but if offers a 93-percent increase in torque capacity, making it ideal for drag cars.”
Moving away from a traditional full-face clutch disc to a puck-style design can increase torque capacity and heat resistance. “Although it sounds backwards, decreasing the friction material surface area through the use of pads or smaller facings actually increases the holding pressure in the clutch. This achieves more holding power, but at the expense of driveability, as these types of facings are generally metallic and engage more aggressively,” Mike Norcia of RAM Clutches reveals. Simple physics dictates how a smaller facing increases clutch bite. According to Will Baty of Centerforce, it’s all about psi. “Take your palm, and press it hard against your arm. Now use the same amount of force and press one finger into your arm. That increase in pressure demonstrates the difference between a full-face and a puck-style clutch.”
In extreme racing applications, puck-style discs boast lower rotating mass and enhanced durability as well. “Puck-style discs are common in larger-diameter, high-performance aftermarket clutches where you need a sintered material for more torque holding capacity and heat dissipation,” Quarter Master’s Jeff Neal explains. “Puck-style disc designs for multi-plate clutches are a good choice with engines and drivetrain systems that are lightweight and exhibit high harmonics, as these types of combinations can cause solid-face discs to crack and fail.”
Increasing clutch diameter increases torque capacity, but the downside is greater rotating weight. Likewise, diameter is limited by the dimensions of the bellhousing. Depending on a car’s intended use, rotating weight can be advantageous or detrimental. “Clutch diameter is critical for driveability, as it provides the mass and inertia necessary to get a car moving from a standstill. On the street, we want enough mass so the clutch does not have to be slipped excessively,” Mike Norcia of RAM clutches remarks. “A lighter, smaller-diameter clutch is great for applications like autocross, where you want the motor to accelerate and decelerate quickly. However, we tend to stay away from this for cars that are predominantly street driven.”
On the opposite end of the spectrum, in road racing and circle track applications, reducing rotating mass can net quicker lap times. “Clutch diameter is an important factor in both moment of inertia and torque-holding capacity. If your racing discipline allows you to run a smaller clutch diameter, it’s advantageous for both acceleration and deceleration,” says Jeff Neal of Quarter Master. “The lower rotating mass allows the engine to decelerate more quickly, which makes it possible to drive deeper into corners and brake more aggressively. That said, when you decrease your clutch size you’ll need to add more friction discs to make up for the loss in surface area.”
Without question, designing a clutch requires balancing a dizzying array of variables in order to reach the best possible balance between holding power, streetability, and longevity. However, the beauty of a multi-disc clutch is that it throws many of these compromises and trade-offs out the window. “Multi-disc clutches offer a higher holding capacity over single-disc assemblies, and in most cases, longer clutch life. You basically have two clutches in one,” Will Baty of Centerforce explains. “Our DYAD twin-disc clutches have an average torque holding capacity of 1,300 lb-ft at the crank with excellent driver control. It feels like a stock clutch, but with way more holding capacity.”
Not surprisingly, added performance comes with added costs. With twice as much hardware as a standard single-disc clutch, twin-disc units can cost twice the price, sometimes more. The obvious question is, at what point is a twin-disc clutch necessary, and when will a single-disc clutch suffice? “Any time you get above the 600hp range, you really should consider a twin-disc clutch,” Bob Scheid of McLeod Racing advises. “For a single-disc to hold more power than that, it must be very aggressive and very tough on the leg. Why put up with that when you can use a twin-disc and get an easier pedal feel and smooth engagement? Twin-discs are no longer that race-only style of clutch. Even the OE’s recognize their superior capabilities.”
“We feel you can use a dual-disc clutch at any power level. The bonus of picking these units initially is that the customer’s combination can grow in power output without having to continually upgrade the clutch every time a performance enhancement is made,” Mike Norcia of RAM Clutches recommends. “Once you get to 500 hp, you are in the zone where your option is going to be using a more aggressively engaging single-disc unit or have the smooth engagement and good driveability of a dual-disc clutch.”
01. In contrast to a single-disc clutch assembly, a twin-disc system adds a second disc and sandwiches a floater plate in between them. This gives both discs a pair of friction surfaces to interface with, thus doubling the surface area. RAM’s Pro Street Dual clutch assemblies hold well over 1,000 hp.
02. Single-disc clutches can hold plenty of power when lined with a full-metallic facing. RAM’s Level 4 Powergrip HD system will hold 650 hp. The tradeoff compared to a twin-disc system is more aggressive engagement.
03. Quarter Master’s four decades of race-bred heritage encompasses active involvement in circle track competition ranging from NASCAR Sprint Cup to the local dirt track. Ideal for autocross and road racing applications, the company’s Optimum-SR single-disc clutches for LS small-blocks are 50 percent lighter than stock and hold up to 700 hp.
04. Available for C5 Corvettes and fifth-gen Camaros, Quarter Master’s racing heritage shines brightly in its stunningly beautiful Optimum-RR clutch system. Its super-small 7.25-inch diameter substantially decreases rotating weight, while triple discs provide prodigious holding power. The pressure plate and floaters are slotted for heat dissipation, and the clutch is fully rebuildable.
05. McLeod’s RXT Street twin-clutch kits include a steel flywheel. While the company’s RST Street twin-clutch kits bolt up to stock flywheels. Organic linings matched with a twin-disc arrangement allow for serious holding power with smooth engagement. Both systems employ a strapped floater to reduce noise.
06. Ceramic friction facing, such as the Miba lining used by McLeod, offer greater holding power than organic linings and better streetability than metallic linings. This makes them very popular in single-disc clutches that will see both street and occasional track use.
07. Centerforce’s DYAD twin-disc clutch systems hold up to 1,300 lb-ft of torque, and are available for most Chevy applications. For ease of installation, the DYAD kit comes pre-assembled from the factory.
08. By using ball bearing fulcrum points on its pressure plates, Centerforce can decrease the overall internal friction that a standard clutch experiences. The resulting decrease in pedal effort allows increasing the release ratio and clamp load. This approach allows tailoring the clamp load and release ratios to the needs of each application.
09. Available with organic and metallic discs, SPEC’s Super Twin clutch kit can hold between 700 and 1,500 lb-ft of torque. The unit is built from a rugged combination of billet aluminum and steel.
10. In the world of clutch friction materials, carbon offers the best properties of both organic and metallic linings. It provides plenty of holding power, resists heat, and yields smooth and progressive engagement.
11. By plumbing RAM’s pedal height adjustment valve between the master and slave cylinders, the clutch engagement point can be adjusted down closer to the floor. Since many hydraulic clutch systems disengage as soon as the pedal is depressed, this valve puts some free play in the pedal travel, much like adjusting the cables in a mechanical release system.