Engaging Options

Dec 2, 2009

In an automotive world slipping more and more into automatic, the clutch continues to thrive in the performance market. “Manual transmissions are dwindling in the OE market,” Chris Hopf, engineer for Zoom Performance Products in Mooresville, N.C., confirms. “Even diesels now have better automatics that hold power and provide easier towing. But the clutch is definitely not dwindling for the performance market.”

To match the demand, manufacturers must thread the needle of performance, reliability and economy to suit each application, but that can be a challenging balance to achieve.

Clutch Time

The market has shifted away from “The Fast and the Furious” aesthetics and back to performance in the last two years, aided by an abundance of readily available performance parts on the market that can be purchased on the Internet and shipped anywhere in the world.

There’s never been a better time. “Cars are building more power from the factory, but the clutches are marginal,” claims Will Baty, Centerforce Clutches media liaison. “If you add a new exhaust and intake, you’ll drive through the clutch.”

When a car owner adds performance enhancements to a car, it is imperative to upgrade the clutch. To illustrate the point, Baty says adding an exhaust, intake and computer chip to an ’05 Mustang exceeds the factory clutch capacity. Add to that the fact that many performance enthusiasts are aggressive drivers, and it’s easy to see why a factory clutch doesn’t last long. “The clutch is the fuse between the engine and the tranny. The OEs deliberately design it as the weakest part. If you’re tearing up other parts, it’s going to get expensive.”

Types of Clutches

The OE may have designed the clutch as the weak link, but after-market manufacturers are producing far more durable, sturdier versions. Clutches come in a variety of materials, designs and number of discs, which most manufacturers market in stages for diverse applications.

Fidanza begins with a traditional design, single-disc clutch with an upgraded pressure plate for an increase in clamp load. One step above stock is the 2.1 level, made of a carbon-Kevlar blend. “If you add a cold air system and a performance muffler to a stock car, this is the clutch you’d want,” says Jeff Jenkins, sales manager for the Perry, Ohio-based company. “Later, if you added headers, softer compound tires, a computer chip and forced-air induction, you’d want the 3.2.”

The 3.2 is constructed of Kevlar, the 4.3 of ceramic and the 5.4 of sintered iron. It’s probably their most popular model, Jenkins speculates. “It’s forgiving once it’s broken in; it retains its original adhesive quality after overheating.” When new, though, it’s known to chatter on engagement and it doesn’t grab the metal correctly. However, its coefficient of friction is substantial and it can handle a lot of power.

Different performance levels deal with different driving levels, but regardless of the performance level, all Fidanza clutches (except race clutches) feature sprung hubs for smoother engagement. “That flexible piece of metal for shock absorption provides a little cushion,” Jenkins explains. Fidanza does offer a 4.3 level made of ceramic suitable for “a little racing” or an aggressive, performance-oriented driver in a more modified car, and a 5.4 sintered iron for flat-out racers.

Zoom also features a wide range of clutch types for a variety of applications, although Hopf cautions that there are trade-offs for drivability. Levels begin with organic materials – a molded-type composition with fiberglass, commonly used by the OEs – that provide smooth engagement.

Next up is Kevlar, which provides a higher coefficient of friction – the all-so-critical element of good performance. “It’s smooth, but cost is an issue,” Hopf admits. But because it lasts five times longer than a stock clutch, the pay-off resides in durability. The Kevlar clutch doesn’t wear opposing surfaces, but does require a longer break-in time. Carbon mixes have a similar break-in period and do a similar job. Popular with the street and muscle markets, it’s also suitable for a daily driver.

Clutches with a ceramic/metallic facing have an even higher coefficient of friction – double Kevlar and double the torque capacity,” according to Hopf. However, due to the brass or copper element, it is very abrasive and can cause chatter in the drive line. On the positive side, it is stable at different heat ranges, holds power and costs less than Kevlar.

Sintered iron has a similar abrasiveness, but handles heat better – a benefit for all-wheel-drive vehicles that build up heat when the clutch slips. But sintered iron is also more expensive and heavier, which is why Hopf says Zoom uses it in a button configuration rather than a full facing.

Like the others, Centerforce Clutches in Prescott, Arizona, whose market is “everything street,” according to Baty, also offers different levels, or stages. Centerforce 1, a “mild” upgrade over stock, consists of organic materials. Centerforce 2 is basically the same disc with additional centrifugal force weight.

Centerforce Dual-Friction clutch is a performance-matched pressure plate and disc combination. The single-disc clutch features two types of friction materials: full-facing organic material on the pressure plate side enhances drivability, while a carbon composite puc style segmented facing on the flywheel side increases holding capacity. Incorporating a patented centrifugal weight system to concentrate weight in one area increases psi for exceptional holding power and performance that is suitable for street and competition applications.

The Centerforce LMC (light metal clutch) series is a specifically designed low inertia performance clutch that reduces engine rotating mass. Originally designed for circle track applications, it also performs well in road and drag racing because of its light weight and fast release ratio. A dual-segmented carbon composite lining assists cooling and torque capacity. The clutch incorporates aircraft-grade billet aluminum pressure plates and patented ball bearing diaphragm and centrifugal weight system to increase grip.

“Our ball bearing pressure plate design increases the release ratio,” Baty explains. “It increases the static clamp load and decreases pedal load.” By using the ball bearings as a pivot point for the pressure plate diaphragm spring, it reduces the strain on the thrust bearing and increases clutch holding capacity. Baty says the lighter pedal load is popular, but some customers are skeptical. “People think a performance clutch has to be stiff, but ours isn’t.”

While some manufacturers focus on leverage and adjust the pivot point, Lancaster, Calif.-based ACT’s design philosophy involves heat-treating the diaphragm instead. That translates into a minimum clamp load increase of 30-34 percent without decreasing leverage, although it does make the pedal feel stiffer.

Breaking away from the peloton, ACT doesn’t market clutches in stages. “That’s too broad,” says Sampson. “It varies between manufacturers.” Instead, ACT, known for performance clutches and flywheels for domestic and sport compact cars, was one of the first to gauge by torque capacity. He contends it’s easy enough for jobbers and car owners to determine their clutch needs. “It’s not hidden.”

Dual Duel

Some jobbers determine that a twin-disc clutch best meets their needs, although Sampson isn’t convinced. “You can’t ignore the twin-disc,” he says. “There’s a demand for it, although most people don’t need it.” For those who do, ACT manufactures a twin-disc ceramic clutch with carbon-on-steel floaters, the cheapest of which cost $1,300-1,600. A new version with carbon-on-carbon floaters was introduced at SEMA. Not yet on the market, Sampson anticipates a price tag in the $4,000 neighborhood and says they “never slip.”

If that figure has you gasping for air, not to worry, he says. Fueling the debate of single- vs. twin-disc, he maintains that for 80 percent of car owners, a single-disc clutch should suffice. “They hold as much torque as needed. The Evo, a Top-10 best seller, makes 400 wheel hp. Its stock clutch is adequate for the street.” ACT’s basic clutch holds 508 lbs. of torque.

To generate torque, Hopf says, you have to look at how strong the spring is in the clutch, the size of the disc and the coefficiency of the material. Friction is one part, he continues, but there are also other parts involved – especially in a dual-plate clutch. “Most clutches are single-plate, but if you add discs, you increase the holding and torque capacity.”

Zoom’s D2 series uses a flywheel-specific floater assembly and pressure plate. Its 8-7/8″ diameter is smaller than a factory Mustang’s 11″, but despite reducing the size and weight and lowering pedal pressure, it retains a near-stock feel. Hopf admits it’s pricey. The two-disc clutch runs a jobber around $1,000 – vs. $300-500 for a single-disc without a flywheel. However, he points out, “some guys put $4,000-8,000 in their engines.”

Centerforce currently offers only single-disc clutches, but the Arizona company has a dual-disc in development. “The extreme guys like dual-discs,” Baty recognizes. Extreme guys aren’t the only ones who like it. Being able to increase the surface area of the clutch and, hence, its ability to perform, while taking up the same amount of space gives the twin-disc an advantage.

“You should not have a big left leg!” Jenkins jokes. All joking aside, doubling the friction area to allow the clutch to handle the power without losing the ability to put the power down is critical.

Jenkins thinks a streetable twin-disc is “sorely lacking,” but believes that twin-disc applications will grow significantly. “It will become the standard in shifting,” he says, making an unintentional pun, “because it has more advantages due to its ability to produce more horsepower in a small space. It can hold up to 800 hp and still be drivable.” Rattle and noise reduction make the new twin-disc versions more beloved than the older style, he adds.

Push-Pull And WeighingThe Differences

Twin-discs aren’t the only space-saving clutch. Some OEs used pull clutches because they have a lower profile. “You can get more high torque in the same space,” Hopf says. But because the after-market is limited to follow factory applications, push clutches are most often produced.

While explaining that most after-market clutch manufacturers use OEM parts and few switch from a push to a pull clutch, Sampson points out that the Honda S2000, Hyundai Tiburon and Mitsubishi Evolution use a pull clutch. “A pull clutch has a softer pedal and better drivability, but it’s more expensive.”

It’s a market Jenkins expects to address “down the road.” Citing the company history of performance flywheel design and the relatively recent foray into the clutch market, he explains, “Our development program looks at the most popular flywheel applications; we’re at the mercy of the car design.”

While saving space is important, saving weight is too. “Lightening up the rotating assembly is critical,” Jenkins reports. “Reducing the weight of the flywheel can free up 20-30 horsepower already in the car, increasing available horsepower, especially to the lower gears. Some manufacturers over-engineer flywheels to suit a general audience, but testing shows a lightweight flywheel improves performance because the engine revs faster, you can accelerate faster in lower gears and you can decelerate faster.” There is, of course, a fine line: reducing the weight too much results in a negative impact on engine performance. “We look for a balance so the car is still streetable.”

Once again in opposition, Sampson says weight isn’t a big issue because they’re working with OEM parts. “We remove the diaphragm and heat-treat it. It’s a given weight. The weight-saving in a disc depends on the customer’s choice of material. The lighter it is, the less inertia and the faster it shifts.”

Engaging The Customer

The market has shifted from speed shops to online purchasing, Sampson claims, where price is the only difference. Simultaneously, the market has become younger, with more income and less car knowledge. Therefore, it is incumbent upon those selling clutches to determine what the buyer needs.”It’s important to find out about the car, its use and any modifications so we don’t over-sell or under-sell,” Baty advises. “You can save money if you install the right clutch the first time.”

To choose the right clutch the first time, Baty says it’s imperative to know the reason for changing the clutch. If the reason is due to clutch failure, Centerforce Clutches wants to know what the failure was. To assist the car owner in deducing the cause, an online FAQ section incorporates pictures to help diagnose problems that lead to clutch failure.

When selecting a clutch, it’s important to match parts to the application. For example, Baty notes that the 1996-2002 Mustang’s stock flywheel is made of gray-cast iron, which is brittle. “It’s not intended for performance use; it comes apart. The new bell housing is aluminum; it won’t retain an explosion.”

Jenkins says it’s easy to match a car with a clutch on Fidanza’s Web site. Although most purchasing is done online with little expert guidance, because there’s “so much technical data people don’t understand,” Fidanza employees are available to walk customers through the buying process. “Enthusiasts like to talk about their cars. That gives us an opportunity to learn what they need. Getting the right clutch is important. Some car owners put too much clutch in a car. That puts the weak link in the drive line. If you have too much traction, you can break U joints and CV joints.”

Sampson guides customers, educating them that “you buy a clutch for torque, not horsepower. You have to determine how much torque the car is making, and if it’s at the wheels or the crank. You have to find out what modifications have been made to the car, and you have to know the purpose: is it a street car or a track car? Guys like to dream about building a race car, but they don’t drive well on the street. An entry-level clutch has the best drivability and engagement for a street car, but there are always trade-offs.”

No matter what type of clutch a customer chooses, Baty warns them to pay attention to the details. “The equipment is only as good as the bolts. You need the proper bolts and torque specs. It’s a safety issue; there’s a lot of force spinning at your feet.”

Shifting Into The Future

Because the after-market clutch industry is driven by the OEM, Sampson looks at trade magazines and follows the hype when considering design applications. “The WRX, Mustang and Evo were highly anticipated, so we knew those would be big sellers.” The Dodge SRT-4, on the other hand, was a limited product. However, the clutch in this powerful car was poorly designed and didn’t make power, so ACT rightly expected an audience for after-market clutches and designed appropriately.

“You have to listen to customer demand and pay attention to trends,” Sampson continues – even if they’re foreign trends. ACT designed a flywheel and clutch assembly for a Japanese car never sold in this country. Also, although most diesels are automatic, he says, Dodge Cummins are often manuals, providing another market.

Hopf says Zoom is “doing constant R&D to stay ahead of the curve.” Although their bread and butter is in street cars, with a focus on domestic performance cars, sport compacts and diesel pickups, Zoom pulls a lot of ideas from the racing industry, specifically, NASCAR. Looking further afield, Hopf mentions new multi-disc button clutches (5-1/2″ and 7-1/4″) for circle track racing and says the company is getting into grassroots circle track racing with a lightweight single-disc clutch.

Zoom also plans to launch a new ring and pinion program this fall. Meanwhile, Hopf says they’re busy developing clutches for new models because there’s a lot of interest in new cars like the GTO, Mustang and Corvette. “After-market parts houses are providing a good supply for domestic muscle cars. That facilitates the need for clutches.”

Fidanza is trying to fill the need for a clutch that will handle 600-900 hp and still feel like stock. “Twenty years ago, a V8 had to work hard to get 300 hp, but a Mitsubishi Evolution cranks 700 hp now,” Jenkins says.

Manufacturers have to look at the ability and the requirements of all vehicles. Jenkins says Fidanza looks at new technology and ideas for more applications all the time. “Our R&D is ongoing.

“The market never ceases to amaze me,” Jenkins marvels. “It grows and grows. Striving for a better wheel has driven men since the wheel was invented. It’s an exciting industry. There’s always something new to address.”