Ford Clutch Guide - Coming Through In The Clutch

In-between the organics and bronze metallics, you'll find Kevlar facings. With a greater friction coefficient than organics but a lower level of abrasiveness compared to metallics, it's not a bad choice for a high-performance street application. Like the others, however, excessive slip and the resultant heat will kill the disc in short order, so it's not always the best choice for all-out race applications.

At the top of the heap are the sintered-iron disc facings. Unlike all the previous facings, the sintered-iron facing material does not significantly lose friction coefficient at high temperatures, so it can withstand major amounts of slip and heat. And like flywheels, the sintered-iron discs can be refaced when worn. Pay attention to disc thickness, though, since it will affect base clamping pressure.

Of course, as always, there's a price to pay for the advantages of the sintered iron. Besides the wear issues on the pressure plate and flywheel, the sintered-iron discs are heavy. Hence, they do not always shift well at high rpm with synchronized transmissions. Drag racers using sintered-iron discs must either have clutchless transmissions (i.e., shifting occurs without releasing the clutch), or have trans-missions specifically designed for quick shifting with heavy discs, such as face-tooth engage-ment systems.

Application and TuningAs with just about everything else, you'll be rewarded with the best performance and driveability if you choose the "right" clutch for your application. If you get it right the first time, you can also save a bunch of money and anguish.

For street driving, it's all about being smooth, affordable, and low-maintenance, but still having enough holding capacity to ensure the clutch does not slip when maximum power is required (without having a monumental pedal effort). Organic discs and diaphragm pressure plates are your best bet. To get more capacity without higher pedal effort, use centrifugal-boost pressure plates to reduce base pressures. For crazy torque levels, consider a dual-disc clutch, where two clutch discs are used between the flywheel and pressure plate (separated by a "floating" plate indexed to the pressure plate cover bolts). This effectively doubles the torque capacity while still using the same clamp pressure. Such a clutch is standard in the '07 Shelby GT500.

In addition to capacity, size and inertia are key requirements for road race, stock car, or autocross applications. Small-diameter clutch assemblies allow you to set the engine/trans lower in the chassis, which gets the center of mass down low for better handling. They also allow the engine to rev up quicker since less mass needs to be accelerated. While quick shifts are nice, they're not as critical in road racing as for drag racing, so the inertia of the disc itself isn't as important as the inertia of the entire flywheel/clutch/pressure plate assembly. Keeping the rotating inertia to a minimum allows greater acceleration potential, so small-diameter, multi-disc setups made from whatever exotic lightweight material your bank account supports are ideal.

For drag-race applications, it gets more complicated. For synchronized transmissions, low-disc inertia is important for quick, clean shifts. For face-tooth or clutchless transmissions, greater disc inertia can be tolerated. For heavy cars without much power, you need plenty of flywheel to get the car off the line, so low pressure plate inertia is not important (since it combines with the flywheel inertia), and vice versa for light, powerful cars. To save inertia, sprung hubs can be dumped if your driveline is up to the abuse or if your clamp pressure is tuned to give a softer initial engagement. Marcel spring discs are all wrong if you want quick, clean shifts. Without the disc compression from the marcel, clutch pedal travel can be reduced for even quicker shifting.

Managing clutch slip in drag racing is critically important. Now, don't simply think that clutch slip won't be a problem if you choose a stiff enough clutch. Slip is something any clutch has to deal with in-between being engaged and released, but for drag racing, it gets much worse. Think about what happens when you dump the clutch at 6,000 rpm on the last yellow. At the instant you engage the clutch, you have a flywheel and pressure plate spinning at 6,000 rpm, but a clutch disc at zero rpm. If the clutch were able to lock up with no slip (besides the fact that the forces and torques generated would be infinite), you'd instantly blow the tires off (not a good scenario unless you're into the ballet of drifting). Realistically, to get the best launch possible, you want the clutch to slip and not the tires. Ever wonder how those Top Fuel cars can run without transmissions? It's all clutch slip, but there's a fine line between toasting the clutch and roasting the tires. See why the slip characteristics are so important?

Managing the rate of slip is everything for the "slider clutches" used in clutchless or transmissionless drag race applications (and sintered-iron discs are mandatory). But for us regular folk who have to shift gears, overkill on the clutch is almost as bad as not enough clutch. Too much clutch can not only spin the tires on launch, but can also spin the tires on the shifts if it hits too hard on engagement. Poor e.t.'s resulting from tire slip can be almost as embarrassing as smoking your inadequate clutch.

For drag racing, when the clutch is on the verge of slipping at maximum torque, usually right after shifting into high gear, it has reached the ideal clutch capacity. Tunable clutches are there-fore mandatory, so we look to the Long-style pressure plate for serious drag action.

Previously, we covered the tuning advantages of Long-style pressure plates, both for base pressure and centrifugal assist. If you have to use the clutch to shift at high rpm, centrifugal boost is a bad thing, since all that extra clamp force kicking in at high rpm makes the clutch engagement too harsh after the shift, and tire spin usually results (not to mention you'll have to push the clutch pedal against all that extra force). Using little, if any, centrifugal assist and tuning the base pressure is the better bet. You may be surprised by the low base pressure many dedicated drag cars run when combined with aggressive discs. Less than 1,000 pounds is typical.