May 1, 2007

For many Mustang enthusiasts, there's nothing quite like ripping off the perfect powershift. We understand. Why wouldn't you want to drive with both hands and feet while being an integral part of the acceleration process? Rowing a stick is that much more fun. If it becomes a little routine after a while, you simply add more power, and the gear changing happens faster and likely with more fury. The fun is restored, until the next power upgrade is required. Eventually, the power upgrades become too much for the old clamp to hold, and out comes the death smoke from the clutch department. Too many of us know the smell of a burnt clutch-it's one you can't forget.

So you open the first clutch catalog you find, skip down to the bottom of the race clutches page, and install the full-competition setup in your street car. Now you have a heavy pedal, and every time you let up the pedal, it chatters your lug nuts loose-once again, the fun is gone. For a brief moment you consider installing an automatic trans, but then you wake up.

Maybe, with a little clutch science, your next clutch choice will hold the power without taking all the fun away. Thankfully, MM&FF is here to help.

Clutch BasicsMore than 100 years ago, those Germans who figured out how to adapt a crankshaft into a cannon-thus creating internal-combustion engines-realized the need to decouple the engine from the wheels in early vehicles so the engine could be started without having the car immediately run you over. Karl Benz is generally credited as the inventor of the automotive clutch we love (or hate, if we have the wrong one for the application).

An automotive clutch is a system that consists of a flywheel, a pressure-plate assembly, a clutch disc, and a mechanism to release the clutch (i.e., release bearing, fork, and clutch linkage-be it cable, hydraulic, or mechanical linkage). The engine is decoupled from the transmission when the clutch is released. When the clutch is engaged, the power from the engine directly drives the input shaft in the transmission. Naturally, different applications require different levels of grip for the clutch to retain power, resulting in no slip.

To accomplish no slip, the clutch disc has friction-material facings on both sides and is clamped tightly between a flywheel face and a pressure plate. As part of the pressure-plate assembly, springs are used to develop the force that clamps the disc tightly between the pressure plate and flywheel, thus preventing slip. Both the flywheel and pressure plate are attached to the engine crankshaft, so the torque from the engine is transmitted through the friction faces to the clutch disc. The disc is then splined to the transmission input shaft. When the clutch is released, the clamping force is withdrawn (the pressure plate and flywheel separate a small distance), and the disc is decoupled from the crankshaft rotation.

Here's the confusing part: When the clutch pedal is released, it engages the clutch (the disc is clamped tight and transmits torque), and the clutch is released when you step on the clutch pedal. Hey, we didn't name this stuff.

If the friction coefficient of the facings or the clamping force on the clutch disc is insufficient, the clutch will slip when it's asked to transmit torque, and a lot of engine power gets converted into heat (death smoke and the evil smell soon follow). Historically, clutch torque capacity was typically increased by using greater clamping spring forces, more aggressive friction materials, clutches with larger diameters (the larger the diameter of the friction ring, the greater the torque capacity of the clutch), or multidisc clutch setups (more on this later). With modern technology, we now have additional methods to increase clutch torque capacity, since all four of the previous techniques have downsides.

Pressure PlatesPressure-plate assemblies come in three basic configurations: diaphragm, Borg & Beck, and Long style. In the olden days, GM, Chrysler, and AMC (real olden days) used the B&B style clutches in any performance application, while Ford went with the much better Long-style arrangement. Each had its own specific bolt pattern, so they weren't easily interchangeable. There was, however, an aftermarket hybrid version that combined the desirable features of the Long style with the B&B bolt pattern, appropriately called the B&B/Long style. Today, just about every OEM uses a diaphragm-style pressure plate because it offers several advantages for street applications.

The B&B and Long-style pressure plates both use several coil springs (roughly the size of valvesprings) compressed between the cover and the pressure ring to develop the clamping force to hold the clutch disc firmly to the flywheel. Both use three levers that the release bearing pushes against to release the clutch disc when you step on the pedal. When you push inward on the levers, the levers pivot on the clutch cover to pull the pressure ring away from the clutch disc.

Besides the difference in bolt patterns, the other big difference between the two styles are the levers: The B&B uses wider, stamped-steel levers, while the Long style uses narrower cast or forged levers. Long-style release levers can include provisions for weights that can be attached to the outside of the levers (often called fingers). These weights are used to increase the pressure plate clamping force as rpm rises. This last difference is significant, so we'll expand on it.