TechTime with Scott M - Transmission Clutches

Porsche has been using clutches to transmit the engine power to the transmission since the beginning. It is my hope that Porsche will continue to offer clutches in their sports car well into the future. To date Porsche is releasing new cars like the 991R only equipped with a manual transmission. So what is the propose of a clutch and why is sought after by many Porsche drivers. For any transmission to shift gears the power from the engine to the transmission must be interrupted. A clutch is basically an off/on switch for power to be fed to the transmission. For this switch to happen there are few vital components. Power in the engine is made with the movements of the pistons. This power is harnessed in the crankshaft in a clockwise rotation. At the end of the crankshaft is bolted a “flywheel”. The flywheel is a very important piece of the clutch assembly. The flat surface of the flywheel is one side of the friction face of the clutch. The flywheel mass also stores energy so the engine does not stall out easy. Modern flywheels are a dual mass design where the face of the flywheel will rotate a small amount to make the clutch engagement smoother. Bolted to the flywheel is the Pressure plate. The pressure plate has a smooth face like the flywheel. The clutch disc floats in-between the face of the pressure plate and the face of the flywheel. The pressure plate is spring loaded so the clutch disc is sandwiched in-between the two faces. In the center of the clutch has splines and is connected to the input shaft of the transmission. So driving down the road pressure plate is clamping the clutch disc and the engine power is going into the transmission and in turn to the wheels on the road. Now when the driver presses down on the clutch pedal the pressure plate face is now moved back a small amount. The clutch disc is no longer touching the faces of the flywheel or the pressure plate and stops spinning inside of the still spinning flywheel and pressure plate. With the clutch disc now not connected to the engine the transfer of power to the transmission is interrupted and the driver can switch gears. One other important piece to the clutch assembly is the release bearing. This bearing spins along with the pressure plate and either pulls or pushes the pressure plate springs to disengage the clamping force on the clutch disc.

Porsche has made some improvements to how the clutch is disengaged. From the beginning to the mid 1980’s when the driver pushed down the clutch pedal it pulled a cable which pulled the release bearing disengaging the clutch. This was a good system IF the clutch cable was adjusted correctly. If the cable was out of adjustment then problems were sure to follow. Next Porsche used a hydraulic system so when the driver pushed down the clutch the master cylinder pushes fluid though a tube that is connected to a slave cylinder which pushes the release bearing arm with either pushes or pulls the pressure plate disengaging the clutch. Porsche got very fancy with the 993 Turbo and designed the clutch slave cylinder to use the power steering hydraulic pressure to make the clutch pedal easy to press in. This design also uses a pressure accumulator which stores hydraulic pressure when the car is turned off so the clutch pedal is easy to press down when starting the car, pretty fancy stuff!

The clutch system is very basic compared to automatic transmissions and is very reliable. The clutch can have problems though, especially when abused. When engaging the clutch the driver must be careful not to over slip the clutch, this wears out the clutch disc. Now the clutch disc along with the pressure plate is a wear item. The clutch disc is usually made fiberglass, silicon dioxide, and some metal. Sometimes, Kevlar, ceramics, and metallic compounds are used. As the disc wears it gets thinner. The thinner the disc gets the more the pressure plate face has to move toward the flywheel to sandwich the clutch disc. At a point the pressure plate can’t clamp the disc and the power from the engine causes the disc to spin inside the pressure plate/flywheel. If you are on the freeway and in 4 gear and jump on the gas and the rpm’s go up but the car doesn’t go any faster it’s a good sign that the clutch is worn out. Another problem that I see at the shop with the 996/997’s is broken pressure plate spring. The spring cracks (most likely due to the driver holding in the clutch at a light) and cannot provide enough clamping force to the disc and it slips. The best way to diagnose this problem is when pressing in the clutch pedal and light pluses can be felt. For drivers who are looking for more performance out of the clutches there are some great options. A light weigh flywheel can really help with acceleration. Some of the later cars (986 and newer) have very heavy flywheels (45+lbs!!!) and cutting the weight down can really help with performance. For drivers who are making more power or are racing a high clamp load pressure plate and race clutch disc is a great idea. The pressure plate has more clamping force so the engine power cannot spin the clutch disc inside the pressure plate/flywheel. A good upgrade for the 996/997 Turbos is a GT2 style slave cylinder and removal of the power steering hydraulic assist. This makes for a much better clutch feel.

For any questions on this article or suggestions for future articles please email me at scott@mcilvainmotors.com