Clutch: Description and Operation

System Description of Hydraulic Power Clutch

General

To be able to transmit the high engine torque of 540 Nm (398 ft. lb.), a contact force of 14000...15000 N (10000 to 11000 ft. lb.) is required at the clutch pressure plate. This would require a very high pedal force if a conventional clutch operating system were used.

For this reason, the 911 Turbo uses a hydraulic power clutch system.

Compared to the 911 Turbo 3.6 (964), this system allows the clutch pedal force to be reduced by 25%.

Design

Instead of the power steering pump, a tandem pump is fitted. The pressure for power clutch operation is generated in the second circuit of this pump. An accumulator is filled via the pressure limiting valve that limits maximum pressure to 70 bar.

The power unit (clutch slave cylinder) is bolted to the transmission instead of the slave cylinder.

This power unit includes the slave cylinder, booster section and the emergency operating device. The pressure accumulator is also bolted to this unit.





Operation of the fully operative system:

When the clutch pedal is actuated, hydraulic pressure is generated by the master cylinder. Across a power valve (2), this pressure is fed to the slave cylinder (3) that is then actuated in the usual manner. From here, pressure is also supplied across the emergency piston (4) to the poppet valve (5). When a certain pedal pressure is reached, the poppet valve (5) opens and releases high pressure from the accumulator (8) to the power valve (2). The pedal pressure and the resultant counter- pressure from the slave cylinder (3) activate the power valve (2) that now releases the high pressure to the slave cylinder (into the free space of the slave cylinder) and thus boosts clutch operation.

Operation of the faulty system:

If high pressure is not available anymore, the clutch can still be operated but requires a higher pedal force.

To keep the resultant differential of forces as small as possible, a small auxiliary pressure for the slave cylinder (3) is built up across the emergency piston (4). This, however, reduces the pedal release travel by 6 mm. When the clutch pedal is fully depressed, the travel still is sufficient for moving off or shifting or to disengage the gear previously selected before the vehicle is started.

Note: The entire system, i.e. the operating, high pressure and power steering hydraulic systems, is filled with mineral oil fluid (Pentosin CHF 11 S).

Pentosin must never be mixed with other service fluids (brake fluid, ATF) and must never be used to fill other steering systems.