At 09:21 AM 7/19/04 +0200, Sigbert Weinberger wrote:
>"being in the process to modify the clutch of my Twin Cam to an MGB unit I'm wondering how much horsepower such a standard clutch will handle. Thinking it is designed for the 90 hp of an MGB, the Twin Cam engine will hopefully have an output of 110 hp plus. Should I go for an uprated unit?"

Forget about horsepower, and think torque, torque, torque.

Clutch capacity is rated with two numbers, output torque and continuous thermal capacity (capacity to dissipate heat when it slips). If you turn it twice as fast when it is engaged it can handle twice as much horsepower. If you turn the input shaft twice as fast when it is slipping it will generate twice as much heat. So you upgrade a clutch to handle increased output torque, or increased heat dissipation (not necessarily for increased engine power).

Peak torque comes when you engage the clutch with engine running at high speed. At high speed the flywheel has a lot of energy, and that energy has to be dissipated as the engine slows down. The torque will depend on how tight the clutch grabs, and how much resistance you have at the propshaft when the tires start to spin, and which gear ratio is selected in the gearbox. The horsepower throughput will be a product of that torque and the speed of the output side of the clutch. Any excess input power would be dissipated as heat in the clutch when the clutch slips.

When the clutch engages, one of three things happens.

a.) Engine is not running fast enough or with enough torque when you engage the clutch, and it kills the engine. This is not good. The solution is to increase throttle setting and/or engine speed, and also to engage clutch gradually to allow it to slip some until ground speed increases and (maybe) engine speed decreases. When ground speed matches engine speed the clutch stops slipping, and you're driving.

b.) For lower gears, multiply the clutch torque capacity by the gear reduction ratio in the gearbox and rear axle to find the output torque at the rear wheels. If the resulting torque is enough to overcome the friction between the tire and the pavement, the clutch will grab instantly and the tires will spin. In this case torque is limited at the wheels by the grip of the tires. If the engine is running faster to start with, the flywheel will be dropping more energy as it slows down, and the tires will spin a little longer. If the engine can develop enough torque at full throttle to spin the tires in the lower gears, the tires may continue to spin until ground speed increases to match engine speed. Any excess energy not being used to accelerate the car will be dissipated as heat of friction at the tires (clutch not slipping).

In this case if you install larger tires or stickier rubber, the tires may get a better grip to provide more friction and more torque at the wheels (and quicker acceleration). If the tires can still spin, the increased torque accelerates the car faster. When it takes less time for the car to get up to speed, the tires will spin for less time, as more energy goes into acceleration and less energy goes off as heat. So the stickier tires can give you better acceleration and less slippage (less heating and wear in the tires). Through all of this the clutch has no problem at all because it is firmly engaged and not slipping.

c.) For higher gears, when the clutch does not have enough torque to spin the tires, the clutch will slip to dissipate energy as the engine slows down (same as the brakes dumping energy to slow the car). As the car is moving, the power throughput is a function of the clutch torque and the clutch output shaft speed. Any excess power is dissipated as heat from friction on the clutch disk as it slips, and the clutch gets hot. If the clutch is intended to slip continuously (hopefully not in your car) it will have a certain continuous thermal dissipation capacity. As long as that limit is not exceeded it can continue to slip in that fashion indefinitely, until the friction surfaces wear out. When the clutch is dissipating more energy than its rated thermal capacity (all cars do this), then it continues to heat up as long as it is slipping, and the temperature increases until something melts or burns (usually the friction lining). Hopefully your clutch will never slip that much to get that hot.

Here's the rub. If the engine continuous power output from throttle only is greater than the clutch power throughput, then the engine will not slow down, and the clutch will continue to slip until the ground speed matches engine speed. This is not good. If the clutch torque capacity is higher than the engine torque output at full throttle, then the engine will slow down. When the flywheel speed slows to match the speed of the clutch output shaft the clutch stops slipping, as all excess energy has been dissipated. This is a required condition for your car, so you don't continue to slip the clutch and burn it out. In short, the clutch torque capacity has to be somewhat higher than the engine full throttle torque output.