The clutch transmits torque — not horsepower — produced by the engine, at the flywheel. It is not affected by how much torque makes it through the transmission and differential to the tires. The only way to accurately measure engine torque is with an engine dynamometer.
Since very few people are willing to pull their engines out to choose a clutch, either a chassis dyno or a realistic estimate is needed to find your vehicle’s torque output. This figure is usually provided in foot-pounds (represented as ft.-lbs.). However, if the figure is provided in Newton-meters (Nm), multiply by 0.74 to calculate ft.-lbs., as the information in this product guide is based on ft.-lbs.
TORQUE (ft.-lbs.) = 0.74 x TORQUE (Nm)
For those with dyno data, engine torque is normally around 15 percent higher than that measured at the wheels. This is because of losses in the transmission, differential(s), wheels and tires. There are many variables related to the type of dyno being used, atmospheric conditions and the methods of the dyno operator.
For the purpose of clutch selection, multiply the wheel torque number by 1.15 (RWD or FWD) or 1.18 (AWD): ESTIMATED FLYWHEEL TORQUE = MEASURED WHEEL
TORQUE x 1.15 [1.18 for AWD]
If no dyno numbers are available, a reasonable torque figure can be estimated by qualified individuals familiar with your specific vehicle and the modifications it already has. If planning on more engine upgrades in the future, be sure to consider those before ordering. Bolt-on mods (intake, exhaust, cams, ignition and so on) generally increase horsepower with little torque improvement. Adding forced induction (turbo or supercharger) and/or nitrous oxide will substantially increase both torque and horsepower, so it is important to understand what effect mods have on your torque needs and clutch selection.
ACT recognizes every driver and engine setup is unique. There are many factors that influence a well-informed decision, such as engine torque, maximum RPM, available grip (i.e., type of tire grip), driving use (i.e., street, racing, both, etc.), driving style, and drivetrain type (e.g., front-wheel drive, rear-wheel drive, all-wheel drive). Most importantly, ACT provides the torque capacity of every clutch kit offered to ensure the ACT clutch you choose can handle the torque produced by your engine.
This is the formula ACT uses when calculating its clutch torque capacity:
T = N x F x P x R
Defined as:
T = torque capacity in ft.-lbs.
N = number of disc surfaces (usually 2)
F = coefficient of friction
P = lbs. of pressure plate clamp force
R = radius of gyration in feet
If any of these factors are increased, the torque capacity will increase proportionally. There are varying degrees of trade-offs and benefits resulting from the change in design. Coefficients of friction vary slightly from one manufacturer to another. However, the average for a standard organic disc while slipping is 0.25. On a good quality, ceramic ACT race disc, the average friction is at least 0.32.