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The Critical Difference

New in Clutch University

What's the best way to drive my manual transmission car?

Chapter 8

Clutch tuning in professional race cars

(click on any image to enlarge)


Professional billet drag racing clutch	An attractive run graph from a Pro Modified car

The most sophisticated drag racing clutch systems allow controlled slippage to get maximum horsepower to the ground with minimum tire spin. These clutch systems are fully adjustable for spring and centrifugal pressure, as well as wear so that the system can be tuned for the car to meet the track conditions.

Adjusting the clutch system requires the use of a data recording computer. Engine, transmission, and driveshaft RPM graph overlays indicate the amount of clutch slippage and tire spin on both the launch and subsequent gear changes that allow us to make proper tuning decisions. Without a data recorder, any tuning decision is merely a guess.

The most important consideration is how the car will react to the clutch settings when the car is launched. On the launch, you want to achieve optimum wheel speed without excessive tire spin or clutch slippage. To accomplish this, the launch RPM needs to be significantly lower than the shift RPM to provide some initial slip while allowing the clutch to gradually applying additional clamp pressure as engine accelerates to the shift point.

TECH TIP - LAUNCH RPM

Since centrifugal assist is RPM sensitive, increasing or decreasing the launch RPM will increase or decrease the clutch pressure at launch without any actual adjustments being made. Additionally, increasing the launch RPM increases the inertia applied to the rear tires from the stored energy of the clutch system.

Once the vehicle leaves the line successfully, the performance of the clutch can be evaluated on subsequent gear changes. If the vehicle tends to have no slip on the gear change, spins, or if the tires shake, reducing the counterweight will provide additional clutch slippage reducing the spin or shake. Without the data recorder, it is extremely difficult to determine exactly what is happening at these points in the run. Examining each successive gear change, we should see the slip time reduced until there is very little clutch slippage in high gear.

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