Chapter 10

Operating a manual transmission performance car

Driving a manual performance car can be a challenge if the proper elements do not come together.  Proper gearing in the transmission and rear end, matching flywheel weight, and driving style are essential.  Extreme street cars will sacrifice some driveability to get the necessary holding capacity in a clutch system, and still work effectively at the race track.

Several factors affect the life and durability of the clutch system.  The key word is load; specifically the higher the load on the clutch the more likely it is to slip.

Chassis Dynos

Most people wouldn't think it, but the chassis dyno provides the single largest load you can place on your clutch system.  There is no tire slip during the run and any lugging the engine at all can cause the clutch to slip.  Your dyno time is more strenuous on your clutch system than actual passes at the drag strip.

At the drag strip

Here are a few suggestions for the drag strip that can greatly improve the life of your clutch system. 

On the burnout make sure the tires are wet but not in the water, and as the tires start to grab the pavement push the clutch in.  Do not attempt to extend the burnout toward the tree.  The point that the tires hook puts a heavy load on the clutch, especially in 3rd or 4th gear.  Trying to drive on out will place a tremendous and unnecessary load on the clutch system.

While it sounds so simple, make sure the car is in first gear before you leave the starting line.  Leaving the line in 3rd gear will pretty much wipe out your clutch system.  Checking up to be sure your in 1st gear prior to pre-staging can save your clutch system.

'Hot lapping' can put severe heat into the clutch.  Some events require this, but try to avoid back to back consecutive runs without allowing the clutch time to cool down.  Focus on making quality runs and not quantity.

Hard starts and downshifting

Hard launches on the street are usually not as hard on the clutch since the tires tend to spin, but still put a good load on the clutch.  If you run a sticky tire that will bite on the street, then it is no different than at the race track, the clutch will be loaded harder and wear is going to be accelerated.

Many drivers misunderstand downshifting. The clutch is not a brake.  However, if you shift to a lower gear and let the pedal back out with no throttle, the resulting 'surge' you feel is loading the clutch braking the engine.  This is extremely harmful to your clutch system and will greatly reduce its service life.  Under this condition the clutch center hub is loaded very hard and can cause the drive center to break in extreme situations.  The straps that retain the pressure ring on diaphragm clutches may also buckle under this severe strain.

The proper method is to 'match' engine speeds on a downshift.  To do this, add some throttle and raise the rpm's as you engage the clutch so the engine and driveline speeds are closer matched when the clutch engages.  Avoid sharp downshifting.  Match engine speed when it is appropriate to achieve optimum service life.  The result is a smoother transition, which does not load the clutch and disc so hard.  If you watch road racers using a clutch, you will see they do this all the time.  With some practice you will be able to make the downshift without even feeling it. 

How long should a clutch last?

It is nearly impossible to predict clutch life as everything including  the driver will affect the service life.  In a street vehicle that is raced often, track passes add up quickly.  This will reduce the life of your clutch system for street use.  To optimize service life it is a good idea to over clutch an application.  Using too weak of clutch system will  lead to premature clutch failure, whereas, over clutching your vehicle will allow to handle additional load you may want to place on your clutch system in the future. 

In full race vehicles it is common to see between 75 and 150 passes on a clutch system between rebuilds.  An optimally tuned system might only get 30-40 passes between major servicing.  Remember, to optimize your clutch system for the fastest run, the clutch operates on the very edge of slippage. 

Street driving an aggressive clutch system

While not easy, it is possible to master smooth operation on the street with an aggressive clutch setup.  The foundation is efficient rear and low gearing and adequate flywheel weight so that the minimal amount of slippage is required on takeoff to make a smooth transition.  Experiment with different RPM levels and the amount of 'pedal' you give the clutch on engagement.   Get it engaged as quickly as possible to avoid excessive wear.

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