Many SCCA competition classes allow for the use of aftermarket clutches, either in the form or an OE-style replacement or a multi-plate racing unit – or something in between. Luckily, selecting the right clutch package is simple; it comes down to what you are going to use it for and how much power you intend to make.
A full-floating, multi-disc clutch won’t be a great choice for an autocross car that is driven to events; likewise, an original equipment replacement unit certainly wouldn’t be preferably for a GT car. The trick is to look for a unit that matches your needs and offers the drivability you require – keeping in mind that the performance gains here often come in the form of weight reduction rather than clamping force. “I am looking for the clutch that has the best moment of inertia,” Jesse Prather, multi-time SCCA Runoffs winner and owner of Jesse Prather Motorsports, told us the last time we were clutch shopping. “We’re trying to keep the weight as close to the center of the crankshaft as possible, and that’s why there are 7.25-inch, 5.5-inch, and 4.5-inch clutches. It’s all about getting the least moment of inertia possible so your motor will rev up quicker.”
But while compact, lightweight, multi-plate clutch setups will help your car rev quicker, they can adversely impact drivability. With less inertia, the clutch pedal can become very sensitive and lead to a less drivable car; in extreme cases, small clutches can become a veritable on/off switch.
In addition to the number of clutch plates and diameter of the clutch package, the material of the lining is also important to consider. “Different types of driving will require different clutch attributes such as weight, friction material, and dampening characteristics,” Evan Cline, Technical Coordinator Aftermarket for Exedy, previously explained to us. “Organic friction material has very good drivability characteristics making it great for vehicles that are mostly street driven; however, the friction coefficient and heat resistance are low when compared to other materials. Cerametallic friction material has a high friction coefficient and excellent heat resistance, but the drivability will be reduced due to the higher friction coefficient and lack of cushion marcel drive plate, which is common in organic clutch discs.”
Carbon friction material is very lightweight and results in extremely quick shifting, as the mass on the input shaft is very low. “Carbon friction material also has a very low friction coefficient when cold versus hot,” Cline warned, “which makes this clutch a poor selection for a street-driven vehicle.”
The other factor when it comes to clutch construction is the hub type: Should you go with a sprung or un-sprung clutch disc? Once again, the answer comes down to what you are doing with the car. For dedicated track cars, a little judder may not be a big deal, but if you are commuting in your car between autocrosses or Time Trials, a sprung hub can help alleviate this.
“A sprung hub will damper the torsional vibration from the input shaft and give it a very smooth, engaging feel,” Richard Barsamian, Vice President Sales & Marketing at Advanced Clutch Technology, told us. “This is ideal for street and everyday driving. Un-sprung hubs have reduced inertia for faster shifts and lower stress on synchronizers, providing race-proven performance.”
Clutches are, indeed, a case of buying only what you need. Do your research right and you will end up with a clutch that works well and offers great performance. Buy poorly, however, and you’ll hate every second of your commute.