C5 Corvette powertrains were designed remarkably well as a system to reliably handle the performance of a 350-400 hp sports car.
When you push the envelope much past that, you begin adding extra stress to driveline components, and invariably begin discovering the weakest links. In a C5 with a manual gearbox, the component that normally takes the most abuse (both from the driver and from the engine), and thus is often the first to go, is the clutch.
When the clutch pedal is pressed, hydraulic fluid is forced into the actuator cylinder mounted to the end of the driveline support assembly, pushing a piston in the actuator cylinder forward and forcing the release bearing against the center of the clutch diaphragm spring to disengage the clutch pressure plate from the clutch disc. As the middle of the diaphragm spring is pushed in, a series of pins near the outside of it cause the spring to pull the pressure plate away from the clutch disc, releasing the clutch from the spinning flywheel. Springs in the clutch plate help isolate the transmission from the shock of the clutch engaging. When your foot is off the pedal, the diaphragm spring pushes the pressure plate against the clutch disc, which in turn presses against the flywheel. This locks the engine to the transmission input shaft, causing them to spin at the same speed. The amount of force the clutch can hold depends on the friction between the clutch plate and the flywheel, and how much force the diaphragm spring puts on the pressure plate.
As soon as we put an intercooled Magna-Charger on Tabula Rossa, our '99 Hardtop, we began to smoke the stock clutch. It already had 65K miles of wear, and before we even finished dyno-tuning the supercharged LS1, the massive amounts of low-rpm torque were enough to overwhelm the friction force the stock clutch was capable of, causing it to begin slipping. In the very first hard launch we made with the blower (and the only one we dared make before replacing the clutch), we smoked the tires and the clutch for about 100 feet!
We consulted several well-respected Corvette builders and racers, including Dick Guldstrand (Guldstrand Motorsports in Burbank, California) and Lou Gigliotti (LG Motorsports in Wylie, Texas), who unanimously suggested an Exedy clutch. For overall use as a daily driver, they recommended an Exedy Hyper Single Disc clutch, which is rated for moderately ludicrous performance of up to 500 hp.
Taking their advice, we contacted Daikin Clutch, the U.S. subsidiary of Exedy (a Japanese corporation). Based on our current and proposed power figures, the Exedy/Daikin engineers recommended that we step up to a Stage 4 Twin Disc clutch, designed for high-powered, hot street action as well as drag, road, and rally racing. The twin plate clutches are rated for above 500 hp to 750 hp, while Exedy offers triple disc clutches for up to and over 1,000 hp. A multi-disc clutch is somewhat heavier, grabbier, and chatters more than a single disc, but it should be nearly indestructible within our performance goals, even when used hard on road courses or drag racing. At a certain point of radical performance, some sacrifices in drivability must be made for durability, and we seem to have reached that point.
Unlike most high-performance aftermarket clutches, which are beefed up and reinforced OEM units, the Exedy clutches are custom-built from scratch. The Exedy Twin Disc clutch, PN GT01SD, is a beautifully machined unit that comes complete with a lightweight, chrome-moly steel flywheel, purple anodized forged aluminum clutch cover, and T5001 cerametallic friction materials for extreme heat resistance. We opted to replace the original pilot and throw-out/release bearings while we were at it, so we ordered a new, OE pilot bearing and clutch actuator cylinder (the only way a new OEM C5 release bearing is available) through Mid America Motorworks. These OE replacement parts are available through your local Chevy dealership, but at substantially higher prices. A C5 clutch replacement generally takes approximately 6-8 hours because the entire rearend and driveline back of the engine must be removed, all of which John Best at West Coast Corvettes took care of for VETTE.