So far, all SL-Cs have used manual transmissions, which means they have some sort of conventional clutch. This page discusses some of the options and issues in general. For more detailed info about the right clutch solution for specific transaxles, see the transaxles page.
Single vs. Multi-Disc Clutches
Most cars come with single-disc clutches for reasons of cost. For very high-torque applications, or those with a requirement to have small-diameter clutches for packaging reasons, clutches with multiple discs are often specified. An additional benefit of multi-disc clutches is the ability to have very large friction area in a relatively small diameter, which usually translates to an ability to rev slightly faster. Alternatively, you could also double or triple the friction area with the same diameter when using multiple discs, allowing much more torque to be applied, usually with less pedal pressure than would be needed with the same clamping force in a single-disc application.
Some high performance cars like the 2005-6 Ford GT come factory-equipped with multi-disc clutches, and these are used on SL-Cs with the very strong Ricardo gearbox. If you are using a Porsche gearbox, you may find that a multi-disc clutch provides more clamping force (which allows you to dump more torque into the rest of the driveline, assuming the clutch is the limiting factor), and a lower rotational weight (as these usually come packaged in small-diameter setups). Here's an example unit for the Ford GT from Stillen:
It's a little hard to tell, but there are two discs in that assembly.
There is another explanation of single and multi-discs clutches at the Mantic site here.
Clutch Limit Stops
Certain transaxles, particularly the Porsche and Mendeola variants, are very sensitive to clutch arm overtravel. The symptoms of this can be as varied as crank walk (in this case, the crank moving forward and backward too much), to graunching noises when selecting gears, to excessive clutch wear, and occasionally cracked clutch arms at the slave cylinder. Apparently even cracked bellhousings do occur.
What happens is that the driver pushes the clutch pedal all the way in, which in an improperly adjusted state, puts too much force on the clutch arm and cracks or breaks it, or pushes the clutch in too hard so downstream things (cranks, flywheel, etc) causing other damage.
While some transaxles are more sensitive than others, every SL-C builder should carefully adjust the clutch limit stops on their SL-C. This is a very important thing to do, and shouldn't be skipped.
Fortunately, the solution is easy. The factory ships an excellent pedal system from Tilton that has built-in clutch pedal adjustment screws. These are very easy to adjust to get just the right engagement. To get the clutch adjusted properly, do the following:
1. With the car in neutral, the clutch OUT, and the engine off, have a buddy gently rock the car forward and backward while at the same time you are putting slight pressure on the gear lever to go into first gear.
2. Once it pops into gear, have your buddy rock the car again while you SLOWLY push the clutch pedal in. When the car starts to move, keep pushing the clutch in for another 1/4-3/8" after the car moves freely.
3. Keep holding the pedal exactly where it ended up at the end of step 2, and have your buddy mark the position on the pedal cluster.
4. Adjust the pedal limit screws on the Tilton pedal assembly to limit the pedal travel to the position you've found is 1/4-3/8" beyond where the car moves freely with it pushed in.
5. Test the position by having your buddy push the car again while you push the pedal all the way in. The car should move freely just before you reach the end of the pedal travel.
In addition to preventing engine or transaxle damage, this also allows you to shift faster as your foot only needs to move enough to disengage the clutch and no more.
Clutch parts are usually available from two main sources: the manufacturer of the original clutch for the transaxle you have, or, in certain lucky cases, the aftermarket. A little Google searching will turn up providers for many popular cars, but Tilton, Kennedy Engineering, South Bend Clutch and Clutch Masters are popular places to start.
For the Ford GT/Ricardo transaxle, there is an uprated clutch available from Stillen that is based on AP Racing internals and engineering. Make sure you have the updated flywheel from Superlite if you use the aftermarket Stillen clutch, or rapid wear can result.
Problems and Solutions
Soft Clutch Pedal
As in most areas of the car, heat is the enemy. For clutches, there isn't much you can do about heat buildup on the disc(s) and flywheel beyond managing the facing materials (and avoiding excessive slippage), but there is often a mis-diagnosed problem with occasional soft pedal.
If your clutch gets a soft pedal (meaning, it goes to the floor without the release action expected) that returns to normal operation when the car has cooled off, the problem is almost certainly heat-related, specifically fluid that got too hot. The fluid then tends to boil, which creates bubbles, which are compressible, unlike normal brake or clutch fluid. Thus, the pedal goes soft, and doesn't disengage the clutch until the fluid cools, and the bubbles are re-absorbed back into the fluid.
There is a short-term and a long-term solution to this problem: The short-term solution is to completely re-bleed the clutch, and replace the old fluid with new. This gets rid of the old fluid, and replaces it with new fluid that has the specified wet boiling point.
The long-term solution is to prevent the problem from occurring in the first place, by keeping heat away from the clutch hydraulic lines. This is a problem that tends to plague many modern mid-engined cars, as there is usually tremendous heat generated by nearby exhaust components, the engine, and even the transaxle itself. Typically, the solution is to either plan to keep exhaust pipes far from the clutch lines (Obviously, this has to be done in advance of knowing there is a problem) or by wrapping all parts of the clutch line in a suitable insulating material. Here's an example of the way that Ford managed their clutch line from heat soak: this is the stock corrugated aluminum cover over the flex line leading to the clutch slave cylinder:
There are many kinds of insulation material available; the usual suspects are DEI, Taylor and Aeroquip. The best choice is probably the kind that has a reflective coating, as much of the heat seen here is the radiant type, and the reflective coating tend to resist that best. Some builders have used a hybrid approach and covered sensitive hose, tubing and cables in a rubber-coated fiberglass sleeve (like the Aeroquip fire sleeve), inside a reflective sleeve (like the Taylor one in the above link).
Black Clutch Fluid
Often the clutch fluid will look black after a period of use. There are several possible causes for this. One is that you may have used EPDM or rubber-based hoses, which often leach the color from the hoses as debris. This is bad as the debris can clog small passages in the system, and cause more frequent rebuilds as well as erratic clutch (or brake) operation. The simple solution is to use only teflon-lined flex hoses for clutch (and brake) lines.
Another possible problem is that dust from the clutch may have passed through the seals in the slave cylinder, and migrated back up to the reservoir. In this case, assuming the seals are really up to spec, all you can do is keep changing the fluid. For the Corvette, GM actually changed the color of the clutch fluid reservoir to black instead of milky white to obscure the color changes in the fluid. In the US, DOT actually requires brake fluid to be amber colored, so you can easily check to see how your fluid color changes over time.