This is an exploded view of a complete clutch assembly. From left to right, you have the flywheel, clutch, pressure plate, and throwout bearing with clutch fork. What we didn't show is the pilot bushing that fits inside the crankshaft that supports the nose of the transmission input shaft.
This is the new age of manual transmissions. The new darling of the street set is the T-56 six-speed, and there is a passel of new five-speeds to choose from, most of which sport some kind of overdrive top gear. Automatics have their place, but 21st-century performance machines are increasingly sporting manual transmissions. That means clutches are back in the spotlight.
Just like a high-performance cam or carburetor, it's important to choose the proper clutch and pressure plate that's matched to your application. The good news is there are literally dozens of different clutch assemblies available to choose from. That's also the bad news because that offers plenty of opportunity to choose the wrong one.
Disc material selection changes the coefficient of friction--or how well the facing grabs the flywheel and pressure plate. The organic compounds (left) generally are less aggressive and offer easier engagement. The metallic compounds (right) can withstand more heat and offer greater holding power but are difficult for daily street driving. Many companies like Centerforce and Hays offer discs that compromise by using both materials, one on each side.
Let's take a look at some clutch basics first, and then we can jump into selecting the best clutch and pressure plate assembly for your car. For this story, we'll concentrate on the clutch disc and pressure plate. Flywheels are part of the system, and weight does play a big role. Suffice to say that those trick lightweight aluminum flywheels may seem like a great idea by reducing the mass the engine has to accelerate. But for a typical street car, that reduced mass also makes light acceleration from a dead stop very difficult. We'll save the rest of that story for a later issue.
This is a side view of the spring hub that helps cushion the engagement. The limit pins prevent the hub from rotating too far. If you see flat spots on the limit pins, you've got problems and should replace at least the disc.
Basics
A clutch only has two basic functions--to momentarily disengage the engine from the transmission and to be able to transfer maximum engine torque without slipping. The clutch must also be capable of performing this job for thousands of miles of easy street operation where a small amount of slippage is necessary to make the engagement of the clutch as hassle-free as possible.
To achieve this compromise between performance and street civility, all clutches operate within three basic parameters: The clutch designer must juggle clamp load, coefficient of friction, and surface area to meet his goals. For example, you can have a clutch with a relatively high coefficient of friction (like a sintered iron race clutch), but it will be a nasty, grabby, terrible clutch for street use. You could increase the clamp load, also called static pressure. Static pressure is the amount of base pressure exerted by the pressure plate ring on the clutch disc when the clutch is engaged. The problem with high static load pressure plates is that this also increases the pedal effort, making it a chore to push in the clutch pedal. This takes a toll on linkage parts.
Always have the flywheel machined when installing a new clutch disc. This ensures the disc will perform as it should. Also torque the crank bolts and use thread locking compound rather than star washers. ARP makes great bolts for virtually all applications.
Alternatively, you could use a larger-diameter clutch. This increases the surface area, which will increase holding capacity, especially if the clamp load is the same. For example, a GM 11-inch clutch disc has a roughly 16 percent larger surface area than the smaller GM 10.5-inch assembly. This increased size does work well, but the tradeoff is the larger clutch requires a larger-diameter flywheel that is heavier and is also roughly 1-inch-larger diameter, requiring more power to accelerate. As an example, Centerforce tells us one of its top-of-the-line 11-inch street clutches can connect up to 620 lb-ft of torque, while a 10.5-inch disc is rated around 530 lb-ft of torque capacity.
So the key is to carefully balance these three variables to create a clutch and pressure plate combination that can hold all the power your engine makes while not requiring superhuman strength to push the clutch pedal in at a stoplight. You also want a clutch with the right friction surface that offers a pleasant engagement action each time you use it, and one that will deliver durable performance over a reasonable lifespan. Let's start by looking at the clutch disc.