Courtesy the Manufacturers
July 18, 2011

Horsepower doesn't really mean much if you can't put it to the ground. There are lots of key components between your flywheel and your tires that can either help get your vehicle moving, or cause lots of chaos and get you nowhere fast. One of those key components is your clutch kit, and the point of this article is to familiarize you with what is available and what fits your particular application the best. For you guys who are currently spinning torque converters instead of pressure plates, please stick around. I plan to talk about some cool stuff that you might find helpful for a project a little further down the road.

Your average clutch kit is composed of just a few major components: a pressure plate that provides clamping force, a disc that provides a friction surface, and a release bearing which provides a method of engagement and disengagement. All clutches are just a variation of that simple group of parts. Sure, you may have a kit with two or three discs, but it's all based around that same general idea.

The manipulation and combination of each component is what gives a clutch its own unique personality. You can get to the same end result by taking several different routes, and that end result is the sole fact that you need enough clutch to fully transmit (when you so desire) the power that your engine is making to the transmission. Since the clutch's ability to do this is derived from clamping force and friction, then one of those routes is a large amount of clamping force (static pressure) combined with a soft friction material, and the other route is a lower amount of static pressure combined with a disc that has a very high coefficient of friction. There are also routes in between those two and they all give different performance characteristics and driving manners.

Pressure Plates

Since we're all Blue Oval fans here, we're basically looking at two different styles of pressure plates: the long-style (three-finger) and the diaphragm-style (multiple fingers). The long-style pressure plates are most commonly available in 10.5-, 11-, and 11.5-/12-inch diameters. They are the preferred pressure plates for competition because of their use of both static and centrifugal pressure to clamp the disc. Clamping forces range from less than 1,000 pounds to more than 3,000 depending on the application. You will find these in most older vehicles, with the 10.5- and 11-inch plates being used on the small-blocks and some FEs. The 11.5-/12-inch pressure plates were generally reserved for big-blocks and some truck applications.

The diaphragm pressure plates are actually a GM design, but you'll find them in a lot of OEM Ford applications as well as the aftermarket. They can be used in competition as well, but are mainly warranted for lower rpm engines. Clamping forces vary, but the majority of choices stay between 1,600-3,000 pounds. If you want to see a popular example of a Ford diaphragm pressure plate, look inside the bellhousing of basically any mid-'80s and up Mustang. The 10.5-inch diaphragm clutch was very popular for the Fox-body crowd and was changed to an 11-inch metric size in 1999.

Keep in mind that when looking for a clutch kit, a lot has to do with the bolt pattern that's on your flywheel. Some flywheels are only drilled for a certain bolt pattern and most pressure plates don't interchange. When dealing with aftermarket applications, it also helps to know the capacity of your bellhousing.

For example, all Quicktime small-block bells are only designed to accept 157-tooth flywheels. If you do some searching, you'll find that 99 percent of the 157-tooth flywheels out there are only drilled for 10.5-inch diaphragm patterns, with a handful of them having a 10.5-inch long pattern as well. This could pose a problem if the wrong parts are selected. Likewise, a 12-inch clutch in a smaller 390 FE bellhousing could cause some internal clearance issues. The point here is to do your homework before pulling out the plastic.

Discs

Here’s where the water gets a little cloudier in terms of selection. You literally have a gazillion choices (well, maybe not that many) when it comes to disc design and friction material. If you happen to choose the wrong type of disc for your application, you could very well end up with a jacked-up mess, or at the very least, end up wishing that you were driving a slush box. (You guys trying to creep into the Valvoline oil change bay with an unsprung hub, paddle style, sintered metallic clutch know what I’m talking about). Let’s look at the anatomy of a disc itself. This will help you to understand how each part affects the manners of your clutch kit.

Each disc is basically made up of a friction surface and a hub. The hub of course is what slides onto the input shaft of your transmission, so it’s a given that you need that hub to match the diameter and splines of your particular input shaft. Hubs are also offered in sprung and non-sprung versions. The springs surrounding the center of the hub act like a harmonic balancer; in the fact that they absorb some of the shock of the clutch engagement. Most street and street/strip versions are sprung as they reduce NVH and are easier on the drivetrain. A non-sprung, solid hub disc should be used for race applications, as they are business-only, and can be very harsh upon engagement.

Essentially, the rest of the disc is the friction surface. The diameter of this friction surface needs to coincide with the diameter of the pressure plate to get the most effect. There are many types of friction materials used for clutch discs. This includes, but is not limited to, organic, sintered bronze, sintered aluminum, sintered iron, ceramics, and Kevlar. Each material has its own coefficient of friction and thus its own performance rating. You’ll also notice that many of these friction materials are arranged in evenly spaced pads around the circumference of the disc.

The organic material is what you would consider an OEM or stock replacement material. These generally have excellent street manners, not exhibiting any form of clutch chatter after break-in. When you get into the dual-friction discs (organic/higher-friction material) and discs composed entirely of the same high-friction material, then there is always the chance of chatter or less-than-perfect behavior. Lighter cars and lower rearend ratios (3.73, 4.10, 4.30, and so on) tend to help that situation, while heavy cars and higher rearend (3.00, 3.27, 3.50, and so on) ratios tend to aggravate it. However, unless you go the twin-disc route, you may have to face it as a necessary evil.

Since a clutch kit is made up of several different components, it’s a little more difficult to put a ranking system on discs alone, but if we had to come up with an order on the most popular choices, they would be classified in this way in order of increasing grip and decreasing street manners:

  1.  Organic
  2.  Dual-friction (Organic with ceramic, sintered metal, or Kevlar)
  3.  Full ceramic/metallic discs (i.e. both sides sintered iron, sintered bronze, and so on)
  4.  Paddle-style, non-sprung hub, sintered metallic/ceramic faces

So Which Clutch Do I Need?

The majority of drivetrain-related emails that we get revolve around picking out the right clutch for his/her application. However, if you arm yourself with a knowledge of static pressures and disc materials, and then combine that with a mental image of what you’re willing to put up with and what the application is, then you can make a very educated decision on where to spend your money. There are a lot of variables that need to go into clutch selection, including vehicle application (street, street/strip, or competition), vehicle weight, rearend ratio, transmission specs, mechanical or hydraulic clutch actuation, what you’re willing to put up with in terms of street manners and pedal pressure, pressure plate pattern, and physical size—i.e. what your flywheel and bellhousing will accept—and of course, price.

Also, a good rule of thumb for most situations is to pick a clutch that will hold your engine's horsepower, plus 10 percent. If you're packing 500 hp, then a clutch rated for 550 hp will be a good match for you.

Let's take a look at two of those application categories:

Street

OEM replacement and daily driver applications fit very well into this category. It's a little hard to put horsepower limits on street clutch kits, as their ratings depend on several things, but we would ballpark this category at around 350-400 hp. Most stock pressure, organic clutch kits will hold their own for quite a while. Usually, the breaking point is when you start adding a few performance mods or start getting aggressive with sticky tires. You can expect a light to moderate pedal pressure with most of these kits, along with no chatter and a gradual engagement. If you need something with a little more room, the RAM HDX series clutch kits are great for lightly modified vehicles and are generally rated to about 450 hp. They also feature 100- percent-organic discs, but the pedal pressure may be slightly increased.

Street/Strip

This category can cover a lot of ground, as there are dedicated race cars that run the gamut from stock engines to the ridiculously modified. For the scope of this article, we'll propose a horsepower range somewhere between 450-1,000 hp. Broad range, right? There are lots of viable solutions for this type of application as well.

It gets trickier to choose a street/strip-style clutch because when the kind of performance levels that are in this category are reached, you start to lose sight of, or the need to have, perfect street manners and clutch engagement. If you insist on a very light pedal pressure or no chance of chatter at all, then a thorough evaluation of vehicle weight, rearend ratios, and more, has to be completed to see if that can be achieved. If none of this is a concern, then it's best to aim for the horsepower ôplus 10 percentö rule, and make a decision based on flywheel/transmission specs and cost.

There are many different high-quality clutch manufacturers that are available to us including RAM, McLeod, Spec, Fidanza, Hays, and Centerforce. As an example, with the McLeod brand, you can pair one of the company's heavier duty long-style or diaphragm pressure plates with one of its 500-series discs and be covered up to more than 500 hp. Pairing that same pressure plate with a 600-series disc will extend that rating up to 600 hp.

RAM's Powergrip series pairs a long or diaphragm pressure plate with an organic/sintered iron disc to achieve a 550hp rating. Typically, this would be an aggressive combination, but RAM uses a lighter static pressure on the pressure plate, so pedal pressure is very light. Along the same lines, the RAM Powergrip HD series uses the same pressure plate but uses a sintered iron disc to allow up to 650 hp to make it through. This author has personally used this clutch on a 7.50-second, 1?8-mile-class Fox Mustang and it was very streetable without any chatter. However, the car also sported 4.30 rearend gears.

When you start dealing with engines with more than 650 hp and you want to stay on the streetable side of things, then the twin-disc clutches are the way to go. Most of the companies mentioned previously now offer twin-disc clutches. The McLeod RST, for example, is rated for 800 hp and sports a diaphragm pressure plate with twin organic discs. It also offers a very light pedal pressure and very good engagement/disengagement manners. The RXT has dual-friction discs and is rated for 1,000 hp.

Most twin-disc setups utilize a specific flywheel as part of the clutch kit--something to consider when you see the higher prices of the twin-disc setups. These specific flywheels are often much smaller in diameter than stock, and help reduce rotating mass. You can even get some of these combinations with an aluminum flywheel.

Of course, all bets are off when it comes to pure competition clutches. Things like chatter, instant engagement, and high pedal pressure don't mean anything when you're not carrying groceries or passengers. The "mission statement" of it is that you want to engage the clutch as quickly as possible (without shocking the driveline on higher horsepower vehicles), and you want the clutch to hold whatever the engine's dealing out.

This author personally prefers to specify clutches for race applications, rather than high-horsepower street applications as you don't have to worry about the customer coming back a month later with a grimaced face talking about chatter. This may be a subject that we will want to cover in a future tech article, as there are many possibilities of combinations and strategies (sometimes you want the clutch to slip) when it comes to a race clutch.

Clutch Actuation

So what happens when you push your clutch pedal down? The gist of it is that when you shove the pedal in, you want the release bearing to press against the fingers of the pressure plate, which spring it out and away from the disc, allowing the disc to rotate freely and independently of the pressure plate. Your clutch pedal is basically connected to that release bearing by three different possibilities: mechanical actuation (mechanical linkages as used in vintage cars), cable-actuated clutches (such as those used in Fox body, SN-95 Mustangs, and so on), and hydraulic clutches (master cylinder and slave cylinder, or master cylinder and hydraulic release bearing, typical for S197 Mustangs).

Mechanical actuation is the simplest way of actuating a clutch system. If you look on the firewall of a vintage Mustang, for example, you will see that the clutch pedal is connected to a series of linkage rods and a "Z-bar" to push on a clutch fork. The clutch fork pivots on the driver side of the bellhousing and moves toward the rear of the vehicle when the clutch is engaged. The release bearing is clipped onto the clutch fork, so when you move the fork toward the rear of the car, the bearing slides forward on the input shaft bearing retainer and contacts the pressure plate fingers.

It's a very simple system and easy to work on. If your older vehicle is already equipped with this style of clutch, you're probably better off using it. However, in higher-performance applications, the linkage rods, bushings, and mounting attachments can all deflect and wear. Sometimes these components have to be replaced with larger rods, spherical bushings, and Heim joints. However, once upgraded, this setup is a bulletproof, reliable method of engaging your clutch.

Cable actuation is next in terms of mechanical simplicity. In this situation, when you push the clutch pedal down, you pull on a cable (much like a brake cable on a bicycle) that actuates the clutch fork. Since the cable has to "pull," you'll find that this type of setup uses a clutch fork that pivots on a ball stud located on the passenger-side of the input shaft. The driver-side end of the fork moves toward the front of the car, pulling the release bearing into the pressure plate fingers.

Like the mechanical method, this is a very simple and easy setup to work on. There are also lots of aftermarket parts available, including heavier cables, beefier cable quadrants, and firewall adjusters so that you can dial in the pedal travel. There exists the possibility, however, that the cables can weaken and stretch when dealing with higher pressure plate static pressures.

Lastly, we have actuation by hydraulic action. As was mentioned earlier, this can be done by combining the use of a master cylinder with either a slave cylinder or a hydraulic release bearing. When you push the clutch pedal down, you're actually pushing on a piston, which moves fluid and creates a hydraulic action. This hydraulic action can be used to extend/retract a slave cylinder (which is connected to a clutch fork) or it can be used to move a hydraulic release bearing, which slides itself along the input shaft bearing retainer.

One of the pros of using a hydraulic clutch system is that the pedal pressure can be fine-tuned. Since you're essentially using two hydraulic cylinders to actuate your pressure plate, you can change those ratios to give you the desired result. Typically, a large master cylinder pushing on a small slave cylinder will give you a harder/firmer pedal, with a shorter pedal stroke required to actuate the clutch. When you decrease the size of that master cylinder piston, you decrease the amount of pedal pressure required as you're pushing on a smaller piston area.

However, the amount of stroke required to accomplish the same task increases. So, if you have a 3,000-pound, hydraulically actuated pressure plate and you find yourself needing to go to the gym to push the pedal down, try a smaller master cylinder. It can make a huge difference, even going from a 7/8-inch master cylinder bore to a 3/4-inch master cylinder bore. The downside to hydraulic systems is that they're comparatively more expensive (especially where hydraulic release bearings are concerned), they're messier to deal with (the systems rely on brake fluid and the lines must be bled), they require firewall strengthening on older vehicles, and if you ever have a hydraulic release bearing failure, you can count on removing the transmission and possibly the bellhousing to replace it.

In terms of "what will work with what," as long as the components are correctly specified, you can run different styles of clutches on different vehicles. With all of the aftermarket parts available by different manufacturers, you can run a mechanically actuated, diaphragm-style clutch in a '69 Mustang with a 1-3/8-inch Top Loader, or you can run a cable actuated, long-style clutch in a '93 Mustang GT with a T-5.

Again, careful consideration must be given to several different specifications, including where the clutch fork pivot is located, the diameter of the release bearing face, the bore in between the pressure plate fingers, and even the bellhousing that you are using. There are very common combinations to be used with each style of car, but clutch kit components can be mixed if a different route needs to be taken.

Hopefully, we've armed you with a little more information on how to choose your own clutch kit. The task can be daunting at first, but once you get a feel of what you really want to do with the car, what you're able to sacrifice in terms of streetability, and what's available in the aftermarket selection of parts, then you can make a very educated decision on the kit that you need between your engine and transmission.

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