Modified Mustangs & Fords
Clutch and Pressure Plate Choices - Clutch Plays
Choosing the right clutch and pressure plate for your modified Mustang or Ford
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.
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.