How to Choose and Install a New Clutch
In The Clutch - Modern Driveline takes the mystery and confusion out of choosing and installing a new clutch
Automotive clutches have been in use for more than a century, including the Mustang for 50 years. When the Mustang was introduced in 1964, manual three-speed and four-speed models used crude three-finger Long Style clutches with six stiff return springs, which meant a hard clutch pedal, excessive wear and tear, and driver fatigue. Over the years, clutch technology has improved significantly to yield a kinder, gentler pedal. New Mustangs are easier to drive thanks to improved clutch design including hydraulic release. For classic Mustangs, options from the aftermarket, including cable and hydraulic release, have made them a lot more enjoyable to drive.
For Mustang driving enjoyment, whether classic or late model, you need to know how to choose the right clutch and release system. We met with Bruce Couture of Modern Driveline for a look at Mustang clutches and release systems.
A clutch is designed to engage and disengage the engine and transmission for start off, shifting, and stopping. The pressure plate applies a powerful spring-loaded clamping force on a friction drive disc that connects the engine's crankshaft and flywheel to the transmission's input shaft and, ultimately, to the driveshaft and drive axle.
Bruce tells us there are basically two types of clutch pressure plate designs—three finger with coil springs (also known as Long Style) and diaphragm. Although the term "Borg & Beck" has also been used through the years, it is little more than a brand name for a Long Style clutch.
Long Style clutches offer excellent clamping pressure, but they are old technology and require a lot of leg muscle, which can make them unpleasant for daily driving. They're also hard on the clutch release mechanism, engine thrust bearings, and crankshaft. Vintage Mustang Z-bar clutch linkages will bend or break if you use a 3,000-pound Long Style racing clutch.
Diaphragm clutches are more common in '74-up Mustangs. Consisting of a Belleville spring diaphragm to load the clutch disc instead of coil springs, a diaphragm clutch offers greatly reduced pedal effort, making it user friendly for the street. Centerforce Clutches are diaphragm-style with flyweights attached to the fingers for comfortable pressure during normal driving but increased pressure at high rpm.
Mustang clutch release mechanisms were cable from '74-'04, then hydraulic release from '05 to current.
Not all clutch pressure plates are created equal, Bruce tells is. When you're shopping for a clutch, you want a ductile iron pressure plate, which is stronger and safer than traditional cast iron. It's what you want if you're going to spin your engine to 7,000 rpm.
Clutch Disc Facts
There's more to a clutch disc than simple engagement and power transfer. A clutch disc is engineered to slip as the driveline and vehicle catch up to engine speed. Examine a clutch disc and you will see how this happens. A clutch disc consists of a hub, a drive plate with friction material on both sides, torsion springs, and drive limit pins. Torsion springs act as shock absorbers during clutch engagement where the hub and drive plate interact with one another with an elastic rebound. If dampening springs are weak, you can expect limited pin contact and clutch chatter.
Another clutch disc dynamic is called Marcel, or "Marceling," which is the amount of disc compression that occurs when the clutch is engaged. Disc compression is the result of the wafer-like Marcel spring action that occurs between the clutch frictions. There is a greater amount of Marcel in an organic clutch disc than in a Kevlar or metallic clutch disc. Choice depends on how you want your clutch to engage. Smooth engagement calls for organic or Kevlar. Quick engagement requires metallic, which performs better in high performance use. Kevlar engages smoothly but applies a firm grip for performance driving. Street clutches with organic or Kevlar friction materials engage more smoothly than those with metallic frictions on both sides.
The type of friction material you use depends on your type of driving and how you want a clutch to hook up. Organic clutch frictions slip and compress more liberally than Kevlar and metallic. One popular concept employs two types of friction material (metal and organic) in segmented pucks on the flywheel side of the clutch disc. Because the pressure plate side of the disc engages first, this is the side more significant to the friction material used.
Metallic clutch disc frictions tolerate heat better than organic and Kevlar. However, metallic clutch frictions connect the flywheel and transmission input shaft more aggressively, which works well in racing but will jar your teeth in street use. Kevlar is a good compromise between organic and metallic because it stands up to heat while yielding the slippage for the street. Kevlar is also good for the weekend racer.
Bruce tells us that a successful clutch installation calls for selection of the right parts and a methodical approach. Take clutch installation step by step and make sure you have everything you're going to need going in.
Because small-block Fords are externally balanced, you need to know which generation of small-block Ford you are working with when ordering a flywheel. Early small-block Fords (221/260/289/302) prior to '82 used a 28-ounce offset balance flywheel. When Ford gave the 5.0L small-block more connecting rod and a one-piece rear main seal for the '82 HO, it had to add more offset weight—totaling 50-ounces—and calling for a 50-ounce offset flywheel and harmonic balancer. If you get this backwards with a 28-ounce flywheel on a 50-ounce engine, or vice-versa, you can count on vibration that will jar the disc jockey's teeth out of your FM stereo.
Clutch Release Systems
Choosing the right clutch release system is as important as choosing the right clutch. There are three basic clutch release choices—mechanical, cable, and hydraulic. Each has its benefits.
Classic Mustangs were factory equipped with a Z-bar style mechanical clutch release, which never worked very well because they were not engineered well for the job. They are hard to operate, especially with a high-performance Long Style clutch with steep spring pressures. If you're going to go with the factory Z-bar system, opt for the Muscle Z-Bar from Modern Driveline, which is engineered to take on the most powerful Long Style clutch out there and operate smoothly thanks to the use of Heim and ball/socket joint technology.