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Clutches - 101

Subaru Clutches 101: What’s Right For Me?

When it comes to replacing clutch on a modified, and even stock Subaru WRX or STI, owner is faced with too many choices and little information on which clutch would be just right for the car. Let’s sum things up and shed some light on the subject of clutch selection to help you select the right clutch for your Subaru’s level of performance.

In brief, a clutch is a link between an engine and a transmission, which can be engaged (clutch pedal released) or disengaged (clutch pedal depressed).

It is always good to upgrade from stock any time you are replacing a clutch. Aftermarket clutches can be stock-like, and give you more clamping power and a longer lasting clutch with stock-like pedal feel/effort and ability to comfortably “slip” the clutch. At the same time aftermarket clutches can be race-only, which would mean clutch chatter, very stiff clutch pedal and extremely harsh engagement not suitable for daily driving.

A clutch should be selected based on your Subaru’s power (torque) levels and your car’s purpose - whether you have a Subaru racecar or a daily driver.

General clutch selection:

Race cars, including track and drag race cars, need to have clutches with quickest engagement, in a race you do not really need to comfortably “slip” the clutch, thus transferring engagement stress to the clutch rather than transmission. Race clutches have quick, harsh engagement with little ability to slip the clutch. Race-type clutches put more stress on the Drivetrain due to their quick and harsh engagement, and should be avoided for non-race applications.

Clutch chatter is a chattering, clunking noise along with jerking that happens when the clutch is partially or fully engaged at lower RPM levels and there is little torque applied.

Drivers who drive their cars daily on highways and around town need the ability to “slip” the clutch to be able to move in stop and go traffic and for overall comfort. Slipping the clutch creates friction that helps synchronize engine and transmission and reduces the stress on the transmission during engagement.

Modified daily driven Subarus, which likely comprise the majority of Subaru WRX/STI cars out there, need the ability to do both “grip” and “slip”. There is no free lunch, however - the more “grip” your clutch has and the more torque it can hold, the less ability to “slip” the clutch is left. And vice versa. Twin- and triple-plate clutches (very expensive) compensate for that somewhat, additionally a compromise can be achieved without sacrificing an arm and a leg by choosing a sprung hub with a puck-style disk.

Clutch disk and pressure plate types:

Clutch consists of 2 different parts: a pressure plate and a clutch disk.

Pressure Plate. First of all, there are few different “types” of pressure plates depending on the engagement/disengagement system. All you need to know is that WRX clutches are “pull” type and RS clutches are “push” type. Then, they classify the pressure plate by the pedal throw length. This is basically how much pedal travel is required to disengage the clutch. Besides the actual number represents the throw length, some pressure plates have special names. For example, a very short pedal throw pressure plates are called “Rocket” clutches or “On-Off” clutches. The pedal travel is minimal and slipping a clutch on purpose becomes pretty much impossible to do. One can drive a car with a rocket clutch very well. It all comes down to practice. Then, all pressure plates are rated based on their springs strength (in lbs or kgs). The “stiffer” the pressure plate, the harder it presses the clutch disk into the flywheel. Higher rated pressure plates can transfer higher horsepower (correction: torque) to the wheels without UNDERPOWER slippage. There is a drawback. The stiffer the springs, the more leg muscle one must have.

Clutch disks.

There are 3 major characteristic of a clutch disk: friction material (organic, carbon, carbon-Kevlar, semi-metallic, metallic), friction area (3 puck, 4 puck, 6 puck, full disk) and its“raciness”. The “raciness” depends on the springs located around the input shaft opening. More springs means that during a fast engagement, the springs absorb some of the initial rotation and prevent the car from jerkiness. “Street” clutches usually have 6 springs while the racing ones have none. The slippy-ness of the clutch disk depends on the friction material. Organic disks offer the most slippage while the metal ones offer very little to no slippage at all. Initial slippage is needed to prevent the car from jerking every time you disengage it. Finally the friction area also determines the slippage-ness of the disk. While metal disks generally tend to be 4 puck and less, the organic ones are usually a full disk. More area = less slip, less area = more slip.

Other types.

There are also few “other” types of clutches such as multi-disk clutches (twin and triple disk) and dampener type clutches. These are the extreme examples of what I have described above.

So now you understand that an unsprung hub4-puck metallic clutch will have extremely harsh engagement with little ability to slip it. An organic, sprung-hub full-disk clutch will have smooth engagement and you will be able to comfortably slip it without having to rev up the engine very high.

Twin-plate and triple-plate clutches, while very expensive, can provide a good compromise between higher clamping force (handling large amounts of torque) and smooth engagement. What those twin and triple plates do is provide an additional clutch disk, which allows the clutch to hold twice amount of torque with the same pedal effort.  Thus you have the smooth engagement and can still hold a lot of torque.

So, which clutch do I buy for my Subaru?

Have a look at our custom made clutches, in the chart at the top of this page, to see the best clutch for your application: http://xcceleration.com/itemdetails.cfm84.htm


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