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Old 05-23-2005, 08:08 PM   #1
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Exclamation Clutch & Flywheel FAQ: Read if you are thinking of buying them!

Clutch FAQ

What is some good background clutch information?
This link is to a Toyota clutch tech article that is applicable for general knowledge.

The first impulse when clutch shopping is to get "too much" clutch. This is often a very big mistake, as there will be compromises in the different types and compositions of clutches.

Clutches hold Torque, not Horsepower:
Most performance enthusiasts relate more to horsepower numbers rather than torque, but clutch capacity is measured in terms of torque. Think in terms of a high rpm 250 HP Honda Civic versus a 250 HP Ford Powerstroke turbo diesel. The truck will need about three times the clutch capacity because the engine produces about three times the torque.

Choosing what’s best for you:
It may be difficult to know what clutch is right for a particular application since there are so many different levels of personal tolerance and many variations in design. Some people can tolerate clutch chatter, or noise, or heavy pedal effort, or shorter clutch life, higher cost, or other trade-offs. But why tolerate unnecessary issues if you don’t have to? Get the clutch that suits your needs.

What are the various clutch materials? Other than unique or specialized compositions, clutches are generally comprised of:

1. Organic
2. Kevlar
3. Ceramic
4. Feramic
5. Carbon (initially invented in 1998 by Alcon Components for the Subaru World Rally team )
6. Sintered Iron

Depending on manufacturer specifications, this list also shows the general order of the amount of force the clutch materials can hold.

Organic: Metal-fiber woven into "organic" (actually CF aramid with other materials), original-equipment style. Known for smooth engagement, long life, broad operating temperature, minimal-to-no break in period. Will take hard use, somewhat intolerant of repeated abuse (will overheat). Will return to almost full operational condition if overheated. Material is dark brown or black with visible metal fibers.

Kevlar: High-durability material more resistant to hard use. Engagement is similar to organic, but may glaze slightly in stop and go traffic, resulting in slippage until worn clean when used hard again. Higher temp range in general, but can be ruined from overheating; will not return to original characteristics if "cooked". Material is uniform yellow/green and may look slightly fuzzy when new.

Ceramic: Very high temperature material. Engagement is more abrupt. Will wear flywheel surface faster, especially in traffic situations. Due to it’s intrinsic properties, ceramic has a very high temperature range. Material is any of several light hues - gray, pink, brown.

Feramic: This unique clutch material is one that incorporates graphite and cindered iron. The result is a friction material that offers good friction coefficient, torque capacity, and smoothness of engagement.

Carbon: Very high temperature material. Engagement is more abrupt. Will wear flywheel surface faster, especially in traffic situations. Slightly more durable and flywheel-friendly compared to other aggressive clutch materials. Material is black.

Sintered Iron: Extremely high temperature material. Engagement is extremely harsh and is generally considered an “on/off switch” both due to it’s characteristics and the clutch types this material is generally associated with. It requires a special flywheel surface. Material is metallic gray in color.

What is a dual friction clutch? A dual friction clutch is when two different friction material facings are applied to each side of the clutch disk. For added performance and service life, Kevlar is added to the pressure plate side of the clutch disk and the other side remains organic. For street and strip, a dual friction disk is often a combo of Kevlar and metal. The one flaw in this logic is that your overall holding power is then limited to the weakest holding material.

Which clutch material is right for my car? This depends on your configuration and the manufacturer's specifications. Each manufacturer has their own "recipe" for each clutch material type so that Manufacturer A's organic clutch material (for example) can be quite different from Manufacturer B's organic clutch material. Many clutch materials can be doped with other materials to provide different characteristics than would be expected of that particular type of clutch material. Changing to a more aggressive clutch material can gain increases of 10% to as high as 60% in the amount of torque they can hold.

As to the rating of clutches, most manufacturers rate their clutches to the point of slip, instead of being able to sustain long term use at specified ratings. Torque ratings are based off of the average torque per crank rotation, if you buy a clutch which is border line with the amount of torque you put out, chances are its going to start slipping sooner than later.

Do I need a sprung or unsprung clutch? Many do not consider this an important issue. A sprung clutch allows it to act similarly to springs on a car. In this fashion, the clutch is “engaged”, slack is taken out of the springs, and then the clutch is fully engaged. The actual amount of travel of these springs in only a few millimeters. The theory is that the springs will dampen the engagement slightly and to soften driveline shock and reduce associated clutch engagement noise. To generalize, sprung clutches are preferred for street use and unsprung clutches are preferred for racing applications.

What are the various clutch construction methods?
a. Full face
b. Segmented*
c. Windowed*
d. Puck (AKA Paddle)
e. Multi Disc (AKA Multi Plate)

*These two construction methods represent the same reasoning and methodology with two different construction techniques depending on the manufacturer.

Full faced: Full round facing of material or full face with slight separations similar to sprung and unsprung clutch photo shown above.
Segmented: Generally associated with Kevlar clutches. The missing sections are designed for greater heat dissipation.
Windowed: Generally associated with Kevlar clutches. The missing sections are designed for greater heat dissipation.
Puck (AKA Paddle): Generally associated with higher power vehicles. Careful consideration should be given prior to using this type of clutch.
Multi Disc (AKA Multi Plate): Generally associated with higher power vehicles. Careful consideration should be given prior to using this type of clutch.

What causes increased clutch pedal pressure? Pressure plate clamp force. Just because you buy an aftermarket clutch does not mean you have to have a heavy clutch pedal. The amount of increase over the OEM clutch pedal pressure is dependent upon what the pressure plate manufacturer's specifications are.

Are there any drawbacks to high clamping pressure plates? Generally speaking, the higher clamping pressure of the pressure plate, the higher pressure you induce on your crankshaft thrust bearings.

Can I use XXX's pressure plate on YYY's clutch? No. All manufacturers plainly state to use the same brand components, so if you go with YYY, use all YYY components.

What is the most transmission friendly clutch? The OEM organic clutch is by far the easiest on your transmission. This is due to the lighter clamp loads of the OEM pressure plate and the organic clutch material. Organic materials, which are bound by resins, will almost always loose friction when they get very hot. This is because the resin melts and becomes almost like a lubricant rather than a bonding agent. Any increase in clamp load or clutch material coefficient of friction will increase the shock load to your gears.

Can I use a 6MT clutch in my 5MT? Yes, but it requires some modification to do so. this link provides detail on how to do so.

Can I use a 5MT clutch in my 6MT? Yes, but you do have to use the 6MT clutch fork.

So this gives me carte blanche to use either clutch willy-nilly then right? It means you CAN use them in either application, it doesn't mean you SHOULD.

How much torque will the OEM 5MT and 6MT clutches hold? Though no one knows for sure, common knowledge say that the 5MT will hold 300 TQ and the 6 MT will hold 400 TQ.

What clutch will hold the most power? Generally speaking, one that has:
a. Highly sprung pressure plate. The more clamping force the pressure plate exerts, the better it will grip.
b. High coefficient of friction clutch material. The higher the coefficient of friction, the better it will grip.
c. Increase the amount of surfaces. This can be accomplished by going to a twin or triple disc design.

What about a "Stage 1 clutch"? Stage 1, 2, 3, etc. clutches are just a marketing tool like Stage 1, 2, 3, etc. power packages for Subarus. Some manufacturers have them, some don't. While a staged clutch may suit your application, DO NOT use a staged clutch package as a methodology to purchase your clutch based on your staged Subaru.

So since I need to buy a clutch based on torque is there a list somewhere? Some asshat put together this list so you can make an idiot proof decision.

Where do I buy a clutch? Every Subaru/Import performance store sells them. For purchasing, support your local economy or the NASIOC Vendors.

How hard is it to install a clutch? Allow around five hours for install time. Professional installation, depending on your area, is around $300. This is one vehicle modification that should be farmed out to a professional unless you have the right tools/equipment and are mechanically skilled.

How do I install a clutch? Refer to the clutch manufacturer's instructions. For clutches without instructions, below are links to some of the better known clutch installation instructions:

Clutch replacement instructions
RS clutch instructions
OBS clutch instructions Clutch replacement instructions
Helpful link
Helpful link
Various SOA clutch/flywheel install instructions
5MB .pdf instructions

Are there any other important things to remember when replacing the clutch myself? Yes, unless using a new flywheel, the old flywheel must be resurfaced on a blanchard grinder or by a professional shop who uses that or other specific type of machinery for resurfacing. DO NOT use emery cloth, a Dremel tool, a regular lathe, or other shortcut. While flywheels work similar in fashion to brake rotors, you cannot get away with the unusual techniques some people use with brake rotors during a brake pad change.

Is there a bed in period for a new clutch? As with new brake pads, there is a bed-in period with clutches. This varies between manufacturers and is generally a period of less than 1000 miles. For more information as to how long and how to properly bed in your clutch, refer to your clutch manufacturer’s recommendations.
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Last edited by Unabomber; 09-28-2009 at 09:32 PM.
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