Clutch & Flywheel FAQ

Clutch FAQ

What is some good background clutch information?

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.

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.

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.

Flywheel FAQ

How does a lightweight flywheel improve performance? A transmission can be thought of as a fulcrum and lever in a car. First gear has a really long lever; second gear has a shorter lever, etc. The lever represents the mechanical advantage that gears give your vehicle. When your car is moving, you have two factors that are present during acceleration, one is driveline losses, which are constant and the variable, which is vehicle weight and the mechanical advantage supplied by each gear. While changing to a lighter flywheel will give the user little to no changes on a dyno, the apparent changes are quite dramatic due to the greater mechanical advantage. Consider these made up figures for consideration: Drive line losses, 45 pounds and vehicle mass (weight) at the driveline (remember your gear’s mechanical advantage reduces your actual car weight). We know that within reason, vehicle mass is a constant. Now imagine if you reduced the driveline loss from 45 to 35 with the use of a lightweight flywheel. Since the engine has less drivetrain losses to compensate for, this means the “gained” horsepower can be applied to moving the vehicle mass. Using mathematics, one can realize that the higher you go up in gears, the less effect that a lightened flywheel will have to the overall equation.

How much will a lightweight flywheel affect my car’s performance?
This Excel document will allow you to find out for your WRX or STi application.

Are there any downsides to a lightweight flywheel? While the performance characteristics of a lightweight flywheel seem to be the perfect solution, there are compromises:
a. Low end performance is affected. This usually means that higher revs are necessary for smooth starts due to the reduced rotational mass. For drag racers, this can be a BIG issue.
b. Possible missfire check engine light.

What causes a missfire CEL with lightweight flywheels? No one is really sure. There are theories though. One theory is that the Crank Position Sensor senses the rotational speed of your crankshaft. Since a lightweight flywheel reduces the rotation mass of your engine, your crankshaft accelerates and decelerates quicker than OEM specifications. Another theory is that there is a missmatch of information from your Crank Position Sensor and the Cam Position Sensor during acceleration and deceleration. This may or may not be caused by the slow reaction of the belt tensioner which will cause enough belt slop to give erroneous readings to the ECU. Since missfire CELs (due to lightweight flywheels) do not occur as often on the STi and the STi has two Cam Position Sensors, this adds further evidence that the missfire CEL has something to do with the Cam Position Sensor since there is only one on the WRX.

Which lightweight flywheel will not throw a missfire CEL? In truth, there is no lightweight flywheel that will not throw a missfire CEL. The odds of you throwing a missfire CEL are higher the lower in flywheel weight you go. Even with a “heavier” lightweight flywheel, you may throw a missfire CEL.

Will the use of a lightweight pulley increase my chances of a missfire CEL with a lightweight flywheel?
Yes. To a smaller degree, lightweight pulleys also decrease rotation mass, adding their quicker acceleration and deceleration into the equation.

How do I fix a missfire CEL with a lightweight flywheel? The safest course of action is to use a portable OBDII code instrument and clear the codes frequently. Once you know the frequency of the missfire CELs, it will aid you in determining if this is an actual code or perhaps something to investigate further. While the missfire CEL code is a “safe code” (just an indicator and won’t throw your car into safe or limp mode), it is never a good idea to drive around with the CEL on for extended periods of time as another, more serious code can be present without your knowledge. Additionally, it is not a wise idea to remove the missfire CELs via engine management software. Missfire codes can be an indication of a problem and removing their presence will remove possible symptoms of an actual problem.

Who is a good or a bad candidate for a lightweight flywheel? Arguably, the users whose driving technique is most highlighted by the benefits of a lightweight flywheel are people who autocross frequently. Many people are also genuinely happy with the performance increase in their daily driven vehicles as well. The only group that is generally dissatisfied are people who competitively drag race as the reduced rotational mass does not lend itself to their severe launch techniques.

Editors Note

My special thanks to the following personnel for their assistance in the formulation and research on this FAQ:

* NASIOC member DrDRum who came to my rescue and made the WONDERFUL Excel chart for lightweight flywheels. I wanted something ghetto, and he created something highly configurable and very pro looking.
* Tom @ Kartboy for giving me hands on experience with a complete clutch assembly and explaining the basics.
* David Koyle at ACT for humoring my questions after I met him at the Rim of the World.
* Andrew Yates for verifying a lot of my information and going over advanced clutch information.

This post was created because I wasn’t able to find a good clutch/flywheel FAQ. I came up with the text based on LOTS of searching here. It was also created to be intentionally brand neutral so that it serves as a stepping stone for further research. Upon reading this you should have an idea of what type of clutch and/or flywheel best suits your needs. The manufacturer is up to you.

If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, “I have XXX’s clutch and it’s great!” or “XXX’s flywheel cracked after 1 month” are not appreciated here, that is what the Car Parts Review Forum is for.

Leave a Reply

You can use these XHTML tags: <a href="" title=""> <abbr title=""> <acronym title=""> <blockquote cite=""> <code> <em> <strong>