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
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
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
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.
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
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
Have a look at our custom made clutches,
in the chart at the top of this page, to see the best clutch for your