How to choose a
The selection of the correct clutch for the intended application
is critical to good operation, including the characteristics of
(but not limited to:
- clutch feel
- operating temperature range
- wear characteristics / durability
- clamping force
- break-in period
Quite often, the first impulse is to get "too much" clutch. This
is often a very big mistake, as there will be compromises in some
or all of the operational features listed above.
The first step in identifying what clutch to get is dependent on
the characteristics of the car. Ask these questions:
- How much power does the car make?
- How is it used? Street driving or track use? If for racing,
what kind of racing?
As we are dealing with BMW applications here, let's use an
widely-understood example - the 1992-2004 6-cylinder 3-series. Over
this 12 year period comprising two different chassis, we are still
dealing with virtually the same fundamental engine and transmission
designs. Power ranges from around 190hp to 333hp. Typical bolt-on
modifications bring many of the early models up to the 215hp-260hp
range and later models around 350hp. Extensive modifications
including supercharging or turbocharging bring power to the
350hp-450hp range, with some examples in the 500hp-600hp range. The
typical BMW enthusiast uses the car primarily for sporty street
driving, and the occasional auto-x or track day. The 3-series is
also popular as a heavily-tracked car and dedicated race car. Some
see time spent drag racing, whether occasionally or as dedicated
So it is apparent that we have a broad range of power and use to
Presented in increasing "aggressiveness", here is a brief overview
of clutch compounds, their power handling, and other
(note that hp figures are generalized, pressure plate clamping
force and torque curves are additional factors)
||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.
||Street-driven cars up to 400hp,
auto-x and track use.
||A 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". Has a break-in period of 500-1000
miles during which slippage may occur. Care must be taken during
this period not to overheat from excessive slipping. Material is
uniform yellow/green and may look slightly fuzzy when new.
||Street-driven track cars up to 500hp,
auto-x and heavy track use. Will take hard use, intolerant of abuse
(will overheat and not recover completely). Due to the
unforgiving nature of Kevlar, it is not recommended for street
cars, especially those that see frequent stop-and-go traffic which
will cause surface glazing of the clutch.
||Same material and characteristics as
solid kevlar above, but segmented (blocks or sections missing) for
better heat dissipation. New generation of kevlar offered by
UUC is resistant to glazing and is an excellent choice for
smooth operation in high-powered cars or those equipped with SMG
||Street-driven track cars up to 650hp,
auto-x, and heavy track use.
||Organic material on one side and a
segmented carbon or ceramic material on the other. The idea is that
the organic side will help smooth the engagement, reducing the
shuddering from the segmented side. Engagement is same as organic,
but still with shuddering. Temperature and hp range is identical to
organic. Carbon/ceramic side will wear flywheel or pressure plate
surface faster and will wear out faster in traffic situations.
Material is organic on one side (described above) and segmented or
completely separate pucks (described below) on the other.
||Can be used in same situations as
standard organic. The "hybrid" design appears to be more of a
marketing gimmick rather than an actual performance advantage. Some
brands are poorly designed and wear unevenly due to flexation of
the clutch disk. For
examples of hybrid failure, click here.
||Very high temperature materials,
usually only found on multi-puck disks. Will accommodate 500hp+
Engagement is more abrupt. Will wear flywheel surface faster,
especially in traffic situations. Carbon is slightly more durable
and flywheel-friendly, ceramic has a higher temp range. Multi-puck
design may result in slight shuddering or "stepped" engagement when
used in traffic situations, although many users report completely
acceptable operation. Material is any of several light hues - gray,
||Street/strip applications for
drag-racing and heavy track use cars up to 500hp. Will take very
hard use, suitable for extreme-clamping applications.
||Extremely high temperature material.
Will accommodate 700hp+ Engagement is on or off. Requires special
flywheel surface. Material is metallic gray.
||Strictly for high-horsepower
endurance racing. With correct pressure plate, capable of extremely
high clamping force. Engagement is like a switch, either on or off.
Does not work well when cold. High-durability flywheel surface
required, standard flywheels will be destroyed quickly.
Back to our original question, how to choose? The answer depends on
the answers to the two basic questions asked earlier regarding
intended use of the car and power output. Based on the answers,
this gauge will help the decision:
To repeat the important point, do not buy more
clutch than you need.
A simple organic disk will handle a wide variety of use - including
street use, auto-x, and even true racing. In fact, SCCA ITS racing
rules require a standard OE-spec organic disk. UUC has
tested organic disks to reliably handle up to 475hp in long-term
street use. M3s regularly run through several auto-x seasons
A kevlar disk is a good choice for a heavily-tracked or road-raced
cars, especially with forced induction.
Carbon/ceramic should be left to high-power cars that see lots of
drag racing, or are dedicated track/drag cars.
Sintered iron clutch disks are strictly for endurance racing.
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