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When
we rate a clutch kit for either severe duty or performance use, we use
engine torque as our defining parameter. Torque is the "twisting"
force exerted on the drivetrain that gets you up and out. It is the force
(lbs.) multiplied by the distance (ft.) (normally expressed in lbs.-ft.).
Imagine using a big wrench to tighten a nut to a certain torque value.
One pound at one foot is 1 lb.ft. Three hundred pounds at two feet is
600 lbs.-ft. The more force or the greater distance increases the torque
value. Horsepower is the force that keeps you moving. One horsepower equals
a horse pulling 180 pounds 181 feet in one minute. Today it is usually
expressed as 550 pounds for one foot in one second. It will keep you moving
until the engine redlines or the rev-limiter kicks in.
A Dodge Ram 3500 Cummins 5.9L Diesel can exert 610 lbs.-ft. of torque
at the engine and only generate 325 hp while a BMW M6 generates 383 lb.-ft.
and 500 hp. In both cases, we concentrate on the torque output of the
engine because it is the twisting force that causes a clutch to slip.
All of our ratings are done in accordance with the OE practice of allowing
for a 20% safety factor for occasional extraordinary demands on the clutch
package. This may be an overloaded vehicle, a heavy trailer, steep hills,
tire size change, improved exhaust, or a mild tuning modification. We
would prefer to have the clutch kit step up and do its job in the time
of need instead of saying "Adios, amigo. Hasta luego".
There are clutch suppliers that state their clutch kit capacity without
the 20% margin to give the appearance of a heavier duty clutch while in
fact it has the same clamp load and mating components as ours. In fact,
they would take a cheap import clutch and call it heavy duty because they
rate it without any safety margin.
We also rate our clutch kits based on engine torque capacity because the
numbers are readily available either through measurement directly or from
credible published sources. Torque ratings at the wheel need to take into
account parasitic drivetrain loss which has been estimated to range between
15% and 25% of the engine torque rating. There are a lot of discussions
on how to determine this loss and on whether it remains constant (i.e.
always 50 lbs.-ft.) or declines as a percentage when a motor is upgraded.
We have decided to stay with the engine torque number because it is more
readily determined and directly affects the choice of the clutch kit.
Other factors that we consider are how things like Power Density affect
the ability of the chosen friction materials to handle the BTU output
of the slipping action generated during the engagement cycle. The failure
of the material to handle the power within its optimum temperature range
can cause excessive heat buildup, coefficient of friction decline, and
rapidly increased wear rates. The engagement characteristics of friction
material can change from moment to moment depending on factors such as
stiction (mating surfaces moving together [static state] or slipping past
one another [dynamic state]), internal bellhousing temperatures, friction
material temperature, heat sink ability of the mating surfaces (heavy
cast iron flywheel versus lightweight aluminum or steel flywheels), material
transfer (proper and adequate bedding in procedures followed), and contamination.
If you have modified your engine and you have an idea of what the horsepower
is, you can estimate the engine torque using the following example. A
chip manufacturer states for example a 30 hp gain for a 2001 Acura Integra
GS-R which has a base hp of 170 hp at 7600 rpm. Use (5252 x hp)/rpm to
get the engine torque. The number 5252 is a constant that happens to be
the rpm where the horsepower and torque values are the same. The new HP
amount is 170 + 30 = 200 hp. It would then be (5252 x 200)/7600 which
equals 138 lbs.-ft. of engine torque.
If you have any questions or concerns regarding the proper clutch for
your vehicle, please don't hesitate to contact us by phone, chat, or email.
We are glad to be of service.
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