Danger of Power Pulleys &
Understanding the Harmonic Damper
BY STEVE DINAN OF DINAN BMW
I have been threatening for a long time to write a series
of technical articles to educate consumers and to dispel misconceptions
that exist about automotive after-market technology. Motivated
by problems with customer's cars resulting from the installation
of power pulleys, I wish to explain the potential dangers
of these products and address the damage they cause to engines.
The theory behind the power pulley is that a reduction in
the speed of the accessory drive will minimize the parasitic
losses that rob power from the engine. Parasitic power losses
are a result of the energy that the engine uses to turn accessory
components such as the alternator and water pump, instead
of producing power for acceleration. In an attempt to minimize
this energy loss, many companies claim to produce additional
power by removing the harmonic damper and replacing it with
a lightweight assembly. While a small power gain can be realized,
there are a significant number of potential problems associated
with this modification, some that are small and one which
is particularly large and damaging!
The popular method for making power pulleys on E36 engines
is by removing the harmonic damper and replacing it with a
lightweight alloy assembly. This is a very dangerous product
because this damper is essential to the longevity of an engine.
The substitution of this part often results in severe engine
It is also important to understand that while the engine in
a BMW is designed by a team of qualified engineers, these
power pulleys are created and installed by people who do not
understand some very important principles of physics. I would
first like to give a brief explanation of these principles
which are critical to the proper operation of an engine.
1) Elastic Deformation
Though it is common belief that large steel parts such as
crankshafts are rigid and inflexible, this is not true. When
a force acts on a crank it bends, flexes and twists just as
a rubber band would. While this movement is often very small,
it can have a significant impact on how an engine functions.
2) Natural Frequency
All objects have a natural frequency that they resonate (vibrate)
at when struck with a hammer. An everyday example of this
is a tuning fork. The sound that a particular fork makes is
directly related to the frequency that it is vibrating at.
This is its "natural frequency," that is dictated
by the size, shape and material of the instrument. Just like
a tuning fork, a crankshaft has a natural frequency that it
vibrates at when struck. An important aspect of this principle
is that when an object is exposed to a heavily amplified order
of its own natural frequency, it will begin to resonate with
increasing vigor until it vibrates itself to pieces (fatigue
3) Fatigue Failure
Fatigue failure is when a material, metal in this case, breaks
from repeated twisting or bending. A paper clip makes a great
example. Take a paper clip and flex it back and forth 90°
or so. After about 10 oscillations the paper clip will break
of fatigue failure.
The explanation of the destructive nature of power pulleys
begins with the two basic balance and vibration modes in an
internal combustion engine. It is of great importance that
these modes are understood as being separate and distinct.
1) The vibration of the engine and its rigid components
caused by the imbalance of the rotating and reciprocating
parts. This is why we have counterweights on the crankshaft
to offset the mass of the piston and rod as well as the reason
for balancing the components in the engine.
2) The vibration of the engine components due to their individual
elastic deformations. These deformations are a result of the
periodic combustion impulses that create torsional forces
on the crankshaft and camshaft. These torques excite the shafts
into sequential orders of vibration, and lateral oscillation.
Engine vibration of this sort is counteracted by the harmonic
damper and is the primary subject of this paper.
Torsional Vibration (Natural Frequency)
Every time a cylinder fires, the force twists the crankshaft.
When the cylinder stops firing the force ceases to act and
the crankshaft starts to return to the untwisted position.
However, the crankshaft will overshoot and begin to twist
in the opposite direction, and then back again. Though this
back-and-forth twisting motion decays over a number of repetitions
due to internal friction, the frequency of vibration remains
unique to the particular crankshaft.
This motion is complicated in the case of a
crankshaft because the amplitude of the vibration varies along
the shaft. The crankshaft will experience torsional vibrations
of the greatest amplitude at the point furthest from the flywheel
Harmonic (sine wave) Torque Curves
Each time a cylinder fires, force is translated through the
piston and the connecting rod to the crankshaft pin. This
force is then applied tangentially to, and causes the rotation
of the crankshaft.
The sequence of forces that the crankshaft
is subjected to is commonly organized into variable tangential
torque curves that in turn can be resolved into either a constant
mean torque curve or an infinite number of sine wave torque
curves. These curves, known as harmonics, follow orders that
depend on the number of complete vibrations (cylinder pulses)
per revolution. Accordingly, the tangential crankshaft torque
is comprised of many harmonics of varying amplitudes and frequencies.
This is where the name "harmonic damper" originates.
When the crankshaft is revolving at an RPM such that the torque
frequency, or one of the harmonic sine wave frequencies coincides
with the natural frequency of the shaft, resonance occurs.
Thus, the crankshaft RPM at which this resonance occurs is
known a critical speed. A modern automobile engine will commonly
pass through multiple critical speeds over the range of its
possible RPM's. These speeds are categorized into either major
or minor critical RPM's.
Major and Minor Critical RPM’s
Major and minor critical RPM's are different due to the fact
that some harmonics assist one another in producing large
vibrations, whereas other harmonics cancel each other out.
Hence, the important critical RPM’s have harmonics that
build on one another to amplify the torsional motion of the
crankshaft. These critical RPM’s are know as the "major
criticals". Conversely, the "minor criticals"
exist at RPM's that tend to cancel and damp the oscillations
of the crankshaft.
If the RPM remains at or near one of the major
criticals for any length of time, fatigue failure of the crankshaft
is probable. Major critical RPM’s are dangerous, and
either must be avoided or properly damped. Additionally, smaller
but still serious problems can result from an undamped crankshaft.
The oscillation of the crankshaft at a major critical speed
will commonly sheer the front crank pulley and the flywheel
from the crankshaft. I have witnessed front pulley hub keys
being sheered, flywheels coming loose, and clutch covers coming
apart. These failures have often required crankshaft and/or
Crankshaft failure can be prevented by mounting some form
of vibration damper at the front end of the crankshaft that
is capable of absorbing and dissipating the majority of the
vibratory energy. Once absorbed by the damper the energy is
released in the form of heat, making adequate cooling a necessity.
This heat dissipation was visibly essential in Tom Milner's
PTG racing M3 which channeled air from the brake ducts to
the harmonic damper, in order to keep the damper at optimal
operating temperatures. While there are various types of torsional
vibration dampers, BMW engines are primarily designed with
"tuned rubber" dampers.
It is also important to note that while the large springs
of a dual mass flywheel absorb some of the torsional impulses
conveyed to the crankshaft, they are not harmonic dampers,
and are only responsible for a small reduction in vibration.
In addition to the crankshaft issue, other problems can result
from slowing down the accessories below their designed speeds,
particularly at idle. Slowing the alternator down can result
in reduced charging of the battery, dimming of the lights,
and computer malfunctions. Slowing of the water pump and fan
can result in warm running, while slowing of the power steering
can cause stiff steering at idle and groaning noises. It is
possible to implement design corrections and avoid these scenarios,
but this would require additional components and/or software.
Our motto at Dinan is "Performance without sacrifice"
We feel our customers expect ultra high performance along
with the legendary comfort and reliability of a standard BMW.
While it is common that a Dinan BMW is the fastest BMW you
can buy, performance is not our only goal. Dinan isn't just
trying to make the fastest car. Instead a host of considerations
go into the development of our products. Dinan puts much more
effort into these other areas than does our competition.
These considerations are Performance, Reliability (Warranty),
Driveability, Emissions, Value, Fit and Finish. We feel that
the power pulley is a bad way to get extra power from and
engine and the potential for serious engine damage is too
This is a simplified explanation meant to be comprehensible
by those who are not automotive engineers. In trying to simplify
an extremely complex topic some precision was sacrificed although
we believe this explanation to be as accurate as possible.
We encourage our customers to educate themselves and understand
the automotive after-market because we believe that our products
are the best researched, engineered, and fabricated products
For those interested in a more in depth and technical explanation
of this topic, the reference book is Advanced Engine Technology,
written by Heinz Heisler MSc,BSc,FIMI,MIRTE,MCIT. Heinz Heisler
is the Head of Transportation Studies at The College of North
West London. His book is distributed in this country by the
SAE (Society of Automotive Engineers).