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 damage.
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 failure).
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.