The supercharger blower is driven by a shaft connected to a pulley which in turn is rotated by a belt that is driven off the engine drive shaft. (This is the same belt that drives the alternator, and the water pump.) Every time the drive shaft makes a complete rotation, the belt around the supercharger pulley is moved a certain distance.
If you remember your basic geometry from high school, the distance around the edge of a circular object like a pulley—the circumference—is determined by the diameter of the pulley. The smaller the diameter of the pulley, then the smaller the circumference of the pulley. With a smaller circumference, less movement of the belt is required to cause the supercharger shaft of the pulley to make a complete rotation.
Or you can think about it another way. If we put a smaller pulley on the supercharger, then the supercharger will spin more times during the same number of revolutions of the engine. And the faster the supercharger spins, the more air is pushed into the engine.
That’s the basis for our next horsepower improvement. By installing a smaller pulley (and the shorter belt that will be required to go with it) we can increase the speed of the supercharger and the amount of air being pushed in. Tuners call this “increasing the boost.” Not surprisingly, since the principles are simple, aftermarket suppliers have developed smaller pulleys that you can substitute.
Of course, there are some limits to how much boost an engine can absorb without blowing itself to pieces, so there are limits to how small a pulley can be used effectively. For this reason, BMW may be reluctant to honor its warranty if you replace its very conservatively designed pulley with one that produces more boost.
Most reputable suppliers supply pulleys that are small enough to make a difference in horsepower, but aren’t so small that they could blow the engine. As long as the pulley diameter isn’t reduced by more than 15 percent, there should be no problems, If the pulley is replaced by one that has a radius of less than 85 percent of the original, it will spin the supercharger in excess of its maxium rated specification, putting the engine itself at risk. Even if the engine isn’t pushed hard, if the pulley is too small, the belt angle will be so acute that the belt life will be significantly shortened.
Replacing the original pulley on the engine is not a simple job, since several other components have to be removed to get access to the pulley, and a special tool is needed to remove the pulley. Even with the special tool, an experienced mechanic may take several hours to do the job the first time. With a little practice, the job still takes about an hour.
So if you decide to replace your pulley with the smaller one, you should probably find a shop that has experience in replacing MINI pulleys. The replacement pulley and belt will cost about $200 and the installation about two to three hours of shop time. In terms of horsepower improvement per dollar, this is probably the most cost-efficient change you can make to the engine.