By: Haitham Alhumsi
For example, if the crank pulley is 4″ in diameter, then for every rotation of the motor the belt moves 2*pi*r which is about ~25 inches. If the supercharger had a snout pulley that were 2″ in diameter, and there was NO BELT SLIP, then 25 inches of belt rotation will cause the supercharger to spin by 25/(2*pi*r) = 2 complete rotations. To simplify things, the supercharger drive ratio is exactly the ration between the diameter of the crank pulley to the supercharger pulley or in our example 4:2 which is 2:1.
This technology did not transfer directly into cars, however we have something similar. Sometimes supercharged cars at high boost can be a handful on the street due to the increased torque at lower RPMs. This becomes very evident for those of us whole live in areas with unpredictable climates that see lots of rain or snow fall. In a street/strip compromise some companies have come up with two piece supercharger pulleys. The inner piece of the pulley is an inner sleeve that is pressed onto the supercharger snot (as you would press on a one piece solid pulley). The outer piece of the pulley is an outer ring that bolts onto the inner sleeve using some high grade bolts. This design allows a ‘quick change’ of supercharger snout pulleys between your low boost street pulley and your high boost ‘race’ pulley in a matter of minutes. You just release your belt tensioner, remove 4 bolts, swap pulleys, tighten 4 bolts, and then re-tension your drive belt and viola a 15 minute quick change. This is a very nice option for large displacement motors running positive displacement roots or twin-screw type superchargers that have more than enough torque to overwhelm their street tiers. This also works well with an assortment of other street/strip products such as drag radials, water injection, exhaust cutouts where you can – on a prepared track – swap pulleys, swap tires, open an exhaust cutout and run open header, and switch on your water injection system for added cooling and fueling. Mr. Jackal and Mr. Hyde mentallity.