bigger underdrive = slower blower smaller underdrive = faster blower Think of the cogs on a bicycle, the underdrive is the crank cog (chainring). The larger diameter makes it easier to pedal, but moves the rear wheel less provided that the rear wheel didn't change cogs as well. *edit* Wait, am I being an idiot? Now I'm second-guessing myself. Is that right up there?
Just like timing gears, or ring and pinion gears, one is bigger than the other. The bigger one turns slower. That's why some people swap pulleys on water pumps, alternators, etc because although it makes those components a little less effective, they free up some horsepower due to the crank pulley staying the same size. The way this applies to blowers is that it slows the rotation of it, which should lower its maximum boost potential. From what I've heard, it is usually not recommended to change the amount of underdrive on a supercharger because of belt slippage and other issues.
If you look at the pulleys, if the top pulley is larger than the crank pulley then the blower is underdriven. If the pulley if smaller than the crank pulley then it is overdriven. Simply if the car is for street use then you run it underdriven, and if it is a racecar you would overdrive it. You can still get up to about 6 - 8 lbs of boost under driven (ballpark as every engine is different), which will make more power then your tires can probably handle, and when it is under driven it is easier on the engine then if it is over driven. Also there is no problem with changing the drive ratios as you can tighten and loosen the play in the drive belt to adjust for the different size of pulleys. Brian
The Jalopy Journal
