underdrive/overdrive pulley

[jpfletch1]

I see threads on these pulleys but i dont know what is over or underdrive. How is this calculated? Is it a ratio or a size? Thanks

[blykins]

When the crank pulley is larger than the accessory pulleys, then it's an overdriven pulley system. Likewise, when the accessory pulleys are larger, they're underdriven.

So of course in an underdriven situation, the accessories are now turning slower than in a stock situation. You have to watch because if you turn accessories slower (i.e. alternator), you might end up with some side effects.

[strictlypersonl]

Underdrive and overdrive pulley sets might better describe the situation.

An underdrive set might drive the water pump and/or the alternator slower than it would normally compared to a standard set of pulleys. The number usually given would be a percent change from the stock speed of the driven pulley. The change can be made by changing the crank pulley or the pulleys on the driven accessories.

For instance, an engine has a stock 6" crank pulley, 5" waterpump pulley and a 3" alternator pulley. You can change the crank pulley to 5", making both the waterpump and alternator underdriven by 17%. Or keep the stock crank pulley and get a 4" alternator pulley for 33% underdrive on the alternator only.

Overdrive systems work in the other direction, but they are pretty rare except with superchargers.