An Underdrive pulley refers to an aftermarket crankshaft or accessory pulley (such
as an alternator pulley) that is designed to drive a vehicle's accessories at
a slower rate than stock. Underdrive from the crankshaft pulley means the pulley
is smaller than the original pulley it replaces.
Overdrive from
the accessory pulley means the pulley is larger than the original pulley it replaces.
Originally they were invented for race cars in the 1950s and 60s, whose
engines had been modified and ran at higher RPMs for longer periods of time than
the manufacturers had intended. It was therefore necessary to underdrive the
belt-driven accessories (among other things like the water pump) to keep the accessories
from burning out and the engines running reliably. |
