An under drive
pulley refers to a crankshaft or accessory pulley (a/c, alternator, power steering, water pump, etc.)
designed to turn at a slower speed than stock. To Under
drive means to
slow the rate of rotation in a system. Under drive is achieved by either making
the crank/main (drive) pulley smaller or the accessory (driven) pulley larger
than the original diameter pulleys.
Under drive
pulleys increase engine output by reducing the draw of the engine's accessories
by slowing them down and reducing the HP they use. Horsepower gains from under drive pulleys can vary by vehicle,
engine, number of accessories and the amount of under drive (Improvements of up
to 5-15 HP at the wheels have been seen). Additional and significant
performance improvements can be seen by reducing the weight of the pulley
versus the original pulley. Gains can range from 3-6 HP per pound of
weight reduced.
Poorly engineered underdrive pulleys
can cause unwanted side effects; this is due to not spinning the alternator,
power steering, and/or air conditioning fast enough. This leads to low
alternator voltage, weak/no power steering assist, and weak/no air conditioning effectiveness, especially at idle/low RPM. The most
commonly seen result is lighting may dim, or the stereo may cut out. Too much
underdrive for a race car is not much of a concern due to the high RPMs they
run at, but for daily driven vehicles it can lead to a dead battery if too much
time is spend at idle or low RPM. Additionally, an underdrive pulley will not
reduce the power consumed by the alternator, as the alternator's load is
automatically adjusted by its control circuit to match the electrical load
regardless of input speed.
Changing the original crankshaft
pulley can also have negative effects if the replacement pulley is not
manufactured properly. A crankshaft or accessory pulley not machined or
balanced properly can cause severe damage leading to thousands of dollars in
repairs.
WORKING