Underdrive pulleys are engine accessory pulleys, which typically include the alternator, water pump and crankshaft drive pulley. These original factory equipment pulleys are replaced with underdrive pulleys that are smaller in diameter in order to slow the engine accessories down when the engine is operating at high revolutions per minute (RPMs). The purpose for this reduction in speed by the underdrive pulleys is to save horsepower instead of waste unneeded power turning the accessories.
Many vehicles that see double duty on the street and the drag strip utilize underdrive pulleys. While the performance gains found by installing underdrive pulleys are minimal in most instances, they cannot be overlooked in a sport where a thousandth of a second may be the difference between winning and losing. Many oval track racers, such as NASCAR drivers, also use underdrive pulleys, but for another reason. In an application such as a NASCAR race where the engine might be turning 9,000 RPMs for 500 miles (310.5 km), it is necessary to slow the engine accessories down in order to prevent them from burning up from extended high-speed use.
Where the average street car is typically operated at 1,500 to 2,000 RPMs, operating the same accessories at very high sustainable speeds will churn the liquid to a froth inside the water pump and burn the diodes and brushes completely out of an alternator. The use of underdrive pulleys will avert this problem while at the race track. There is, however, a downside to using this type of system on a street-driven vehicle.
On a street-driven vehicle, running the accessories at a reduced speed while cruising at highway speeds is not very noticeable. The problem begins to display symptoms when driving a vehicle equipped with underdrive pulleys in city traffic. Over-heating and non-charging electrical systems resulting in dead batteries are often the result. The manufacturers spent a lot of money to test equipment and make sure it will work at typical driving speeds and road conditions. Underdrive pulleys throw all of that research out the window in search of an increase in performance at a very narrow margin of engine operating speeds.
Many racers choose to temporarily remove the serpentine belt, thereby disabling the engine's accessories instead of running the pulleys on their vehicle. This allows the engine to run with added power while at the track. By simply replacing the belt, the vehicle operates at proper temperatures and charges the battery as intended on the drive home.