What is an “underdriven” crank pulley?
An “underdriven” crank pulley is reduced in diameter compared to the stock counterpart in order to reduce the gearing and free up power. This however can cause sacrifices in reliability and comfort of all those accessory systems and over time can even sacrifice longevity of the engine due to the decrease flow of oil & coolant going to the engine. Although power can be made this way via underdriving, it is not recommended due to the necessary sacrifices that have to be made. Roughly 15% of gains from underdriven crank pulleys are from underdriving alone, the other 85% comes from reduction in mass. In addition, UD crank pulleys can irritate their drivers by causing lights to flicker at night, stereo systems not having as much power, increased stiffness in steering, and other annoyances.
