Simply put, Everything! At its most basic level, an engine is an air pump.  More air entering the engine increases the efficiency of the combustion process creating more horsepower and torque. Horsepower is a measure of the engine's maximum power while torque measures how quickly you can accelerate.

The K&N Filtercharger® is designed to increase engine performance in both horsepower and throttle response by reducing air flow restriction.

Maintaining optimal, unrestricted air flow becomes a problem when it must pass through a filtering medium. The level of air resistance varies depending on the size, surface area and physical attributes of the filtering medium.