Fitting a K&N air filter to a vehicle usually increases the engine output by 1 to 4 horsepower, according to the K&N website. The actual increase depends on factors that include the structure of the existing air system, overall engine load and the driver's driving style.
Automotive air filters prevent dust, grime and other abrasive particulate matter from entering the cylinders of an engine. Such materials cause mechanical wear and contaminate engine oil. According to K&N, high-performance replacement filters allow air to flow into an engine more easily, increasing the air pressure within the cylinders, which leads to an increase in overall power output.