K&N Custom Air Filters can increase horsepower up to 12%. They are available in a wide range of sizes, designs and shapes for virtually every application. K&N Custom Clamp On Air Filters are usually not used with OEM air boxes because of their larger than OEM size. They allow considerably more air flow than a stock air box set-up. Generally up to 25% increase in jet sizes will be required because of K&N filters increased air flow. K&N Clamp on Filters are not street legal.