K&N have 3 main product ranges. K&N panel filters, K&N 57i induction kits and K&N Typhoon intakes.
Why would you fit a K&N panel filter?
K&N panel filters are a direct replacement for the factory air filter. The K&N air filter is the same size as the original air filter and requires no modifications
The number reason is that you want a K&N panel filter is that you are looking for an air filter than will last for the life of the vehicle.
When the K&N filter gets dirty you simply wash it with water and cleaning shampoo to remove the dirt and re-oil the filter. Thats it.
The second reason is that you want a filter that gives an even flow rate between cleaning. The factory paper filter is great but it gets blocked much sooner than a K&N air filter meaning it flows much less air towards the end of its service life. A K&N panel filter gives a much more even flow rate.
Why would you fit a K&N 57i Induction Kit?
You want to improve the throttle response of your car.
Your car has a restrictive filter from the factory
You have tuned your car and the factory air filter box has become a restriction
Why would you fit a K&N Typhoon Intake System?
For the same reason as you would fit a 57i induction kit but because the Typhoon system replaces the factory intake piping the power gains are more noticeable.