A couple of reasons you may be asking if a K&N panel filter is the right choice.
- You think your filter box could be undersized from the factory
- You have modified your engine and you are not sure if the filter box is still big enough
There are 2 ways to calculate if you air filter box is big enough
- Using a vacuum gauge (mounted between the air filter and the turbo on a turbo engine). Measure the gauge reading with the engine off. The reading should be the same at full throttle throughout the rev range. If not, there is a restriction between the filter and the engine (or the filter and the turbo on a turbo engine)
- Using this calculator. It is not designed to be the last word, it is designed to tell you if the filter box you are using is in the ball park for the amount of air your engine needs, just enter your filter dimensions (this is specifically for filter boxes using panel filters)
See this article for more information about this air filter size calculator