Yeah, but at what particle size, and what flow rate? A rock is a 100% efficient filter, but the flow rate is abysmal <img src="/forums/images/graemlins/wink.gif" alt="" />. For example, a HEPA filter, such as is required for asbestos and lead cleanup, is specified as filtering 99.97% of all particles down to 0.3 microns in size.

I have always suspected that a K&N does not filter small particles as well as a paper filter, but I have nothing to back that up. But I do know that a K&N works on a different capture principle than a paper filter. It is primarily an inertial impactor, where the particles impact the sides of the convoluted passages as they pass through the filter and are retained by the sticky oil. A paper filter works by simply having passages too small for the particles to fit through, although electrostatic attraction plays a role as well.

This means that a paper filter actually becomes more efficient as it becomes dirty, because the passages through the filter are getting narrowed by the caught particulates. But, this is at the cost of reduced flow rate. Whereas a K&N generally will retain most of its flow until it becomes very dirty, because the passages are larger.

I have also always wondered if the capture efficiency of the K&N-type sticky oil is as good at subzero temps as it is in warmer temps...I suspect not, but again have no hard data to back that up.