I think a foam pre-filter is a good idea for K&N air filters considering they don't seem to filter so great.
My experience is I've been using a 14" cone K&N filter on my Grand National for years. It's a cold air system that get's a clear shot of outside air so it picks up quite a bit of debris.
Anyway, whenever I'd service the air filter and intake tract, I would always notice a dirty and oily residue on the Mass airflow sensor. I figured the oil was from the filter and the dirt was from small air leaks around connections etc. I never suspected the K&N filter as being the cause of the dirt since K&Ns past marketing always praised it excellent filtering ability
Since the outside of the filter was always gunked with all kinds of debris, I decided to give the K&N prefiltering foam a shot. My main reasoning was for less frequent cleaning and maintenance.
After making it fit and work with my application, I was happy that I was unable to detect a difference in feel, performance or sound. Since I was using the prefilter I also used less oil on the actual filter. I carefully oiled it just enough to barely get it light pink. I also spread a thin amount of K&N oil on the inner part of foam prefilter to compensate.
But the best part was after about 10,000 or so miles I finally opened up the intake to inspect and clean and I couldn't detect a bit of dirt, dust or residue. Even with a clean cloth, I could not get a spec of dirt or oil to show up.
Point of this all being, if anyone has invested in a K&N air filter but unsure about it filtering ability, a foam prefilter might be a solution to balance the best benefits between K&Ns and good paper filters.
