I run an open K&N right behind the front left tire with no prefilter on my kid's mini. It gets sprayed frequently. Just installed a 53mm aluminum cylinder kit and the stock cylinder was spotless. Was ridden 95% of the time in sand though. I'm convinced sand can't get through.

Don't have much experience with other conditions, so I can't argue. Never ridden in dirt on a regular basis with a K&N.

 

All factory teams ie honda, kawasaki, yamaha run foam filters for the simple reason of stopping the most dirt from getting in your engine. In extremely dusty desert conditions foam is the best choice. Even K&N markets a foam outerskin for its filters for use in "extreme dust conditions" that should be a red flag to a consumer.

 

I f you want maxi mum airflow then do this.

Buy a UNI filter 2x as big as you need and buy/make an adapter for it. problem solved.

Look at the 450r the stock foam filter is 50% bigger this year.