This is kind of an entertaining 33-minute long video where the owner of the Best Rest Products accessory company tests different types of motorcycle air filter mediums. He uses a stock BMW paper filter in his tests.
https://www.youtube.com/watch?v=JgZPkggKNiI&You can go to minute 25 to see the conclusion of the air filter severe contamination results.
The stock BMW paper air filter seems to be the best filter that was tested for keeping fine contaminates out of your engine and also did a decent job of flowing air as any other filter when clean, although that was a little hard for me to evaluate (at minute 15). However, it appears that the Guglatech filter that they sell at their store, flowed more air than the other filters when both new and after filtering some of their filter compound.
Based upon this "test" (and I will let you be the judge of how scientific it is), personally I would stick with a typical stock paper filter for street riding, although the Guglatech filter might be a less expensive option over time because it is washable and reusable. I was surprised that both the Uni and K&N filter didn't seem to perform very well as air filters. In any case, Guglatech filters are not available for Royal Enfields, so you don't have to be concerned that you are missing an opportunity to buy one of those.
The last couple of minutes of the video are pretty funny.
Topic: Best Rest Products tests various air filters (Read 2579 times)