It doesn't really matter whether its fine or not, dirt particles are carried through the air. The way I see it is that manufacturers spend millions of pounds a year on research into filters making them flow well and filter, why spend £2 to fuck it all up? The restriction a branded air filter makes these days is very little and the power you would get from running without a filter probably around 2bhp, you'll soon claw that 2bhp after 500 miles without running a filter through worn piston rings. Even racing engines use air filters and their run on supposedly a clean smooth track, they value their engines and their only run for a for what a total of say 2000 miles a season including testing and qualifying etc. I would like to see a the inside of a road engine thats been run in traffic, round dirty country lanes, behind other cars etc which has been run without a filter. It really depends how much you value performance over reliability at the end of the day. Just my opinion