Rhinoman I've always respected your opinions and knowledge, and would tend to agree with you on this except for one thing: you seem to contradict yourself when stating " they are rubbish at filtering dust and fine particles" then later in the same sentence you state:" for outright flow, then clean and oiled they are probably the best filter."
I only just saw this reply.
I probably should have said "clean and oiled they are probably the best performance filter". The dyno tests I did comparing cone filters showed that an oiled K&N gave the best flow/bhp, with a big enough filter I got no loss of power over an open intake, K&N claim that the filtering gets better the dirtier it gets which I'm sure is true but as it gets dirty the flow drops off and you lose power. Some of the other filters clearly had a more open gauze/element which would give worse filtering but also flowed less, this seems to be down to the K&N having more of a "bellmouth" shape to the moulding where the air enters the carb.
On my road bike running a K&N element in the stock airbox I was seeing dust/dirt in the airbox which never happened with the stock filter, this was the same gauze used on the cone filters. I had to clean it at roughly the same interval as I would have replaced the stock because the performance tailed off noticably.
I used to run bikes on open intakes but you can get a lot of muck in the engine, particularly when its wet and there is a lot of spray. I put the filter on the racer after dirt in the carb caused the slide to jam open, very nearly causing a big high speed off. On susbsequent engine rebuilds I didn't see the same scoring on the bores that I used to get running open.
Like a lot of tuning its a compromise between engine longevity and power.
