THis works in dusty conditions for engine intake!
 

Donaldson 7" Cyclonic Pre-Filter - Expedition Portal

excelent results in dusty conditions, allows use to monitor the filling of the filter through the glass. And these are cheap!

can be found on most tractor and ground machine service places.

I have used these for years before the cars orginal filter, these seem to work fine also in fast moving vehicles (cars)

datasheet:
http://www.sterlingfiltration.co.uk/...nical_Data.pdf

calculator for choosing the right size prefilter:

http://lovehorsepower.com/joomla/ind...tuff&Itemid=49