Slapshot, I think they are called a swirl-filter because the air inlet is made so the air entering is forced to swirl around and centrifugal force causes heavy particles to collect and drop into the jar while cleaner air then goes through and enters an oil bath air cleaner. The swirl filter functions only as a prefilter just as others have stated. I've also heard them referred to as a Mason Jar filter. /forums/images/graemlins/smile.gif