Open Filters

The Basic Science Of An Air Box


This page will try to explain the sciences behind why the correct air box for a 2 stroke gives better power than an open "performance filter".




As most people know, as the piston moves up in the barrel it creates low pressure in the crankcase, this forces the reeds open and sucks fuel and air mix into the crankcase:



What people don't realise is that on the down stroke of the piston, when the reeds snap shut, a burst of air is shot backwards through the manifold, through the carburettor, where it picks up fuel and oil then back into the air box. The air box, being a vast space, pressurises and generates a reverse pulse (back through the carb and to the reed block). This happens so quickly that it stops the majority of the mix escaping into the air box and then out into the air:



This is what happens with an open filter or no filter: