No VDUBS here in206

so ive read a few forums about vented valve covers, looking for what purpose they serve, and ive caught that they relieve the engine of pressure so pressure doesnt get in the way of things, but this is only a basic understanding.
how do they function?, why do you need the "catch can"?, can you loose hp from this?, can you gain from it with no turbo setup?
sorry if this is a noob question, i just want a better understanding, thanks :]

skaternak

its not legal in califonia because it lets the fumes from the crankcase escape to the atmosphere. that is why there is a pcv system. there is one on the valve cover to let the fumes go into the intake and get it burned