Fra GGR sin side Q&A Database Results Why do GGR breathe from the crankcase, & not the cam cover? - Breather System Most of the pressure is created in the cylinder block by piston ring blow-by. If the engine is vented from the top, the rush of crankcase pressure to the top of the engine will prevent the oil fed to the cylinder head from draining back into the sump. Admittedly, the oil vapour is more dense in the bottom of the engine, this is why we have the oil / air separators.