Engine Case Pressurization

[pglaves]

The vent-out and no vent-in serves a very useful purpose. A significant part of the "pressurization" of the crankcase as both pistons retreat to bottom dead center is absorbed by the partial vacuum in the crankcase. This minimizes the volume of air and oil mist that passes out the PCV valve. This minimizes oil consumption, stress on seals, and the mess in the carb(s) from that oil mist.

 

[Omega Man]

OM -

The BMW Airhead is vented but only "out". There is no purposeful orifice to allow air in...or so it seems. Air has to find its way back in by hook or crook!!

The vent-out and no vent-in serves a very useful purpose. A significant part of the "pressurization" of the crankcase as both pistons retreat to bottom dead center is absorbed by the partial vacuum in the crankcase. This minimizes the volume of air and oil mist that passes out the PCV valve. This minimizes oil consumption, stress on seals, and the mess in the carb(s) from that oil mist.

I’m familiar with the PCV (positive crankcase ventilation) valves being vents. On the old slant-6 Chrysler engines we replace the PCV valve at tune-up time. When an owner failed to replace it, the engines puked brown all over the engine compartment.
I suspect the PCV valve could have sealed when brand new……but past brand new, I doubt it. The PCV valve was usually connected by hose to the base of the carburetor, IIRC
Not being familiar to airheads, I will defer to your knowledge.
OM

 

[pglaves]

I’m familiar with the PCV (positive crankcase ventilation) valves being vents. On the old slant-6 Chrysler engines we replace the PCV valve at tune-up time. When an owner failed to replace it, the engines puked brown all over the engine compartment.
I suspect the PCV valve could have sealed when brand new……but past brand new, I doubt it. The PCV valve was usually connected by hose to the base of the carburetor, IIRC
Not being familiar to airheads, I will defer to your knowledge.
OM

BMW doesn't call their Airhead crankcase vent a PCV valve but that is what it is. Positive crankcase pressure is allowed to escape via two valve types: one is a round fiver disk over a hole, pushed down by a little coil spring. Later, they used a metal reed type valve.