My understanding of PC venting.
The ventilation system for the motor gets clean filtered air into the even bank engine cover from the inlet manifold, then circulates it down through the engine's crankcase area, then is removed from the engine via the odd bank engine cover towards the inlet manifold. The vacuum present in the inlet manifold creates this draw of air through the motor. The vacuum will usually be maximum at engine idle and on a closed throttle slow-down for a normally aspirated motor, and reduces as the engine loads up and builds up RPM and engine load. Long duration overlap cam timing, supercharged or turbo motors will have a different vacuum characteristic and have problems getting rid of crankcase combustion products, so most people fit catch cans in both lines. One to stop the inlet air tract becoming fouled by oil which then coats the throttle butterfly and throttle body. The other is to catch or minimize the amount of airborne material getting into the inlet manifold.
The alloytec does not have a spring loaded PCV valve in the odd bank engine cover. Just a 1.5mm hole (approx.) for the vapours to travel through. The "valve" is permanently locked into the valve cover at manufacture. I check the 1.5mm hole is clear by using a pipe cleaner to pick up any debris around the metering hole and push through into the engine cover space. Hopefully if I see gunk then it is an early warning for crankcase gunking.
By taking clean filtered inlet air, the motor is not getting solid particles such as dust into the engine's vital areas. The flow of air through the motor picks up any blow-by from the piston rings, eg. combustion gases, steam or raw fuel vapours and any oil vapours and takes them through the inlet manifold to be burnt. This is an antipollution feature, and the source of problems when an engine gunks up the PVC vent in the odd bank engine cover, and the combustion products cannot be removed from the engine.
