Type Half Case
Crankcase ventalation is needed because the volume of air inside the engine is constantly changing. The crankcase builds pressure as the engine heats and air expands. The pistons also displace air to some extent on each cycle. The crank air is an oily mist and as it expands needs to go "somewhere". The problem on a 2-cylinder opposed engine is that both pistons are going up or going down at the same time, so the volume of air in the crankcase is compressed and expanded with each revolution of the crank. In a 4-cylinder engine with a crankcase that is open between the cylinders, the crankcase volume remains nearly constant, because there are two pistons going up at the same time two pistons are going down. If the crankcase is simply vented to the outside through a short tube then rather strong air pulse moves in and out of this tube with each revolution of the engine. Since this air is an oil mist, a lot of oil will be carried out of the vent.
Aircraft engines vent their crankcases to the atmosphere, like the old cars. They do not have a PVC valve. Continental engines like the 65 and up, use a 90 Degree angled hose fitting that is fitted into the crankcase wall.
The reed valve is a one-way (check) valve that allows air to exit but not enter the crankcase. This valve opens at the pressure peaks (Bottom Dead Center) letting a small amount of air out. The reed valve closes as soon as the pistons move up in the cylinders again and the pressure in the crankcase drops. The result is a reduced pressure in the crankcase for most of the crank revolution, but no inflow of air through the breather tube. At the next BDC, only the small amount of excess gasses (from piston blowby) raises the crankcase pressure above the outside air pressure and the reed value again opens briefly. The result is that instead of strong air pulses flowing in and out of the vent tube, only weak one-way pulses of air/oil mist flow out of the tube. This greatly reduces oil loss out of the vent tube. This works reasonbaly well on 2 cyinder engines; most 4 cylinder engines don't need to use a reed valve to control oil loss.
Prior to 1965-1970 most our cars vented their crankcases directly to the atmosphere under the car. The vent gases included oil mist and combustion blow-by gases, including un-burned hydrocarbons, acids, and other pollutants. Actually, the oil fill cap was also usually vented, so air could flow through the crankcase to help carry the pollutants out of the engine. A simple solution to reduce this pollution is to connect the crankcase vent to the intake manifold, so that everything coming out of the crankcase vent goes through the combustion chambers and gets burned.
The trouble is that at idle, the intake manifold is at high vacuum, and the rapid flow of air through the crankcase and into the intake manifold may exceed the flow of air through the carburetor, which completely messes up the air/fuel ratio delivered by the carburetor, and the engine stalls or has idle problems. To prevent the stalling problem, a small orifice was inserted into the crankcase vent tube to restrict the flow of air at idle. However, this created a new problem. At full throttle, the manifold pressure is nearly atmospheric and the blow-by increases dramatically. The small orifice would restrict the flow of gases out of the crankcase into the intake manifold to the point that gasses would come out of the oil fill cap, which is effectively venting to the atmosphere again. A PVC valve was later added to help solve this problem.
The PVC valve contains a plunger held off of its seat by a weak spring. The plunder also has a small orifice in it. (or an orifice around the seat, so that the seat "leaks.) When the manifold pressure is low (idle) then the rapid flow of air sucks the plunger against its seat, and the orifice restricts the air flow. When the manifold pressure is high (acceleration or cruising) the spring pushes the plunger off of its seat, opening a low-resistance path for the higher volume of crankcase gases to reach the intake manifold. When the engine is running at higher throttle settings, the volume of air through the crankcase ventilation system is small compared to the air/fuel going through the carburetor, so the higher volume of crankcase gases do not bother the operation of the engine.
One method to vent the VW is to install a tube in the exhaust pipe. This takes advantage of the exhaust vacuum pulse to pull a lower than atmospheric pressure on the crank case. A vacuum gauge may be installed in the cockpit to monitor the action of this modification. This replaces the reed valve, and a standard in line vacuum valve can be used. Some Ford Mustangs (1980) had this setup.
Reed & Collection Bottle
Many are having good results with the collection bottle crank case venting system. This engine has a plate and reed valve arrangement on the top of the engine, leading to a collector bottle for oil that gets through. The collection bottle should be vented so no pressure is created.
PCV Valve & Collection Bottle
Also, many VW use a PCV valve installed in each valve cover. The oil filler hole has a simple plate and plastic filler tube with cap. Originally I was using the PCV valves which allow one-way air flow. You can pick a PCV valve off a AutoZone shelve that is low cost. Some prefre to cut them so it is just an open port to allow two-way air flow. Use a fitting that is attached to the valve cover where the fitting has a nipple so you can connect a hose. Then route the hoses down to a catch bottle or just leave them open. There is plenty of air flow through this but ther does not seem to be significant oil with it.