nkaminar
Well-known member
Positive Crankcase Ventilation
What is it
Positive Crankcase Ventilation, or PCV, is a method of removing vapors and moisture from the crankcase of an engine by sucking them out using vacuum from the intake manifold. Fresh air is sucked into the crankcase to dilute the vapors.
Why is it important
There are two functions that are important for a Model A engine.
First, it sucks out the blow-by that passes the piston rings that contains combustion products and unburnt fuel. The blow-by is corrosive and contaminates the oil, reducing the useful life of the oil and, more importantly, the useful life of the engine. Every piston engine has some degree of blow-by. Water in the blow-by is supplied from the combustion process and forms acid in the oil. Moisture also enters the crankcase through diurnal temperature changes and can form acid even if the engine is never started. The moisture and other blow-by gasses that condense into the oil are a source of sludge. The oil in a Model A or B engine never gets hot enough to fully evaporate the moisture in the oil.
Second, it reduces the pressure in the crankcase to below atmospheric pressure and thus reduces or eliminates aggravating leaks. No more oil spots on my garage floor after I installed my PCV system.
Although properly designed road tubes, that extend into the airstream below the car, can carry away a lot of the vapors and oil mist, the Model A does not come with this system. The road tube system usually has a means of supplying fresh air to the crankcase using a scoop and the air from the fan to push the air in. With a PCV system, the vapors and oil mist are burned in the engine and not just dumped into the environment.
History
The PCV system was initially used on tank engines during WWII to prevent water from entering the engines through the road tube when fording rivers. In the 1950’s it was discovered that the emissions from vehicle engines was a major cause of smog. In the early 1960’s the PCV system was mandated on new vehicles in the United States.
The PCV system adapted to Model A use was presented in a seminar at the MAFCA national meet in Pendleton, Oregon on June 2, 2026 by Gary Bartman. I used his article, below, as a reference for my PCV system on my 1930 sedan. My system is the same as he published with the exception of the fittings and a different bracket.
PCV system as applied to a Model A or B Ford engine
Figures are at the end of the article.
Figure 1 is a schematic of how the PCV system works on a Model A or B engine.
The heart of the PCV system is the PCV valve. When the engine is idling the vacuum in the intake manifold is high and the flow from the crankcase must be restricted in order for the engine to idle correctly. When under load, with the throttle open, the flow can be increased because the vacuum is less and engine performance is not effected, as long as the flow from the crankcase is not too much. The valve controls the flow during idling and the flow when the engine is operating under load with the throttle open.
The flow from the PCV valve is fed to a tap at the center of the intake manifold. The center is used so that no cylinder is overwhelmed.
The fumes from the crankcase are sucked out of the vent tube using a sealed cap with a small tube. The fumes first go to to a catch container (can) that is designed to trap any oil that comes out with the fumes. This is necessary to prevent carbon buildup on the valves. The catch container must be emptied periodically. The catch can must be mounted using a bracket.
Fresh air is supplied through a tap on the valve cover. The air is sucked through a filter so that foreign material does not enter the crankcase. The flow through the crankcase must be cross flow with fresh air entering opposite where the fumes are sucked out.
All this is connected using automotive rubber hose.
Parts used for the PCV system on a Model A or B engine
I am running a Burtz engine. I have drilled 1/8-inch holes in the bottom of each trough in the oil pan tray but others have drilled 1/2-inch holes. This keeps the oil mist down. The rods are pressure fed and do not need the dippers that stock engines use.
Off-the-shelf PCV valves are designed for modern engines and will not work on a Model A or B engine. An adjustable valve must be used. I used the one that Gary Bartman used, a valve supplied by M/E Wagner Performance. See Figure 2. The flow at idle must be set and the transition from idle to flow under load adjusted. The flow under load is not adjustable. This is a quality valve. It is machined from aluminum bar stock.
I also used the catch can that Gary Bartman used. It is from Trick Flow Specialities. The M/E Wagner valve sits in a gasketed hole in the top of the catch can (Air/Oil Separator) so that no extra plumbing is needed. See Figure 3. The catch can comes with a spigot at the bottom with a valve for draining the oil and a sight gauge half way up to know when it is time to empty the container. I welded up a bracket out of 1/4 thick mild steel that mounts to the two bolts that hold on the coolant inlet at the side of the engine, see Figure 4.
I drilled and tapped a 1/4-18 NPT hole in my valve cover for fresh air inlet. I used a 45 degree elbow and an air filter I bought from Amazon. See Figure 5.
The oil filler cap must be sealed except for the connection that runs to the catch can. I used one from MotoRad. It has an O ring for a tight seal in the stock tube. A little grease helps get it on and off. My friend who is using a B tube has a small tube welded to the side of the B tube to connect to the catch can. If you use this option be sure to find a cap that seals. See Figure 6.
I am running a Weber down draft carburetor with an aluminum manifold. I tapped a 1/4-18 NPT hole in the bottom center of the manifold and am using a 90 degree elbow, see Figure 7.
Adjusting the valve and performance
The valve comes with instructions and the Allen wrenches and fittings needed. It was easy to adjust but took a few tries. You have to supply your own vacuum gauge. My car pulls 20 inches of vacuum at idle.
The performance was unaffected by the addition of the PCV system. The catch can and valve got hot after a short test drive (a couple miles) so I knew the system was working. The catch can caught about a half tea spoon of oil during the short test drive so I know the catch can will be important and will require checking at each fill-up. The B tube may be better in this regard. I had to richen up the idle air/fuel mixture a tad due to the small amount of flow from the PCV valve.
Let me know if you decide to use this on your car. And let me know if you have any questions.
Attachments
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Figure 1, PCV System Schematic.jpg -
Figure 2, M:E Wagner Performance Dual Adjustable PCV Valve.jpg -
Figure 3, Trick Flow Specialities Air:Oil Separator (Catch Can) with PCV Valve.jpg -
Figure 4, Catch Can Bracket.jpg -
Figure 5, Air Inlet Filter on Valve Cover.jpg -
Figure 6, Sealed Oil Filler Cap with Fitting for PCV.jpg -
Figure 7, Vacuum Fitting on Bottom of Weber Manifold.jpg
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