Positive Crankcase Ventilation (PCV)

[nkaminar]

Air and fumes are being sucked out of the engine (crankcase). Fresh air is introduced (sucked into the engine) on the opposite side of the engine. On a V8 engine it is common to have the fresh air introduced at one valve cover and the fumes mixed with that air being sucked out of the other valve cover and fed to the intake manifold. The PCV system introduces a partial vacuum in the crankcase to suck the fresh air in.

 

[nkaminar]

The PCV system introduces a partial vacuum in the engine (crankcase). That sucks fresh air into the engine. The fresh air mixes with the fumes and both are sucked out of the engine and fed to the intake manifold. A flow through the engine is needed to keep fumes from building up. The fresh air inlet and the outlet for the combined fumes and air is located at opposite sides of the engine. For a V8 engine the valve is located on one valve cover and the fresh air inlet is located on the other valve cover.

 

[nkaminar]

I apologize for two responses. There has been some issues with the site so it looked like the first one did not get published.

 

[Fullraceflathead]

From a performance standpoint there's hidden horsepower in having a crankcase that's running under a vacuum You will have enormous gains in ring seal and oil control let alone potential oil leaks Although the model A. does not have a conventional rear main bearing seal like more modern engines!!!

 

[nkaminar]

Full Race, you bring up a good point. With the stock engine there will be air sucked in at the rear labyrinth. I had not thought about that. I wonder how much air that would be.

 

[alexiskai]

Full Race, you bring up a good point. With the stock engine there will be air sucked in at the rear labyrinth. I had not thought about that. I wonder how much air that would be.

1. Use a T junction to attach a vacuum gauge somewhere in the PCV circuit
2. Start and idle the engine so it's in a high vacuum state
3. Remove the fresh air intake filter and plug the intake with your finger
4. See if the gauge moves

If the gauge doesn't move then the rear labyrinth is supplying fresh air.

 

[ModelA29]

PVC systems were nothing more than an early emissions device. Instead of having an open "road tube". It take the oil and gas fumes from inside the engine and routes them into the intake stream to be burned rather than just dumping the fumes into the atmosphere.
I prefer the late 30s solution to the .gov mandated 60s solution.

 

[ModelA29]

From a performance standpoint there's hidden horsepower in having a crankcase that's running under a vacuum You will have enormous gains in ring seal and oil control let alone potential oil leaks Although the model A. does not have a conventional rear main bearing seal like more modern engines!!!

A PVC system will never move enough air for that to happen. That requires moving a lot of air. You'd spend more hp running the pump than you'd ever gain on a Model A.
My GT1 car has a dry sump oil system and the scavenging is aggressive enough to pull a vacuum in the crankcase. Top Fuel dragsters started the vac pump idea and now they are pretty common (when the rules allow) in all the pro race series.


https://inthegaragemedia.com/horsepower-from-nothing/

 

[nkaminar]

Update: Model A vent tube vs. Model B vent tube.

I changed out the Model A vent tube for a Model B tube. I brazed in a 1/2 OD steel tube to fit just short of half way into the inside diameter and just below the new cap I bought. See photos. The cap is MotoRad 6035-02 and fits perfectly into the B tube. The seal compresses when you twist the T handle to completely seal the tube. The 1/2-inch steel tube connects to the catch can.

With the A tube I got 1/4 cup of oil out of the catch can in 60 miles (photo on top of head light). With the B tube it was reduced to about 1/2 tea spoon in 60 miles. With that amount of oil coming out of the engine it could be possible to not use the catch can when the B tube is used.

Added: Even with fresh, clean oil in the crankcase, the oil that the catch can caught was black. This indicates to be that what is coming out is contaminated with the blowby and that the positive crankcase ventilation system is doing its job.

 

[philosofriend]

I believe that the air is being sucked out of the crankcase and into the intake manifold, not into the crankcase. That’s the way it’s done on a “modern” car too.

Fresh and filtered air is sucked into the crankcase. Then air mixed with blow-bye gasses is sucked out of the (other side of the) crankcase and into the intake manifold.

 

[30Tudor]

With the A tube I got 1/4 cup of oil out of the catch can in 60 miles (photo on top of head light).

A 1/4 cup of oil in 60 miles is a lot going up the A fill pipe! Sounds like the B-Tube is the only way to go?