model b -- P C V positive crankcase ventilation

[Milton]

"In a turbo or super charged engine (intake manifold has pressure, not vacuum) how do you make a PCV system work properly?"

By taking the vacuum from between the carb and blower/turbo.

"run a draft tube from the upper rear of the valve cover to a venturi attached to the end of the tailpipe, or perhaps within a slightly enlarged pipe somewhere after the muffler. The slight suction produced by exhaust past the venturi would do the work, but again, wouldn't pass modern emissions."

I would hope the slight suction could overpower the flame front in the unburnt fuel component of the blowby.

 

[Milton]

What is the difference between a Crankcase Ventilation system, and a Positive Crankcase Ventilation system?? Do we think the terms are, --or can be used interchangably?

Crankcase ventilation works only when the car is moving (draft tube).

Positive Crankcase Ventilation works whenever the engine is running.

 

[Milton]

Why is that?

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The fresh air supply prevents the crankcase vacuum from building up to match the manifold vacuum (minus the blowby).

 

[Ron Engen]

Mike, you are right on with the exhaust creating the vacume to relieve the crank case pressure. I did this on my model b engine and it works 100 percent. Moroso sells a kit with the exhaust bung and a check valve and the breather cap with the fitting on it. It uses a five eights high temp oil line for the relief. I also added another breather to the motor for cross flow breathing. AS I said I have a model b engine ,so I added a breather to the block where the fuel pump would normally be. Life is good
Ron

 

[bbrocksr]

First off, I have no problems whatsoever with you'all disagreeing with me on this because I have not done actual research to know exactly why my way OR your way would --or would not work.

Lets start off by the PCV name. With a Model A crankcase, why is, --or isn't its present configuration "Positive"? Since the pistons moving and compressed air is leaking past the piston rings, the crankcase likely has a slight "Positive" pressure where vapors can/could easily escape the oil filling tube by that force. So "what" is the PCV system doing that overcomes this?

I am of the opinion to control windage, you must create negative pressure in the crankcase. To use someone's "house" illustration, with the front door left standing open (oil fill tube) and drafty windows (rear main seal, dipstick, front seal), I just cannot see how there is enough manifold vacuum volume to overcome these "leaks" to create negative pressure in the crankcase. As far as removing corrosive gasses, think about how those gasses are getting churned in the rotating air from the crankshaft, ...and think about the path those gasses would need to travel to get into the valve chamber area where they can be vacuumed out.


From my experience with racing engines, a scavenging effect can be made by mounting a tube into the exhaust collector at an angle which will create a siphon effect. Also, some racing applications use a pump tr create a positive vacuum. With regard to un-naturally aspirated engines, usually there is still an air inlet somewhere in the induction system that can create negative pressure.


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In a racing engine It's called "Crankcase Evacuation" Which is lowering the pressure in the crankcase.
Positive Crankcase Ventilation is Fresh air in Old air out through a PCV valve to meter the flow into the intake manifold and prevent backfire into the crankcase. This also works when the car is not moving.
Crankcase ventilation is fresh air in old air out through the road draft tube, Works only when the car is moving.
In this case "Positive" probably means whenever the engine is running it is being ventilated, Not just when the car is moving.
Bill

 

[Chris in CT]

Hey Guys, I've written about this in previous threads on this subject, but a straight-up marine-type of flame arrestor is the way to go. It is simply a vent from the top of the valve galley cover to the carb intake which allows excess pressure to be vented directly into the carb intake - no pvc - no nothing. In my own case, I fabricated a little baffle on the valve cover fitting to ward off splash and oil mist, etc. Works like a charm. No burping oil, no oil slick in the engine bay, and no crankcase fumes into the atmosphere.
Piece of cake... Happy Motoring, Chris

 

[AL in NY]

Chris in CT - sounds like a great idea, but I'm not familiar with "straight-up marine-type flame arrestor". Could you possibly post a photo? Would a marine parts supplier carry something like this?

 

[1931 flamingo]

Chris in CT: Ditto: pictures please. P/n's and where you purchased the parts.
Paul in CT

 

[bbrocksr]

Hey Guys, I've written about this in previous threads on this subject, but a straight-up marine-type of flame arrestor is the way to go. It is simply a vent from the top of the valve galley cover to the carb intake which allows excess pressure to be vented directly into the carb intake - no pvc - no nothing. In my own case, I fabricated a little baffle on the valve cover fitting to ward off splash and oil mist, etc. Works like a charm. No burping oil, no oil slick in the engine bay, and no crankcase fumes into the atmosphere.
Piece of cake... Happy Motoring, Chris

And no ventilation. I guess it depends on whether you want to properly ventilate the engine or just blow the fumes down the carburetor.
Bill

 

[Chris in CT]

And no ventilation. I guess it depends on whether you want to properly ventilate the engine or just blow the fumes down the carburetor.
Bill

I do not know how you arrived at that conclusion!! The cap on the filler tube provides fresh air going into the crankcase, and the fixture from the top of the valve galley cover to the choke bore of the carburetor provides a slight negative pressure to draw fumes into the choke bore, and ultimately re-burn them in the combustion chambers.

To those who want pictures, I'll try to take these and post them, but have not done this before. If I can figure out how to do it, I'll be glad to post them for you.

Chris

 

[bbrocksr]

I do not know how you arrived at that conclusion!! The cap on the filler tube provides fresh air going into the crankcase, and the fixture from the top of the valve galley cover to the choke bore of the carburetor provides a slight negative pressure to draw fumes into the choke bore, and ultimately re-burn them in the combustion chambers.

To those who want pictures, I'll try to take these and post them, but have not done this before. If I can figure out how to do it, I'll be glad to post them for you.

Chris

Why not use manifold vacuum and a PVC valve and have a true PVC system
Which would be more efficient than a slight negative pressure.
Bill

 

[brokenspoke]

Any updates?

 

[ursus]

Hey Guys, I've written about this in previous threads on this subject, but a straight-up marine-type of flame arrestor is the way to go. It is simply a vent from the top of the valve galley cover to the carb intake which allows excess pressure to be vented directly into the carb intake - no pvc - no nothing. In my own case, I fabricated a little baffle on the valve cover fitting to ward off splash and oil mist, etc. Works like a charm. No burping oil, no oil slick in the engine bay, and no crankcase fumes into the atmosphere.
Piece of cake... Happy Motoring, Chris

What Chris describes is basically the crankcase ventilation system used in the 24 million Volkswagen air cooled 4 cylinder engines made in the last century. I got 140,00 miles out of one before having to grind the crankshaft. By then it was noisy but it started every day in all kinds of weather. Simple is good!