In comments from recent posts, I received instructions on how to plumb the PCV components in my 63, and I just about have all the pieces I need. Now, I am trying to understand how the PCV valve works. From some reading, I understand (or think I do) that at idle (high vacuum, low blow by) the valve is almost closed, and with increasing RPM (lower vacuum, more blow by) the valve opens in a controlled fashion. Where I get confused is when I read that in the event of backfire, the manifold pressure closes the valve to prevent combustion of the vapors in the crankcase. If high vacuum closes the valve, how does high pressure cause the same result? Or is it that in the case of backfire, the mere presence of the pintle in the valve causes enough restriction to prevent combustion in the crank case?
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Re: PCV education
Steve,
on that.
Here is the simple way to look at them all PCV valves...
At zero manifold vacuum it is closed. Then as you increase RPM, generally you go from a lower manifold vacuum to a max manifold vacuum.
The valve opens according to the strength of the spring inside the valve reaching a full open position according to the design parameters of that specific valve.
I've not had a vac gauge on the car when it back fired but I'd guess that the manifold vacuum is pretty close to zero so the valve would be closed. -
Re: PCV education
The light spring inside the PCV valve is unloaded when the engine is off and the pintle valve is wide open at that point. As engine vacuum increases, the pintle is drawn toward the vacuum, working against the spring and stretching it and restricting the cross sectional area as vacuum increases. The pintle is usually cone shaped. On the opposite side of the spring is a check valve. When the engine backfires, pressure in the intake manifold forces the pintle valve the opposite way (compresses the very light spring) and presses the flat check valve against another orifice on the opposite side of the spring.Comment
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Re: PCV education
Steve,
Hold the PCV in your hand and blow against the flow arrow. This is the same as a backfire and you will see how the PCV valve pintle shuts flow.
Now, get a small screwdriver and install it through the direction of flow to push the pintle against the spring and suck on the other side and you will feel the air restriction that the pintle allows for idle (high vacuum). This high vacuum does what you just did with the screwdriver and as the vacuum lowers, the spring pushes the pintle to a more centered position allowing more air to pass through to the carburetor.
Same thing as Joe say's, I hope it's easier to understand.
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Re: PCV education
And what about the PCV systems that have NO VALVE, but instead, they have a metered orifice? Thus, there is constant flow, regardless of manifold vacuum, such as 64-65 375hp FI engines.
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Re: PCV education
Thanks. I thought I understood the operation of the pintle, but I had somehow missed the fact that there is also a check valve arrangement. It makes sense now.Comment
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Re: PCV education
Tom,
In those arrangments like you mentioned with the orifice I guess when the crankcase pressure overcomes the flow of the orifice, the flow reverses and the venturi vacuum pulls from the larger rear port on the air cleaner.
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Re: PCV education
That's why they have a flame arrestor in the air cleaner.
The 0.090" orifice which was used in all 1964 and 1965 Corvettes doesn't have enough capacity to properly scavenge crankcase fumes at WOT, especially in engines with high blowby due to worn rings. By virtue of its fixed cross sectional area, I have to assume that it causes a far from ideal, too large vacuum leak at idle as well. For these reasons I recommend replacing the fixed orifice elbow with an AC CV679 or Purolator PV679 PCV valve, in Corvettes that are driven.Comment
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