PCV in 1963

[Doug W.]

I purchased a new and "correct" PCV for my 63, 300HP. When connected, it sucks all the time, creating a huge vacuum leak. Is this correct? The old PCV (which was "incorrect" for the car), did not suck air all the time. The engine vacuum would suck the internal valve closed on the old one, but the new one's operation appears backwards. But since the valve scews in only one way, it cannot be installed backwards.

My transmission guy is telling me this valve is bad (even though it is brand new), causing very low vacuum to the powerglide, causing hard shifts. Plus, the car has a terrible flat spot when accelerating, also potentially a result of a vacuum leak.

My very limited understanding of the old PCV's were that they were supposed to be sucked closed by the engine vacuum, unless the crankcase pressue built up to the point of opening them. But maybe the PCV on this car is supposed to put the crankcase under vacuum at all times...? But then air is sucked in the oil fill tube and the tube that goes to the air cleaner, which is pretty much atmosphere pressure. So there would still be a huge vacuum leak in the system, right?

Is my new valve bad, or am I missing something here? Thank you.

Doug

[Joe L.]

Re: PCV in 1963

I purchased a new and "correct" PCV for my 63, 300HP. When connected, it sucks all the time, creating a huge vacuum leak. Is this correct? The old PCV (which was "incorrect" for the car), did not suck air all the time. The engine vacuum would suck the internal valve closed on the old one, but the new one's operation appears backwards. But since the valve scews in only one way, it cannot be installed backwards.

My transmission guy is telling me this valve is bad (even though it is brand new), causing very low vacuum to the powerglide, causing hard shifts. Plus, the car has a terrible flat spot when accelerating, also potentially a result of a vacuum leak.

My very limited understanding of the old PCV's were that they were supposed to be sucked closed by the engine vacuum, unless the crankcase pressue built up to the point of opening them. But maybe the PCV on this car is supposed to put the crankcase under vacuum at all times...? But then air is sucked in the oil fill tube and the tube that goes to the air cleaner, which is pretty much atmosphere pressure. So there would still be a huge vacuum leak in the system, right?

Is my new valve bad, or am I missing something here? Thank you.

Doug

Doug------


Can you post a photo of the valve you have? It should be an AC-590C.

[Joe C.]

Re: PCV in 1963

I purchased a new and "correct" PCV for my 63, 300HP. When connected, it sucks all the time, creating a huge vacuum leak. Is this correct? The old PCV (which was "incorrect" for the car), did not suck air all the time. The engine vacuum would suck the internal valve closed on the old one, but the new one's operation appears backwards. But since the valve scews in only one way, it cannot be installed backwards.

My transmission guy is telling me this valve is bad (even though it is brand new), causing very low vacuum to the powerglide, causing hard shifts. Plus, the car has a terrible flat spot when accelerating, also potentially a result of a vacuum leak.

My very limited understanding of the old PCV's were that they were supposed to be sucked closed by the engine vacuum, unless the crankcase pressue built up to the point of opening them. But maybe the PCV on this car is supposed to put the crankcase under vacuum at all times...? But then air is sucked in the oil fill tube and the tube that goes to the air cleaner, which is pretty much atmosphere pressure. So there would still be a huge vacuum leak in the system, right?

Is my new valve bad, or am I missing something here? Thank you.

Doug

The PCV valve should be sucking slightly at idle, and it can be tested by placing a finger over the end. To my understanding, there should be a SLIGHT drop in idle speed when this is done. I assume that "slight" means about 50.

[John D.]

Re: PCV in 1963

Doug, I agree that you got a bad valve. An original 63 PCV was a 590. No C as this is a later replacement. I have seen two lengths of this PCV valves. One is short er than the other. The short one is for a 63 and early 64.
Blow compressed air thru it Doug.The end for the rubber hose should be a straight thru shot with air. The threaded area is another story.
Just tested 8 of AC 590C and 2 used 590. Not pleased with my test. My air compressor is probably set too high for this test. Not pleased because no two valves acted the same way. Go figure. I have one on the desk here that hardly any air (compressor set at 90) will blow thru the threaded end. (brand new SR valve). I have a bunch of other new ones-new means NOS AC that are all over the board. Need a more accurate way to test these I think.
Maybe lower air pressure.

Old junkers can be brought back to life if they are cleaned in carb cleaner you know. I hate blackening a zinc plated PCV as I don't like getting the insides of the valve rusty.
Now for more scientific answers from the gang.
I like Joe's way of testing them.

[Timothy B.]

Re: PCV in 1963

Doug,

I have a PCV 590c on my 300hp car and you can feel high vacuum at idle but if you put your finger over it the engine speed will only slow a little bit. I think the valve by calibration will suck closed at idle but there is still air that's supposed to pass.


I have been trying to find out why the engine on my 300hp 3461 carter idles with the throttle blades shut. The idle is probably only 600 rpm but I have eliminated all possible vacuum leaks, the only thing I can figure is the combination of the PCV and the choke piston calibrated vacuum leaks.

This same carburetor is used on the 340hp engine so it needs to breath more. If you put you finger over the choke piston vacuum fitting at the choke coil the engine slows to a point where it needs more throttle.

I think in 64-65 the pcv is a restricted fitting at the carburetor and no valve is used so these afb's may have some different calibrations..

[Joe L.]

Re: PCV in 1963

Doug, I agree that you got a bad valve. An original 63 PCV was a 590. No C as this is a later replacement. I have seen two lengths of this PCV valves. One is short er than the other. The short one is for a 63 and early 64.
Blow compressed air thru it Doug.The end for the rubber hose should be a straight thru shot with air. The threaded area is another story.
Just tested 8 of AC 590C and 2 used 590. Not pleased with my test. My air compressor is probably set too high for this test. Not pleased because no two valves acted the same way. Go figure. I have one on the desk here that hardly any air (compressor set at 90) will blow thru the threaded end. (brand new SR valve). I have a bunch of other new ones-new means NOS AC that are all over the board. Need a more accurate way to test these I think.
Maybe lower air pressure.

Old junkers can be brought back to life if they are cleaned in carb cleaner you know. I hate blackening a zinc plated PCV as I don't like getting the insides of the valve rusty.
Now for more scientific answers from the gang.
I like Joe's way of testing them.

John------


Yes, I now find that the original valve was a CV-590 with no "C" suffix.

The point that I was really getting at earlier was that some of these early valves used a 2 piece body with each end threaded into the other. However, they do not APPEAR to be so-configured. If there are wrench flats at both end of the valve, I would try to see if the ends could be separated. It may be that if the valve is of this type that someone had previously opened it up and either re-assembled it incorrectly or left parts out.

I THINK that when the valves changed to the "C" suffix, they went to a crimped together style valve which cannot be disassembled.

[Joe C.]

Re: PCV in 1963

Doug,

You should get yourself a replacement AC CV590c, Fram FV100, Standard V100 or equivalent for test purposes, and any of which should be less than 5 bucks.

Inside the pcv valve is a spring which works against manifold vacuum, and behind the spring is a tapered pintle valve. Under high manifold vacuum, such as idle and closed throttle coasting the spring is pulled compressed against its stop which corresponds to the tapered pintle forming the smallest possible passage between it and the valve housing. As manifold vacuum drops, the spring begins to extend and as it does so, the pintle moves to a larger and larger diameter adjacent to the pcv housing which allows a progressively larger cross sectional area for flow. At WOT and other situations where manifold vacuum is at a minimum, the spring is ALMOST fully extended against the opposite stop, and the cross sectional area for air passage is at its largest. In short, at high manifold vacuum the valve allows small passage for flow, but this flow passes through at a comparably high velocity; at low manifold vacuum the valve allows a large passage for flow but at a comparably lower velocity.

If you remember your college physics: Q = rho v A
where Q is mass flow rate, rho is density, V is velocity, and A is the cross sectional area

In instances where there is positive manifold pressure, such as during a backfire, the spring extends fully and pushes the valve against the opposite stop, closing the valve fully to prevent a possible crankcase explosion.

[John D.]

Re: PCV in 1963

Joe Ciaravino, Saw your explanation on Wikipedia. Maybe they got it from you. We can learn a lot reading yours and their info on PCV's.

LI handles a really nice blackened cast AC590 which should pass judging. It works quite well also.
Replacement AC590C for 5 bucks JC? Where do you buy them that cheap? JD

[Stuart F.]

Re: PCV in 1963

I know the whys and wherefors of a crankcase explosion cause I had one on my 63. The L-76 is the worst one w/o a flame arrestor in the air filter housing. My car had been in storage for several years while I was in SEA. When we pumped out the Wynns, re-set the collapsed valves, changed the oil and put in new plugs, we gave no thought to the PCV. Bad Move! That's why I have the imperfect valve covers on my engine (die parting line). It was below zero that day in Jefferson, WI which may have contributed to the situation. We were in a garage with a low ceiling (door open), and you should have seen those suckers fly and bounce off the ceiling. Something I'll never forget.

Stu Fox

[Joe C.]

Re: PCV in 1963

Joe Ciaravino, Saw your explanation on Wikipedia. Maybe they got it from you. We can learn a lot reading yours and their info on PCV's.

LI handles a really nice blackened cast AC590 which should pass judging. It works quite well also.
Replacement AC590C for 5 bucks JC? Where do you buy them that cheap? JD

John,

It's been a long while, and I don't remember where I read/learned about PCV valve operation. Wikipedia does have a good although not all inclusive explanation.

Here are just 2 examples of where to buy them:

http://compare.ebay.com/like/160442558138?ltyp=AllFixedPriceItemTypes&var=sbar&rvr_id=182426302628&crlp=1_263602_304662&UA=WX%3F%3F&GUID=091fd36e1250a0e2027362d3fe8c327f&itemid=160442558138&ff4=263602_304662

http://auctions.overstock.com/NOS-AC-CV590C-PCV-VALVE/Cars-Parts-Vehicles/item/46163403

[Duke W.]

Re: PCV in 1963

It might be good to review this ten-year old thread:

https://www.forums.ncrs.org/showthread.php?t=4325&highlight=PCV+valve&uid=3713

I think a slightly edited version was also published in The Corvette Restorer not long after.

PCV valves are designed to meter a range of airflow based on engine size and manifold vacuum characteristics and differ physically based on how they are connected to the system - threaded, clamped, etc.

Unfortunately, I've never seen and actual flow data for any, but measured a few myself.

In particular, because my Cosworth Vega ingested oil through the PCV system another CVOA member who had access to lab equipment that could be used to test them tested a couple of other valves of the same geometry and found one that flowed less.

Later I completely redesigned the PCV system, which eliminated the valve completely and the oil ingestion problem.

There appear to be a lot of "restoration parts" that have the correct number, but do not functionally perform as the originals. I believe this is the case with some vacuum advance controls.

In the case of the PCV valve, buying one from a reputable suppliers catalog base on year/model/engine may be a better way to go than from some "reproduction vendor".

Duke

[Joe C.]

Re: PCV in 1963

It might be good to review this ten-year old thread:

https://www.forums.ncrs.org/showthread.php?t=4325&highlight=PCV+valve&uid=3713

I think a slightly edited version was also published in The Corvette Restorer not long after.

PCV valves are designed to meter a range of airflow based on engine size and manifold vacuum characteristics and differ physically based on how they are connected to the system - threaded, clamped, etc.

Unfortunately, I've never seen and actual flow data for any, but measured a few myself.

In particular, because my Cosworth Vega ingested oil through the PCV system another CVOA member who had access to lab equipment that could be used to test them tested a couple of other valves of the same geometry and found one that flowed less.

Later I completely redesigned the PCV system, which eliminated the valve completely and the oil ingestion problem.

There appear to be a lot of "restoration parts" that have the correct number, but do not functionally perform as the originals. I believe this is the case with some vacuum advance controls.

In the case of the PCV valve, buying one from a reputable suppliers catalog base on year/model/engine may be a better way to go than from some "reproduction vendor".

Duke

Because I have reconfigured/upgraded the L76 engine in my Corvette, one of the concerns was to re-engineer the PCV system by upgrading from the fixed orifice (.090") used for 1964-65 to a regulated flowpath. I could not find any quantitative comparison anywhere, relative to PCV flow characteristics. I looked at PCV valves used in both the 1967-1969 Z28 Camaro, and the 1970-1972 Corvette with LT1, reasoning that the engine characteristics based on idle vacuum and displacement were fairly close to my 331 cubic inches. I used the CV746c. Air/fuel ratios at idle, cruise, and WOT are right where I want them, so that is verification that the system is satisfactory.