How is a PCV Valve rated?
- Thread starter BIGREDRASA
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66 Fastback
Famous Member
I tried posting a couple of times yesterday, but they did not take.
XPC66, I have learned something and I did not know there were some valves that operated in the manner that you described. I apologize if I came across too strong. Before you dismiss Wiki:
http://en.wikipedia.org/wiki/Pcv_valve
If you read the full description at Wikipedia, it describes a valve that somewhat operates as you describe. The valve is normally held open by the weak spring and allows full flow into the manifold. However under higher level of vacuum, it begins closing and passes less crankcase vaports into the manifold. The valve profile is tapered such that it flows less as it closes, but it does not completely shut off. The valve has two valve seats and if a backfire were to occur, the valve would close on the other end preventing flow back into the crankcase. Apparently there are more advanced PCV valve designs than some of the old ones. So you are correct on your observations.
However, last night I pulled two new PCV valves that I had laying around. One was off of a '66 Pontiac and one was off of a '65 Lincoln. Both of them operated fully open at high vacuum and operated as simple check valves with little to no flow being allowed back into the crankcase. Neither valve shut off or reduced the flow with a high pressure differential across the valve in the direction of flow, which corresponds to what other people have observed on their valves.
While the PCV 101 thread that I posted may not be 100% accurate, it gives a history similar to the Wikipedia history, but it does not describe the actual valve mechanics. I think your comments that it "smacks like a piece of revisionist history" are incorrect. I do agree that the flame arresting is a secondary consideration and as it points out, many fuel injected cars eliminated the check valve. But your postings that may be ignoring the earlier, simpler PCV valve construction may be more of a revisionist nature. Not all PCV valves had the two valve seats.
But as I said, I learned something new yesterday and for that I thank you.
Doug
XPC66, I have learned something and I did not know there were some valves that operated in the manner that you described. I apologize if I came across too strong. Before you dismiss Wiki:
http://en.wikipedia.org/wiki/Pcv_valve
If you read the full description at Wikipedia, it describes a valve that somewhat operates as you describe. The valve is normally held open by the weak spring and allows full flow into the manifold. However under higher level of vacuum, it begins closing and passes less crankcase vaports into the manifold. The valve profile is tapered such that it flows less as it closes, but it does not completely shut off. The valve has two valve seats and if a backfire were to occur, the valve would close on the other end preventing flow back into the crankcase. Apparently there are more advanced PCV valve designs than some of the old ones. So you are correct on your observations.
However, last night I pulled two new PCV valves that I had laying around. One was off of a '66 Pontiac and one was off of a '65 Lincoln. Both of them operated fully open at high vacuum and operated as simple check valves with little to no flow being allowed back into the crankcase. Neither valve shut off or reduced the flow with a high pressure differential across the valve in the direction of flow, which corresponds to what other people have observed on their valves.
While the PCV 101 thread that I posted may not be 100% accurate, it gives a history similar to the Wikipedia history, but it does not describe the actual valve mechanics. I think your comments that it "smacks like a piece of revisionist history" are incorrect. I do agree that the flame arresting is a secondary consideration and as it points out, many fuel injected cars eliminated the check valve. But your postings that may be ignoring the earlier, simpler PCV valve construction may be more of a revisionist nature. Not all PCV valves had the two valve seats.
But as I said, I learned something new yesterday and for that I thank you.
Doug
66 Fastback
Famous Member
I may find that I am wrong, but I think I'll cut some old valves open as Jamyers did. I suspect the one off of my Pontiac matches his old GM model.
With regards to the calibration and port size, all I have ever seen is that the PCV system and the carburetor were designed with orifice sizes calibrated to each other. Other than that, I have not seen any specifics.
Doug
With regards to the calibration and port size, all I have ever seen is that the PCV system and the carburetor were designed with orifice sizes calibrated to each other. Other than that, I have not seen any specifics.
Doug
In regards to the original reason for the thread, I had the same question when adapting the road draft system to PCV on my Y-block last year.
There are approximately eight or nine thousand different PCV valves listed. I never found any "specs" either. So I chose one that made fabrication a little easier and a little cheaper.
Didn't work. Not enough flow. I didn't figure out why not enough flow, too many variables, but it was obviously not enough. I thought, well a 292 is about the same as a 300 displacement-wise, and it's the same old tech type of engine, so I bought one for a 300-6. Redid my system to make it sit in its normal position (upright, which is also important, some valves are designed to operate horizontal, some aren't) and it all worked perfectly.
Years and years ago I thought the only thing the PCV valve did was act as a check valve. While that may be one of functions it provides I am no longer sure that was even one of the design parameters. Not sure at all. The lowly PCV valve is actually a nicely tuned little piece of work, set just so shape-, weight- and spring pressure-wise to do it's job on a particular engine. There's a lot going on in there, quite accurately controlled from one end of the vacuum range to the other.
I can only say to find the right one may take an educated (word used loosely) guess and then some trial and error. I got lucky and only bought two including the one that worked.
There was a guy in my former car club that had a shop-built system on his 327. Blowby all over the place. He thought there was something wrong with his engine some way or another, but when I looked at his set-up I saw what looked like the same valve I had tried on my engine the first time. He probably used it for the same reason I did.
I got the feeling he did not believe me about simply switching to a bigger valve (easier said than done) and I know he he did some work on the engine looking for his problem but I don't know if he changed the valve. I'll see it again soon enough, bet he did. He'll have his own reasons, not because of my suggestion. (One of those guys.)
Roger
There are approximately eight or nine thousand different PCV valves listed. I never found any "specs" either. So I chose one that made fabrication a little easier and a little cheaper.
Didn't work. Not enough flow. I didn't figure out why not enough flow, too many variables, but it was obviously not enough. I thought, well a 292 is about the same as a 300 displacement-wise, and it's the same old tech type of engine, so I bought one for a 300-6. Redid my system to make it sit in its normal position (upright, which is also important, some valves are designed to operate horizontal, some aren't) and it all worked perfectly.
Years and years ago I thought the only thing the PCV valve did was act as a check valve. While that may be one of functions it provides I am no longer sure that was even one of the design parameters. Not sure at all. The lowly PCV valve is actually a nicely tuned little piece of work, set just so shape-, weight- and spring pressure-wise to do it's job on a particular engine. There's a lot going on in there, quite accurately controlled from one end of the vacuum range to the other.
I can only say to find the right one may take an educated (word used loosely) guess and then some trial and error. I got lucky and only bought two including the one that worked.
There was a guy in my former car club that had a shop-built system on his 327. Blowby all over the place. He thought there was something wrong with his engine some way or another, but when I looked at his set-up I saw what looked like the same valve I had tried on my engine the first time. He probably used it for the same reason I did.
I got the feeling he did not believe me about simply switching to a bigger valve (easier said than done) and I know he he did some work on the engine looking for his problem but I don't know if he changed the valve. I'll see it again soon enough, bet he did. He'll have his own reasons, not because of my suggestion. (One of those guys.)
Roger
66 Fastback
Famous Member
That PCV 101 post off of the Corvette site had mentioned that the '61 High hp models equipped with a PCV system had problems with oil blowing out the breather caps at high rpm's. The engineering "fix" or change order was to install a non-vented cap on the high rpm engines. This was the first year of the PCV system on the California Corvettes. Other modesl still had the road draft tube. But it points out that the Crankcase vapors over-powered the PCV system at high rpm's and blew vapors out onto the engine.
Doug
Doug
66 Fastback":17eifvfi said:
I'd guess this had more to do with it than anything else. They probably had made an educated guess and hadn't done enough trial and error yet.
This is could be the type of thing that began turning the list of available PCV valves into what it is today.
R
Just for giggles, I'm going to find a 4 cylinder valve that has the same physical dimensions, and try that. I'll also try the V8 type.
In a previous thread about Holley Webers we had discussed the effects of the PCV Valve on AFRs. My major issue is an extremely lean condition that caused severe plug fouling (the flaky, white kind).
Before assuming the cause was simply lean jetting, I want to make sure the rattling PCV valve is not part of the problem. This is the same valve that worked well with the Holley 1100 and LOM dizzy. The one change evident is the slightly lower vacuum reading.
Since I lack the sophisticated test equipment required to plot flow characteristics, I'll just go with trial and error. Maybe together we can generate our own reference table.
In a previous thread about Holley Webers we had discussed the effects of the PCV Valve on AFRs. My major issue is an extremely lean condition that caused severe plug fouling (the flaky, white kind).
Before assuming the cause was simply lean jetting, I want to make sure the rattling PCV valve is not part of the problem. This is the same valve that worked well with the Holley 1100 and LOM dizzy. The one change evident is the slightly lower vacuum reading.
Since I lack the sophisticated test equipment required to plot flow characteristics, I'll just go with trial and error. Maybe together we can generate our own reference table.
66 Fastback":1hffy2up said:
Doug I was just having a go at Bort,..... because I can. I was trying to break through a mindset and throwing in distractive comments like pissing competitions dilutes the importance of the message, almost infers my input is comedic.
I still stand by my comments about wiki, even it there is an article that supports my content, there will be another that doesn't, which just allows too much wriggle room.
The PCV valves do vary. The Daihatsu three pot since the early seveties, for instance, has the PCV valve built into the oil filler cap and a baffle box behind it. But it still operates the same way.
Any of the valves are poor when it comes to sealing against positive manifold pressure, so when I have installed turbochargers to engines I have always installed an inline check valve.
If there is too much blowby for the valve to cope under light load conditions, the solution is to fix the blowby. Under heavy load conditions the breather hose will pretty much always take up the slack and feed the vapours back into the carby/throttle body, even on new built engines. It's the "positive" method of drawing vapour in under negative pressure that gives PCV it's name.
On the non PCV "open ventilation systems", many of us older blokes grew up with, the rocker cover was always coated with oil and dust and the cabin smelled of fuel and oil. The problem with these open systems was condensation inside the engine.
When PCV became the new thing, the auto community did what it still does and delved into it's origins and found that Ford had been using PCV valves on army truck engines for nearly two decades, but they called them "metering valves" because they regulated the flow according to engine load. So you can understand why I can't come to terms with Ford using a simple check valve on their cars when they already had a history of using variable PCVs.
The PCV valve may have been introduced in the dometic market to combat emmissions, but it's original purpose was to reduce condensation. The introduced problem of flame arresting was a secondary consideration that was probably solved by default rather than by design.
Thus my comments about revisionist history.
BIGREDRASA":1yykgsvs said:Just for giggles, I'm going to find a 4 cylinder valve that has the same physical dimensions, and try that. I'll also try the V8 type.
........
Since I lack the sophisticated test equipment required to plot flow characteristics, I'll just go with trial and error. Maybe together we can generate our own reference table.
Sounds like a plan. I have vaccum pumps and high end measuring equipment, so I might take a trip down to the wreckers and pull a few different types for test.
66 Fastback
Famous Member
I broke out the Dremel & cut open a PCV valve from the '66 Pontiac. It only had one valve seat. Under a backfire condition, it would seal off flow back into the crankcase. It did not have a ball or disc. It was more like a fat metering rod with the fat end toward the Crankase. The other end the had a stepped down reduced ID that fit into an orifice. In this valve, the light spring normally kept the valve closed at the crankcase end. Any pressure from the crankase would open the valve and flow to the manifold. Under higher vacuum conditions, it would pull the fatter part of the metering rod into the orifice and reduce flow. But there was not a valve seat on the orifice side and flow can not be totally shut off.
With regards to the vapors spewing out of the crankcase vent, my '66 Pontiac Shop Manual had two configurations. Normal cars had a vented oil cap and under high speed operation with lower engine vacuum or times of low engine vacuum, the PCV valve would not pass enough vapor and flow will vent back out the breather as part of the design. However, it stated that California Emission vehicles would install a non-vented oil cap. On the other rocker cover they would run a vent line to the air cleaner. That way in those times when the PCV valve cannot handle all of the flow, the vapors vent back into the air cleaner and do not make a mess on the engine.
Doug
With regards to the vapors spewing out of the crankcase vent, my '66 Pontiac Shop Manual had two configurations. Normal cars had a vented oil cap and under high speed operation with lower engine vacuum or times of low engine vacuum, the PCV valve would not pass enough vapor and flow will vent back out the breather as part of the design. However, it stated that California Emission vehicles would install a non-vented oil cap. On the other rocker cover they would run a vent line to the air cleaner. That way in those times when the PCV valve cannot handle all of the flow, the vapors vent back into the air cleaner and do not make a mess on the engine.
Doug
Using my non-calibrated mouth, I tested a 200/6 valve and another I had lying around. The latter had "EV 68" and "Motorcraft 8112" on the outside, and "22" stamped into the poppet/pintle. The latter valve is black, with a washer crimped into the bottom to form the seat for the poppet/pintle. It was also heavier than the shiny, one-piece stamped valve. Both would rattle when shaken.
When held upside down, I could blow back through both with no perceptible restriction. It appears the the poppet's weight was enough to overcome the spring. However, when held right side up, it took more pressure to blow the poppet off the seat, and increasing pressure resulted in increased flow. Both valves worked about the same, as far as I could tell. Then, again, using my lungs only, it's hard to judge a difference of a couple of PSI or In./Hg. difference.
I may just play with one using a vacuum gauge teed into a hose, with a little shutoff valve inline with the intake manifold. Intersted in seeing what the gauge and observation of the poppet tell me.
When held upside down, I could blow back through both with no perceptible restriction. It appears the the poppet's weight was enough to overcome the spring. However, when held right side up, it took more pressure to blow the poppet off the seat, and increasing pressure resulted in increased flow. Both valves worked about the same, as far as I could tell. Then, again, using my lungs only, it's hard to judge a difference of a couple of PSI or In./Hg. difference.
I may just play with one using a vacuum gauge teed into a hose, with a little shutoff valve inline with the intake manifold. Intersted in seeing what the gauge and observation of the poppet tell me.
XPC66":3ij3j8km said:
Between the board acting up and my ancient camera, I can't seem to get any pics to post here. The valve I hacked open was stamped "176", which should go to my '71 Buick LeSabre.
It has a spring on the vacuum side, and a "short fat nail-shaped" plunger with the larger diameter "head" going toward the crankcase side. There isn't much of any back seat, although it looks like the plunger could get close to the body of the valve (but it doesn't look like it'd restrict flow that much). It's an old valve, maybe the backseat is missing?
By "checkvalve", I mean that when there's no engine vacuum on the manifold side, the spring pushes the plunger to the end of the valve and pretty much blocks off any flow that would be going from the manifold to the crankcase - but would freely allow flow in the other direction.
I had the Buick running this morning, and pulled the pcv valve out of the crankcase just for grins. At idle, there's a lot of vacuum, and quite a bit of flow - enough that taking it loose changed the idle speed a good bit. I'll add that on both the Buick and Ford engines, at idle there's enough vacuum in the crankcase to suck your hand onto the oil filler cap hole.
66 Fastback
Famous Member
My valve is probably similar to jamyers valve being of similar vintage. The fat end sealed up against a washer, not much of a valve "seat", but it can shut off flow.
Until this discussion started, I just thought PCV valves were simply check valves. I did not realize that even the old one basically acted like a fat metering rod. Nor did I realize that newer ones actually allowed flow both ways or had two seats.
Doug
Until this discussion started, I just thought PCV valves were simply check valves. I did not realize that even the old one basically acted like a fat metering rod. Nor did I realize that newer ones actually allowed flow both ways or had two seats.
Doug
jamyers":3p4iezts said:
It may not form a perfect seal, but it will restrict the flow markedly. I have never bothered Googling for PCV valve info before this thread, but there are some very interesting articles out there that indicate it is a subject that many enthusiasts have no idea about, so we are in good company.
I found this article yesterday:
http://www.filtercouncil.org/techdata/tsbs/94-2R1.pdf
This whole thread is an eye-opener for me. Makes me think that the pcv valve on my Buick may be the culprit causing some minor but pesky issues.
Also makes me want to get ahold of a spec sheet. Surely to goodness such an animal exists somewhere on the planet...
Also makes me want to get ahold of a spec sheet. Surely to goodness such an animal exists somewhere on the planet...
Here is the verbatim explanation of the "Positive Closed-Type Crankcase Ventilation System", including the operation of the "crankcase ventilation regulator valve at the rear of the valve cover" on the Ford 240-300 six engines, taken from the Ford factory service manual for 1968.
"Ventilating air enters the crankcase through the oil filler (breather) cap at the front of the valve rocker arm cover (Fig. 8)."
[Fig. 8 is a drawing of a cut away six with arrows showing the flow direction as described.]
"On a closed crankcase ventilation system, the oil filler cap is sealed to prevent air entrance, but the top of the filler cap is connected to the engine air cleaner by a hose to admit clean air to the crankcase. On a closed system, if the crankcase ventilation system becomes restricted, a backflow condition will occur, thereby venting the crankcase gases into the air cleaner and induction air system.
The air flow is metered by the regulator valve. A divider in rocker arm cover diverts air flow from the valve rocker chamber, through the pushrod holes and into the pushrod chamber. Air also flows between the crankcase and rocker arm chamber through the oil drain holes and the pushrod holes in the cylinder head. Air flows to and from the cylinder front cover and crankcase through the front cover drain holes at the front of the cylinder block.
From the valve rocker arm cover, the ventilating air passes through the crankcase ventilation regulator valve [heretofore erroneously called the PCV valve] at the rear of the rocker arm cover and into the the intake manifold through a connecting hose and tube. The air, laden with crankcase vapors, is dispersed in the induction air and fuel mixture in the intake manifold before entering the combustion chambers.
The amount of regulator valve opening or restriction is governed by the intake manifold vacuum pressure. (Fig.9)"
[Figure 9 is the same one xpc66 shows of the valve here.]
"At idle speed, intake manifold vacuum is high. The high vacuum overcomes the spring pressure and moves the valve to the Low Speed Operation position (Fig.9). With the valve in this low-flow position, the ventilating air passes between the valve (jiggle pin) and the outlet port. In this position there is minimum ventilation, but it never completely seals off the air flow.
As engine speed increases and manifold vacuum decreases, the valve spring forces the valve to the full open position (Fig. 9). This increases the flow of ventilating air."
So there you have it. I'll point out (with no malice toward anyone) that no backfire prevention properties are mentioned (this is what made me change my views of what PCV valves are doing) and that the only time vapors are supposed to be going from the breather to the air cleaner is when something is wrong. Also that if you must remove the baffle (divider) in your valve cover it would be a good idea to find a new place for it not too far away.
Roger
"Ventilating air enters the crankcase through the oil filler (breather) cap at the front of the valve rocker arm cover (Fig. 8)."
[Fig. 8 is a drawing of a cut away six with arrows showing the flow direction as described.]
"On a closed crankcase ventilation system, the oil filler cap is sealed to prevent air entrance, but the top of the filler cap is connected to the engine air cleaner by a hose to admit clean air to the crankcase. On a closed system, if the crankcase ventilation system becomes restricted, a backflow condition will occur, thereby venting the crankcase gases into the air cleaner and induction air system.
The air flow is metered by the regulator valve. A divider in rocker arm cover diverts air flow from the valve rocker chamber, through the pushrod holes and into the pushrod chamber. Air also flows between the crankcase and rocker arm chamber through the oil drain holes and the pushrod holes in the cylinder head. Air flows to and from the cylinder front cover and crankcase through the front cover drain holes at the front of the cylinder block.
From the valve rocker arm cover, the ventilating air passes through the crankcase ventilation regulator valve [heretofore erroneously called the PCV valve] at the rear of the rocker arm cover and into the the intake manifold through a connecting hose and tube. The air, laden with crankcase vapors, is dispersed in the induction air and fuel mixture in the intake manifold before entering the combustion chambers.
The amount of regulator valve opening or restriction is governed by the intake manifold vacuum pressure. (Fig.9)"
[Figure 9 is the same one xpc66 shows of the valve here.]
"At idle speed, intake manifold vacuum is high. The high vacuum overcomes the spring pressure and moves the valve to the Low Speed Operation position (Fig.9). With the valve in this low-flow position, the ventilating air passes between the valve (jiggle pin) and the outlet port. In this position there is minimum ventilation, but it never completely seals off the air flow.
As engine speed increases and manifold vacuum decreases, the valve spring forces the valve to the full open position (Fig. 9). This increases the flow of ventilating air."
So there you have it. I'll point out (with no malice toward anyone) that no backfire prevention properties are mentioned (this is what made me change my views of what PCV valves are doing) and that the only time vapors are supposed to be going from the breather to the air cleaner is when something is wrong. Also that if you must remove the baffle (divider) in your valve cover it would be a good idea to find a new place for it not too far away.
Roger
XPC66":31ghwgp9 said:BIGREDRASA":31ghwgp9 said:Just for giggles, I'm going to find a 4 cylinder valve that has the same physical dimensions, and try that. I'll also try the V8 type.
........
Since I lack the sophisticated test equipment required to plot flow characteristics, I'll just go with trial and error. Maybe together we can generate our own reference table.
Sounds like a plan. I have vaccum pumps and high end measuring equipment, so I might take a trip down to the wreckers and pull a few different types for test.
I have seen Ford 2.3 engines (Pinto) that had a valve either between the PVC valve and crankcase or PVC and carb.(I think it was between PVC and carb) It was a quarter turn valve, like a ball valve. I have often wondered if it was used for emissions or idle quality. I know that it could affect the idle but figured it was for fine tuning for emissions.
I mention this because it possibly relates to the original question to some extent, and might be worth adding to your test. It might just be a bandaid, but if it was, it was a factory bandaid.
Thanks guys for sharing some interesting info.
XPC66
I won't go into detail on the ford supercharged lightning, that is covered on a different forum.
The lightning & the supercharged cobra's have a problem where oil is sucked up into the blower intake & clogs the coolant intercooler below the blower.
The only fix i know of that works is a oil seperator which goes into the right valve cover.
This seperator is marketed by L & S out of Baton Rouge, LA
It is the only product that works on these engines.
http://www.lnsauto.com/index.shtml
To get back to the original topic, i believe small engines have a small orifice under vacuum & larger engines have a larger orifice. Bill
I won't go into detail on the ford supercharged lightning, that is covered on a different forum.
The lightning & the supercharged cobra's have a problem where oil is sucked up into the blower intake & clogs the coolant intercooler below the blower.
The only fix i know of that works is a oil seperator which goes into the right valve cover.
This seperator is marketed by L & S out of Baton Rouge, LA
It is the only product that works on these engines.
http://www.lnsauto.com/index.shtml
To get back to the original topic, i believe small engines have a small orifice under vacuum & larger engines have a larger orifice. Bill
