PCV Valves - not all the same?

[Zed Head]

I was going to post this in CO's "relocating the PCV valve" thread but it's fallen off the "Activity" page and I can't find it.

Have y'all noticed that there is a wide variety of part numbers for PCV valves but they mostly look identical?  Why is that?  Why doesn't one size fit all?  And will a PCV valve specified for a vertical application work the same if it's located horizontally?  Is there a test for PCV valve function that is ore sophisticated than blowing air through it and shaking it to make sure it rattles?  Why is there a spring only on one side of the internal valve?  When does the valve close (it is a "valve" after all)? 

Just wondering if relocating the PCV to the side of the valve cover is as easy as it seems.  I've pondered these things in the past.and realized some things but mostly ignore the valve entirely.

 

Edited March 3, 2017 by Zed Head
is for are and vlave

[wheee!]

Good point. I would think the pcv valve we have right now needs to be installed vertically to work.

[Captain Obvious]

I'm no PCV expert, but I think I can answer some of the questions. It is my understanding that there are three "modes of operation" of the PCV valve and the little plunger (the part that rattles) inside will be in one of three general positions.

First mode is under high manifold vacuum conditions like at idle or light cruise. Under this condition, the plunger will fight against the spring and be pulled towards the intake manifold until it makes contact with a valve seat inside. However, it doesn't make a perfect seal. Either the plunger or the seat will have some cuts in it to allow a known and predictable small amount of air to flow through the valve. At idle and light cruise, there is (should be) a small amount of blow-by produced and that small amount of air through the valve should be enough to keep the crankcase ventilated while not screwing up your mixture ratios.

Second mode is under lower manifold vacuums when the engine is being asked to do more work. Under this condition, there is more blow-by, and less manifold vacuum. The reduced manifold vacuum is not high enough to fight the spring and the plunger will be pushed off the metering seat and allow more air to flow. The lower the vacuum, the more open the valve will become. Also in this mode, if the amount of blow-by is greater than what the PCV can deal with, the flow direction in the vent side (the side off the valve cover) will change direction and push the excess blow-by into the intake tract.

Third mode is any condition that pressurizes the intake manifold, such as a front fire, Under this condition, the plunger will be pushed towards the crankcase (away from the intake manifold), and make contact against a valve seat on the other end of the valve. This seat has no cuts in it and is intended to be a full seal.

So... The PCV completely closes in one direction and almost closes in the other direction, and will pass a variable amount of air between those two points depending on the differential pressure across it.

Why are there so many? Because the flow rates are calibrated and known and predictable. Even if they look the same on the outside, they may flow different due to internal features. I agree, however, that there sure seems to be more than we really need. Seems to me the auto manufacturers ought to be able to standardize a little. I found a catalog or website at one point that actually listed the flow rates, but I can't find it. If it turns up, I'll make sure to post that.

Since they are rated for different flow volumes, I would assume a more thorough test (more thorough than the "rattle" test) would involve measuring the flow.

And about the spring, it fights the intake manifold vacuum and because of that, the spring force is important. And since you want it to close instantly in the case of a front fire, you only need a spring on the one side.

For something so seemingly simple.....

[Captain Obvious]

Forgot about mounting position... I theorize that gravity should have some effect, but I bet it's negligible compared to the internal spring. In other words, I haven't tried it, but I bet it won't matter much. (Famous last words?) 

[Chickenman]

They are calibrated for different flow rates for different engines. But I don't think it really matters a lot on modified cars. Just as long as you are reasonably close. Carbed cars can easily adjust the idle mixture to compensate for a little more or little less flow. PCV operation at idle speed has the most affect on vacuum flow.  280Z/ZX EFI cars sometimes need a bit of tweaking at the air bypass screw on the AFM if a different flowing PCV valve is installed . Modern feed back EFI systems with WB O2 sensors adjust on the Fly. 

Edit: Different calibrations probably have more to do with emissions control figures than anything else. Those have to be bang on. 

Just an FYI.  A  number of big boost Audi and VW engines, at Vortex/VW forum,  are now experimenting using Datsun 280ZX Turbo PCV valves. The factory Audi/VW 1.8T OCV system post 2000 is overly complicated with a lot of plastic parts in the PCV system. Including the PCV valve itself. These plastic parts often crack and fail with heat.

The 280ZX Turbo PCV valve is nearly indestructible. It can be plumbed into a modified system and works much better than stock. The Turbo PCV valve also has a silicone " flapper seal ". The silicone seal  is needed on boosted cars , so as to not pressurize the cranckcase under boost. Under boost, a secondary venting system ( utilizing factory design )  is used that bypasses the PCV valve. So far the 280ZX Turbo PCV valve has worked very well. 

Edited March 4, 2017 by Chickenman

[madkaw]

I thought for the longest time that the design ( shape) was pretty much the only difference . Then I realized the amount of bypassed air was huge on some of them. I swear my triples idled better with I heavy by pass type valve vs the OEM.


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[Mark Maras]

 Perhaps I've over simplified the pcv system in my own mind. Thought it was to relieve excess pressure in the crankcase and under the valve cover caused by leaking piston rings (usually the culprit). In the old days the oily air was vented out of the crankcase down towards the road.. Later a catch can was added to catch the oil. No pcv valves back then. I believe pcv valves were added when the crankcase vapor was plumbed back into the air intake to be burned. I assumed that the size of the valve was related to cubic inch displacement, and a vertical pcv didn't need a spring.

 I don't doubt that some of these assumptions could use some tweaking but what I really don't understand is how the pcv system can affect engine vacuum. Being old school, (carbs) I'm picturing the pcv leading into the back of the air filter. Were some of these systems plumbed into the intake manifold? That's the only way I can think of that would affect engine vacuum. We used to pull the oil cap (and pcv valve) when the engine was running to check for excess crank case pressure when looking at used cars. Thanks in advance for schooling me.

[Zed Head]

A more correct name for it would probably be "scavenging" system, based on what appears to be its design purpose.  The fresh air is meant to move gases out of the engine internals and in to the intake system. PCS?

I found a few interesting articles out there, one about choosing a different PCV valve after modifying an engine for more power.  It didn't make a lot of sense since the crankcase volume was the same.  Not clear to me how the desired flow rate is determined.  If it's based on blowby then we should all be changing our PCV valves as engine wear increases blowby.

I tried to find some old PCV valves yesterday to examine but I've removed them all and put them n a container somewhere, from the last time i wondered about them.  I was pretty sure that the spring was on the bottom, or crankcase side and the pintle sat on top of it.  Which doesn't fit CO's description, since then gravity is all that keeps the valve open at low vacuum.  That's why the thought about vertical versus horizontal came to mind.

All of these years of diagnosing people's engine problems but we rarely say "check your PCV valve".  Who knows.