PCV Flow Direction - Can I Reverse?

[Captain Obvious]

Is there anything magical about the direction of the flow through the PCV system? I think I could clean up the engine compartment and hose scheme if I were to reverse the flow direction of the system. So do we have any engine vent experts in our midst?

Currently, under low flow conditions (low blow by and high manifold vacuum), it pulls from the tube in the block, through the PCV and refreshes clean air into the nipple on the top of the valve cover. This direction already reverses under high flow conditions, so is there any issue with reversing this direction for the low flow (default) condition?

[Zed Head]

Maybe not magical, but practical?  That large  chunk of mesh at the PCV hose port in the block acts as an oil most separator, I think.  We used to build them from pipe and steel wool to separate oil on vacuum pump exhausts.  If we didn't have them they'd spray a substantial quantity of misted pump oil.  You could add mesh at the valve cover entrance/exit probably.  Or do a catch can. 

Seems like both top and bottom see the same thing, exhaust gases from blowby, either rings or valve seals, and oil mist from moving parts.  

 

[JSM]

Try it I'm curious. What are you planning on doing? Also how will this help clean up anything?

Edited February 26, 2017 by JSM

[Captain Obvious]

Zed, I agree that both connections should see pretty much about the same thing. What I don't know is if maybe the system was optimized for one direction or the other. I'm pretty sure it will "work under most conditions" in either direction, but not sure about the optimization. If my understanding of how PCV works is correct, then there will already be flow in both directions depending on engine conditions and loading, but the reverse flow might be less common than the default direction.

As for the oil separator, I believe there is also supposed to be one stuffed into the hose that comes off the valve cover. Problem is, however, that most people have had to replace that hose because their originals cracked, and they probably didn't know about the separator and chucked it in the trash with the old hose. How effective is the baffling inside the valve cover? I've never seen one with the cover removed.

JSM, Just musing at this point... The thought was that if I could connect the rubber nipple on the duct between the AFM and the throttle body to the lower PCV connection (on the block), and then relocate my PCV to a new drilled and tapped hole on the top of the manifold, I could connect the valve cover hose to that. Would eliminate some of the long hoses draped across the intake system and might look cleaner. But the default flow direction would be reversed.

[Zed Head]

I think that thing in the valve cover hose (edit - left out the "hose" earlier) is called a flame arrestor.  It is kind of odd how Nissan decided to put that port on the very top of the valve cover.  Seems like they could have shot it out the side and saved an inch or two of engine height.

I wonder if the valve cover is designed to distribute the fresh air over the rocker arms, purging the area evenly, leaving no dead zones for sludge buildup.  You could get fancy inside the cover and seal up the air supply area, creating a type of manifold, then place the fresh air inlet at the back or front of the cover to feed the new manifold.  Run the air supply hose next to the fuel rail, or underneath the intake, to streamline things.

Edit 2 - looking at JSM's picture I realize that the filler cap is also high.  So the PCV elbow height isn't that significant.

Edited February 27, 2017 by Zed Head
left out a word

[JSM]

I'm playing with this idea of routing using a piece of bent steel tubing for the breather cover.  I'm wanting everything super clean, neat and hidden.  I was thinking of routing it like this. Blue is one option, orange is another.  Just depends how space goes.

I'll be running the open N42 intake so going under shouldn't be an issue.

[Captain Obvious]

Basically, what I was thinking was that if you were to relocate the PCV valve to the inner face of the intake manifold (kinda above the exhaust manifold), then you could use a short tube that looks like just the first piece of your orange reroute. Short hose from the valve cover to the PCV valve.

And the other hose that goes from the block to the stock PCV would go from the block to the intake duct instead. You still wouldn't see it.

[wheee!]

Relocate your vent out the side like mine! I will have to plumb everything carefully, but the intent is to hide as much as possible on the top of the engine and maybe add a catch can. The new side vent still threads into the portion of the valve cover that has the internal baffle (not shown).

[JSM]

Nice look forward to seeing it done.  Are you going to add the cover over that hole to keep the oil in the catch can down? On the side like that I could see you getting a lot more. 

[wheee!]

The baffle covers the breather tube just like the original setup, I just removed it for the photo. There will be no more oil than normal in there.

[Captain Obvious]

Mark, So when I said I had never seen one with the baffle removed, I was mistaken. Thanks for the great pics and the reminder. And you're right... Relocating the vent fitting to the side would allow you to hide the system even more. Nice idea! So was there anything else inside the baffle area, or is it just covered by the plate?

Using your idea, one could plumb straight out the side of the valve cover. About one foot long straight tube, right into the PCV relocated to the side of the intake manifold. And then hide the other connection completely under the intake tract. I've recently replaced my rubber intact duct with an alternative from another car (yet another off season project), and I've got a (currently capped and unused) nipple connection on the underbelly side of the duct.

Then if I could come up with a way to get rid of the AAR hoses. I could get rid of one of them by using a newer AAR with the hole in the bottom... and then I could... Oh look! Shiny!

[wheee!]

Yup! The side vent is in the optimal position right now as it is protected by the baffle. There is nothing under the baffle plate at all. I will try and hide my hoses too.