Jump to content
SAU Community

Postive Crank Ventilation Valve

joc

By joc,
in Engines & Forced Induction

 Share

Recommended Posts

r33_racer Grand Master

r33_racer

Posted
Posted

I believe that the plastic bung with the hose tail on it on the side of the rocker cover has a one way ball bearing valve in it, so when on boost or any positive pressure it stays closed and only opens during vacuum. Either way its a bad thing having the engine ingest oil vapours, and its also possible if the valve shits itself and gets stuck open you can get serious blow by and lots of wasted power....even though its not really blow by.

dead32 Mentor

dead32

Posted
Posted

the oil gets churned up in the sump of the engine so oily vapors(air+ oil) gets forced up/rises to the top hence the breathers are located at the top, pcv stays closed at idle and opens as throttle is applied,

GeeTR Grand Master

GeeTR

Posted
Posted

Not sure exactly how its setup on a RB25, but the concepts are all similar.

I assume the breather is at the top, as hot blow by gases rise. There are two processes to be aware of.

- The crankcase / upper head area shares its space, via a hose to just in front of the turbo, so there’s a constant vacuum pulling vapors into the engine. Older engines under load can pull a lot of vapor into the motor, increasing detonation and hindering performance. Hence most people disconnecting this.

- The crankcase / upper head area also shares its space with the intake plenum via one way PCV valve, which i believe works in the opposite way then what dead32 described.

Its under most driving conditions, ( idle, partial throttle, cruise) that the valve is open, ingesting fumes (specially on high RPM over run) then on positive pressure, the valve closes, so as not to pressurize the crankcase.

Hope this helps clarify - there’s 101 articles on this forum regarding bloody oil / air separators. Almost every setup imaginable has been spoken of, with diagrams, pictures, Visio diagrams all here to help you.

M

blind_elk Grand Master

blind_elk

Posted
Posted

The oil that is pumped into the rocker covers (among other areas) to lubricate the camshafts, flows back to the sump under gravity through various passageways in the block.

If oil can flow down these passageways, oil vapours can flow up the same passageways from the sump.

I can't see any reason to block off the PCV valve - the catch can is simply a place where the oil vapours can condense, leaving "clean" air to proceed to the PCV valve.

In my own setup, I only have the catch can on the induction side so I don't ingest vapours back through the turbo side. Since the PCV only operates under low vacuum / low load situations, I'm not concerned about ingesting a few vapours in a situation where detonation isn't going to be a problem.

dead32 Mentor

dead32

Posted
Posted

just read over what i posted, my point was that the engine ingests the fumes under vacum, didnt think it was open at idle because the extra air would fluctuate the idle to much fumes dont burn as good oh well.

GeeTR Grand Master

GeeTR

Posted
Posted
just read over what i posted, my point was that the engine ingests the fumes under vacum, didnt think it was open at idle because the extra air would fluctuate the idle to much fumes dont burn as good oh well.

me thinks ring blow by would be minimal to nothing at idle, and its not pulling against a random leak; rather the other opening is between the AFM and turbo… = already metered and between two vacuums.

Don’t get me wrong, I know wot your saying, and perhaps I was a little quick but the PCV valve in my rb26 is by default open, positive pressure in the intake m.fold closes it.

I guess teh Q: is, what pressure difference either side of the PCV valve is required for it to close and when does this happen, rpm load etc - my 2 cents :laugh:

M

mad082 Grand Master

mad082

Posted
Posted

the load/pressure required to close the pcv is minimal. it is just a ball and when there is vacuum it is sucked to one end. when there is pressure it is pushed to the other end. i'd say by 1 psi its closed. i can't remember if there is a spring in there, but i'm pretty sure there wasn't.

GeeTR Grand Master

GeeTR

Posted
Posted

the load/pressure required to close the pcv is minimal. it is just a ball and when there is vacuum it is sucked to one end. when there is pressure it is pushed to the other end. i'd say by 1 psi its closed. i can't remember if there is a spring in there, but i'm pretty sure there wasn't.

Id say it’s significantly less then 1psi. I wrapped my lips round it ( spare the jokes plz) last night and was able to close it with a very gentle vacuum.

As I mentioned, unless mine is broken, by default its OPEN. As in, off the car, the valve is open and needs some air flow, not just positive pressure to close it.

Would there be enough of a + pressure difference in the intake compared to the turbo mouth area to…

- close at idle

- close at cruise

Course not, cus the way I see it, its open to recalculate always, with only a + pressure condition closing it… so unless im crazy... :laugh:

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing


  • Similar Content

  • Latest Posts

    • GTSBoy
      Mistakes were made, my R34 Story

      By GTSBoy · Posted

      Yup. You can get creative and make a sort of "bracket" with cable ties. Put 2 around the sender with a third passing underneath them strapped down against the sender. Then that third one is able to be passed through some hole at right angles to the orientation of the sender. Or some variation on the theme. Yes.... ummm, with caveats? I mean, the sender is BSP and you would likely have AN stuff on the hose, so yes, there would be the adapter you mention. But the block end will either be 1/8 NPT if that thread is still OK in there, or you can drill and tap it out to 1/4 BSP or NPT and use appropriate adapter there. As it stands, your mention of 1/8 BSPT male seems... wrong for the 1/8 NPT female it has to go into. The hose will be better, because even with the bush, the mass of the sender will be "hanging" off a hard threaded connection and will add some stress/strain to that. It might fail in the future. The hose eliminates almost all such risk - but adds in several more threaded connections to leak from! It really should be tapered, but it looks very long in that photo with no taper visible. If you have it in hand you should be able to see if it tapered or not. There technically is no possibility of a mechanical seal with a parallel male in a parallel female, so it is hard to believe that it is parallel male, but weirder things have happened. Maybe it's meant to seat on some surface when screwed in on the original installation? Anyway, at that thread size, parallel in parallel, with tape and goop, will seal just fine.
    • Kinkstaah
      Mistakes were made, my R34 Story

      By Kinkstaah · Posted

      How do you propose I cable tie this: To something securely? Is it really just a case of finding a couple of holes and ziptying it there so it never goes flying or starts dangling around, more or less? Then run a 1/8 BSP Female to [hose adapter of choice?/AN?] and then the opposing fitting at the bush-into-oil-block end? being the hose-into-realistically likely a 1/8 BSPT male) Is this going to provide any real benefit over using a stainless/steel 1/4 to 1/8 BSPT reducing bush? I am making the assumption the OEM sender is BSPT not BSPP/BSP
    • funkymonkey
      Jacks that fit under GTR lips that can reach the subframe?

      By funkymonkey · Posted

      I fashioned a ramp out of a couple of pieces of 140x35 lumber, to get the bumper up slightly, and then one of these is what I use
    • GTSBoy
      Mistakes were made, my R34 Story

      By GTSBoy · Posted

      I wouldn't worry about dissimilar metal corrosion, should you just buy/make a steel replacement. There will be thread tape and sealant compound between the metals. The few little spots where they touch each other will be deep inside the joint, unable to get wet. And the alloy block is much much larger than a small steel fitting, so there is plenty of "sacrificial" capacity there. Any bush you put in there will be dissimilar anyway. Either steel or brass. Maybe stainless. All of them are different to the other parts in the chain. But what I said above still applies.
    • Duncan
      Mistakes were made, my R34 Story

      By Duncan · Posted

      You are all good then, I didn't realise the port was in a part you can (have!) remove. Just pull the broken part out, clean it and the threads should be fine. Yes, the whole point about remote mounting is it takes almost all of the vibration out via the flexible hose. You just need a convenient chassis point and a cable tie or 3.
×
×
  • Create New...