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Postive Crank Ventilation Valve

joc

By joc,
in Engines & Forced Induction

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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
  dead32 said:
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
  mad082 said:

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.

  Quote

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:

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