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
      One of these is not like the others 😂

      By GTSBoy · Posted

      You are perhaps suggesting that it is not a super car? Well....it's not an AU.
    • The Bogan
      NC MX5

      By The Bogan · Posted

      Dropped the MX5 of this morning to get the hardtop colour matched and some PDR done, they are also going to give it a buff to remove some "stubborn" swirl marks and light scratches in the clear coat Over the last few months the poor little thing has copped some shopping centre car park abuse, I try to not park in them, but hitting Westfield's a few times over the last few months has taken its toll, nothing really bad, but enough of a trigger for a "while we're in there" thingie  Once it's back and all the one colour, with a few dents and mark's removed I'll give some of these products a go and see what they're like  
    • PranK
      One of these is not like the others 😂

      By PranK · Posted

    • V35_Paul
      Rv37 400r questions

      By V35_Paul · Posted

      A locally delivered Infiniti have remote start installed but don't come with the remote start key. You need a new remote start key and they just need it programed. And this is different to making/programing a new key that just starts the car.  
    • Lithium
      Shoota's R33 GT-R RB30/26

      By Lithium · Posted

      Cheers, and cheers for sharing so much of the build and also sharing the glimpse of what turns out to bring a lot of us poor decision makers together haha.  I do recommend learning more about how to manage it, if not considering getting a  formal diagnosis.  The discussion with the psychiatrist I got my diagnosis through was quite eye opening, things I'd not even considered to be ADHD related and hadn't mentioned were things she asked about out of the blue and were common themes with people with my flavour of ADHD.   It's not a label for people who are hyperactive and ill-attentive, there's more to it than that and some of it can be much more challenging or damaging - though there are of course two sides to the thing, and a lot of the stuff we have to go through and work on to live with it make us effectively "better" at other things as well. Aside from the fact that there is some argument I could have a bit of ASD seasoning in there (came up during the diagnosis, and neurodiverse things seem to not stay as a cookie cut) I suspect you need to learn more about ADHD if you are puzzled about how hyperfocus could possibly apply.  I *do* personally use "superpower" with quotes deliberately, but it's 100% an ADHD thing due to the exact reason that lack of focus is also an ADHD thing... Loosely speaking the inattentive side of ADHD isn't the inability to focus, it's the inability to control where the focus goes.  Not being able to sleep because brain is more interested in thinking about a stupid thing I said to a girl I liked 30 years ago, not being able to focus on work because my brain is more keen on putting together the torque management strategy we're going to try out with a drag car next weekend, not being able to focus on a conversation with someone I WANT to listen to and respect because there is a flickering light in my peripheral vision.    If I could just stop work and build the torque management setup right there and then I'd not hear anything else until it was done.  
×
×
  • Create New...