PCV Valve / Rocker Cover Breather

Discussion in 'ENGINE' started by Aidan@EP-styling, Nov 25, 2008.

  1. Well all,

    Ok I know that the pcv valve opens only under vacuum bla bla bla...

    But both the pcv valve and rocker cover breather expell gasses that have blown down past the piston rings. So do the 2 things do the same job? Why is there two breathers???:confused:
  2. Dub-Se7en

    Dub-Se7en Paid Member

    the pcv valve will spend a lot of time closed as the the engine goes into positive pressure (ie when you hit boost) , this is where the secondary breather helps ;)
  3. Sean_GT

    Sean_GT Paid Member

    Would the breather on the engine not having a filter or anything cause any problems because mine is just the rubber tube with nothing on the end of it
  4. Dub-Se7en

    Dub-Se7en Paid Member

    not really just might draw fumes into the car and splash some oil out.
    2 people like this.
  5. but then I dont see the point of having a pcv valve in the first place if the rocker cover breather can do the job on its own???
  6. davehart

    davehart Paid Member

    I suppose its there as a back up in case one gets blocked up.
  7. weeJohn

    weeJohn Paid Member

    The pvc valve is there to do 2 jobs. Remove oil and water vapour in the hot air at the top of the head. This is why there is a second skin to speak inside the rorcker cover, to allow these vapours to accumulate there and as a backsplash as well. Its second job is to help relieve positive crankcase air pressure.

    Remember that the second pipe on the rocker, the one we fit beathers to, is where the stock recirculating dump valve used to connect onto.

    Fitting a breather on there prevents the build of of crankcase air pressure. If its just a pipe there it will do the same thing, but you also get negative pressure sometimes as well, which means that dirt could get sucked in there and into the oil and into the bearings as well. The filter also allows air to flow through it when the pvc valve has vacuum applied to it, preventing a negative pressure in the crankcase and allowing good vapour release.

    It makes no difference to performance sucking these vapous in at idle. We fit catch cans to prevent the heavy oil vapours getting through, as when the air is sucked into the can it drops slightly in pressure and allows the heavy vapours to accumulate in there.

    The pvc valve is a 1 way valve to prevent boost pressurising the crankcase as was said before. If it is connected to the intake manifold, even if its through a catch can, on idle it will still suck the bad vapours out. The vapours will not get sucked out if vacuum is not applied to the pvc valve.

    Really we need both for a belt and braces job.
    Last edited: Nov 25, 2008
    4 people like this.
  8. MWP

    MWP Registered User +

    The valve is also used on NA engines...
    It also has the purpose of stopping backfires into the inlet manifold from igniting the crank case gasses (=BIG bang).

    The benefit to pulling the gasses out of the crankcase is it helps prolong the life of the engine oil.

    It still amazes me how many people install catch cans wrongly.

    Actually most road going turbo cars are only under boost a fraction of the time (say 5%-10%?).
    Th rest of the time the inlet manifold is at vac, so the PVC system is in operation.
    Last edited: Nov 26, 2008
  9. weeJohn

    weeJohn Paid Member

    I agree completely, people just disregard the pvc valve.

    Someone went to the bother of designing a catch can so as there would still be this sort of system in place, but the can would prevent the less desirable heavy oil and water vapours getting into the intake.

    A catch can is an improvement to the system, not an alternative.
  10. Mike_ep91

    Mike_ep91 Paid Member

    so can the pcv valve coming from the rocker be connected to a catch can and have the outlet on the inlet mani blocked off??
  11. MWP

    MWP Registered User +

    No. This is what 99% of people do and is wrong.

    Do this:
    PCV -> catch can inlet
    catch can outlet -> inlet manifold

    The catch can should be the sealed type (no air filter on it).
  12. Mike_ep91

    Mike_ep91 Paid Member

    what about the other breather then??
  13. MWP

    MWP Registered User +

    Goes to the same place it does from the factory.
  14. weeJohn

    weeJohn Paid Member

    If you have removed the standard recirculating valve, which is where the other breather was attached to, you can fit a small filter onto the rocker cover there.

    When it was connected there it was connected to the efi pipe, so it "breathed" through the main inlet filter.
  15. Gee

    Gee Paid Member

    So mine is wrong? (Picture below)

    I was told this would help the engine 'breath' and I have seen other big HP Starlets setup like this?

  16. munday

    munday Paid Member

    The problem with venting the catch can back into the inlet is that you still allow oil vapour back into the inlet causing octane dilution
  17. Gee

    Gee Paid Member

    Would this cause a loss in MPG?
  18. munday

    munday Paid Member

    Possibly, id be more worried about detonation. TBH you probably wont be getting very much octane dilution, but i wouldnt want to risk it ;)
  19. weeJohn

    weeJohn Paid Member

    Thats the whole point of a catch can mate, when the vented gasses pass through the catch can the vapours are "dropped" in the can to prevent them going into the inlet.

    Not 100% efficient, some vapours will still get through, but the majority wont.

    Gee in my opinion your set up is wrong. There is nothing in your set up that will remove the vapours from the crankcase into the can. Have you ever had to empty it?

    This is my setup, please dont think this is a way of showing how yours is wrong as its different from mine, this is the way I do it.


    I use 2 pvc valves so as the valve nearest the inlet mani closes under boost, preventing the can getting pressurised. The valve in its normal place stops anything in the can being sucked back into the crankcase. The breather filter does as it named, its on the other pipe off the rocker, where the standard dump valve hose was connected.

    I get an emulsified oil in my catch can, like a melted bar of fudge.

    I empty it about every 6 months, the ammount in it varies.
    Last edited: Nov 30, 2008
    1 person likes this.
  20. Gee

    Gee Paid Member

    OK, so my setup is wrong.

    The outlets are welded in place, so can't do nothing about that now unless another rocker cover is used.

    How would you 'suggest' my catch can is setup?

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