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Crankcase breathing system

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    Crankcase breathing system

    I'm running a Holley on my Federal '73, and I have no complaints whatsoever. This is the setup that came with the project so I never considered going to the Strombergs or the Weber. An old-timee mechanic (since retired, darn it) told me that the throttle shaft is worn making it difficult to active a low RPM idle, but other than that it is fine.
    I have no leaking in the V, but do weep from the sump. If I understand it correctly, a breather system uses manifold vacuum to relieve pressure from the crankcase, which should reduce the oil's tendency ( preference?) to leak from the sump.
    So in my case, why not simply run a hose from from the RH cam cover to a vacuum port at the Holley, or at the banjo connection t hat services the brake servo. Then perhaps add a one way valve in the hose. Is there any reason to add an oil separation tank when the fumes are being cycled back to the Holley?
    Final question - Will this throw off the fuel/air mix, or is the effect negligible.

    Thanks for your help


    this will be a possible cause of servo malfunction.
    if possible drill into the adaptor plate for the carb and fit a small tapping there.
    an oil catch tank in line may help


      You need a PCV in that vacuum line. Crankcase breather systems using PCVs are totally standard stuff for pushrod yank V8s, I am surprised it causes such issues here..

      Basically - breather in one rocker/cam cover, PCV into the other, then from that direct to manifold vacuum. Only thing you have to be careful of is to ensure the PCV doesn't suck up oil rather than fumes, there are "baffled" grommets that will do that for you.

      Just google "PCV valve" and things like this come up:
      Header tanks - you can't beat a bit of bling.



        "Why not simply run a hose from he RH cam cover to a vacuum port at the Holley"

        At low engine rpm there is a very large / high vacuum developed on the vacuum port on a carb. The vacuum ports are below the throttle body in the inlet manifold. So at tickover the vacuum draws in a large volume from the engine which bypasses the carb and the resultant mixture sent to the cylinders is very weak. Tickover becomes very erratic though at higher rpm when driving the lower vacuum and resultant smaller percentage of air bypassing the carb is smaller and the engine will generally run OK.

        Fitting a PCV valve in the line between the cam cover and the carb restricts the flow at high vacuum (tickover) and the engine runs nicely.

        A PCV valve cost varies from about 2.50 to 12.00 and can fit into the line as you describe. The valve needs to be mounted vertically as they contain a weighted valve which lifts under high vacuum and restricts the flow, keeping the combustion mixture "normal", there is reduced crankcase pressure at tickover and less needs to be extracted from the crankcase.. Gravity pulls the weight down and opens the valve at higher revs (low vacuum) and more is extracted from the crankcase.

        PCV valves tend to have the two connectors of different sizes, on a Triumph engine Stag a short 1/2" hose from the RH cam cover will fit directly onto the base of some PCV valves and the PCV's narrower top connection can easily fit directly into a smaller hose straight to the carb. Just buy a PCV valve with suitable diameter connectors.

        If you really want to see what is being sent to the inlet manifold then you can fit a catch tank and marvel at the rather nasty brown acidic water condensed from the combustion gasses with a touch of oil that is trapped, but a catch tank available mounting location for is never easy and the longer pipe runs reduces the throughput of the pipework. - and you also have the dubious pleasure of emptying the tank, and wondering when it is full. Most modern cars just send the gasses to the inlet manifold to be re-burned and exit via the exhaust.

        Just remember PCV valves are working in a high temperature, acidic, nasty environment and need to be replaced every 20 - 50k miles or so - perhaps every decade in a classic car.



          Phil, wilf and Alan
          Thank you for that excellent feedback. It's exactly what I needed to move ahead.


            Hope these might help you Tank was a ebay but from a Honda I drilled and the inserted pcv. One connection to vacuum port on holly other to can cover breather.

            Cheers Glenn .
            Attached Files


              The only problem I have found with PCV systems is that the small bore of the piping involved can actually make leaks far worse if the car is driven hard, the pressure simply cannot escape down the bore of the pipe used.
              When the engine is run at full throttle no vacuum is being generated and piston ring blow by is much higher.

              My first attempt at fixing a leak by fitting a pcv system turned a drip into a dribble. I found the only way to fix it was to fit a one way valve into a T piece between the cam cover and the pcv valve.

              If the pressure in the pipe goes above atmospheric the one way valve opens and dumps the pressure to the front of the throttle disc so the gasses are drawn into the engine rather than creating a stink.

              If the pcv valve is on the outlet side of the catch tank then the one way valve can be tapped into the catch tank, my example was fitted to the inlet side so I had to T into the line instead. The next four systems I built (three for Stag engines, one for a Rover) just used pipework with no catch tank, but it is surprising how much oily gunk is blown down the pipe into the engine intake. On one of my injected engines all the oil tends to run to the left rear intake which results in a puff of blue smoke from the left exhaust on start up, and this one would definitely benefit from a catch tank.

              TV8, efi, fast road cams and home built manifolds. 246bhp 220lbft torque



                What one way valve did you use?



                  I generally use brass plumbing or air valves, there used to be a fish tank valve that was very good but I can't seem to find them any more.
                  I often find the spring in the valve is too strong and needs to be clipped down to provide the minimum opening pressure.

                  Can't really recommend an easy fit one i'm afraid.
                  TV8, efi, fast road cams and home built manifolds. 246bhp 220lbft torque