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Venting breather tube into exhaust

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  • Venting breather tube into exhaust

    I'm building a 4-place Bearhawk and using an IO-540. I was looking up the skirt of the CubCrafters XCub the other day and noticed that its crankcase breather tube vents right into the exhaust collector. That seems pretty slick, since supposedly the oil mist gets vaporized instead of coating the belly.

    Anyone done this? Does someone like Vetterman build custom exhausts to allow it?

    Is it the kind of thing where you would also use an air-oil separator in combination?
    Last edited by Zzz; 07-03-2017, 11:45 PM.

  • #2
    I'd want to make sure that you had a plan to keep the vent tube from getting blocked by burned oil deposits, and to make sure that you weren't going to have higher pressure at that spot in the exhaust than in the crankcase. Perhaps those are non-issues?


    • #3
      I have no experience but have done some reading. People are messing around with careful and watchful for the clogged breather. has some threads on this topic.
      The Backcountrypilot thread echoes Jared's Concern

      Last years Oshkosh seminar about Engine Blueprinting discussed this. The speaker primarily had a Drag Racing engine builder background, and one of his suggestions involved venting the crankcase to the exhaust and some how created suction at that point. A crankcase breather that has a suction on it produces more horsepower. I forget if the shape of the crankcase vent protector of the exhaust tubing created the suction....

      Brooks Cone
      Southeast Michigan
      Patrol #303, Kit build


      • #4
        The exhaust has a lot of solids in it, lead mostly, which coat the belly anyway.
        I would expect the belly will still get oily, just a more even distribution of oil.

        I wouldn't see blockage as a problem provided the connection was large enough. But that might lead to a lot of "communication" between the two systems.... I wonder what the changes in exhaust pressure would do to the crank case? You don't want nasty exhaust products perculating around your crank / cam / gears....

        My oil breather has the most basic of dog-legs in the system, with a small step down in pipe size. I see almost zero oil on the belly, with a free breathing system like this. General seepage from the engine connections is more of a factor, but still incredibly minor.
        Last edited by Battson; 07-04-2017, 03:05 PM.


        • #5
          If an engine crankcase breather tube becomes blocked by insects/ice or whatever - that scenario gets very bad quickly. Some builders put a hole up near the top to prevent a total blockage. Mark


          • #6
            I know a race car builder that uses the breather into the exhaust to draw a vacuum in the crankcase for more horsepower - design details are important to not have exhaust into crankcase and to have the desired effect. Definitely put the "whistle" hole in the breather up high to prevent blowing seals in the event of icing. It should work the same in the event of carbon buildup in the breather as well (which I have read of multiple occurrences of that). Make it easily inspectable/cleanable if you do it. I wouldn't see much need for an oil separator with it.


            • #7
              If you decide to pursue this, check out He sells a kit to tie the breather into the exhaust and most importantly a bypass option to prevent problems if the breather gets clogged.

              John Ciolino
              Patrol Builder


              • #8
                I have tried this trick on a race car. The HP gain is not that much.

                What you will likely find is an increase in oil consumption because you will be sucking the oil filled air from inside the crank case. The reason I quit doing this on my race car was the occasional plume of oil smoke that I would expel from the exhaust. Race officials will not let you run if you are putting out that amount of smoke.

                I used an small muffler from a lawn mower engine as an oil air separator. I did not think to have a separate drain for the oil from the muffler. So every so often the oil in the muffler would go out the exhaust pipe.


                • #9
                  My reasoning for doing such a thing would not include power increase. It was explained to me that the XCub uses it to simply to easily vaporize oil expelled from the breather. This is a fully Part 23 certified Cub-type aircraft that's meant to work.

                  I'm not even sure what kind of loss to expect out the breather on a 540. My old O-300 wasn't too bad, but the belly did seem to have a perpetual sheen to it.

                  It seems the point of introduction in the exhaust would be a critical design factor.

                  JBC is this the kit you're speaking of?
                  Last edited by Zzz; 07-05-2017, 11:14 AM.


                  • #10
                    On my race cars, I use an air/oil separator with the oil return to the sump tank. It works very well with a dry sump oil system. It should work with a conventional oil system as well. The oil return has to be below the oil level in the sump.

                    The one advantage that I can see with venting to the exhaust collector is the simplicity of the system. You just have tubes from the case to the exhaust collectors. Ideal might be some combination of both a oil recovery system and using the exhaust to generate a negative pressure in the crank case.


                    • #11
                      Some interesting reading:



                      • #12
                        Yeah, I would NOT return oil from the separator to the engine. Take a look at the contents of that separator before you decide to return it to your oil sump – t's pretty nasty. Several friends of mine run little "collector" tanks (vented on top to the atmosphere) and just clean them out periodically. The stuff that comes out is truly NASTY.

                        As for what kind of loss to expect from your O-540... I had a Commander 114 that had the IO-540. Not exactly the same engine, but pretty close... If I filled the oil up to the 8 quart line and flew a 1/2 hour, it would nicely paint the belly of the plane with a coat of oil, and I would find that I was down to 7 quarts or so... By trial and error, I found that 6 1/2 quarts was the "sweet spot" for my engine. Any oil I added above the 6 1/2 quart line would find its way onto the belly within 45-60 minutes of flight. But if I filled it only to the 6 1/2 quart level, the plane's belly stayed clean (well - mostly - there was still some exhaust "slime" that would slowly build up) and the engine used only about a quart of oil every 25 hours!
                        Jim Parker
                        Farmersville, TX (NE of Dallas)
                        RANS S-6ES – E-LSA powered by 100 HP Rotax 912ULS


                        • #13
                          I can put 12L of oil in mine (close enough to 12 quarts) and it doesn't spit a drop more than usual (which is practically nothing, not enough to see).

                          It does spit out oil if we get into a negative g-force situation.

                          My research suggests every engine is very different in terms of oil consumption and breathing oil.

                          Mine uses about a litre / quart of oil every 25 hours, no matter how much is inside (within limits).

                          Happy to share my breather design if you like.


                          • swpilot3
                            swpilot3 commented
                            Editing a comment
                            I'd like to see you breather design Battson. Thanks!

                          • huntaero
                            huntaero commented
                            Editing a comment
                            I would like to see what you have come up with.

                          • Battson
                            Battson commented
                            Editing a comment
                            It's nothing special!!
                            Just a pipe with a step and dog-leg in it. I just wanted something simple, light, and (seemingly) effective. The idea of a big heavy seperator didn't appeal.
                            I'll post photos below

                        • #14
                          On my RV6 the breather tube terminates directly above the exhaust pipe, so the oil drips onto the out side of the pipe. It does a great job of keeping th belly clean.


                          • #15
                            What’s everyone making their oil breather tubes from ?

                            I’ve got the Vans FF kit that includes an aluminium tube designed to run down the firewall, with a flexible hose at the top that attaches to the breather outlet. Simple and lightweight, except that I’m struggling to fit it in the Bearhawk as the engine mount design is different.

                            Any ideas appreciated.
                            Nev Bailey
                            Christchurch, NZ