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Breathers, separators and catch cans


Well-Known Member
Sep 20, 2003
Corona CA
Sometimes I can't believe how primitive the breather system is on our engines. A rubber hose dangling out of the cowl and spewing oil everywhere . Having done a bit of research on the subject, I figured the must be a better way.

There are separators/catch cans available for aircraft and they are also frequently used in aftermarket installations in souped up cars.

Apart from simply catching any oil that escapes and keeping the belly clean(er), creating a slight vacuum has all kinds of benefits. It improves ring seal and stops oil being forced out of seals, to mention just two. And it's so dirt simple, I can't believe that only a few airplanes have them.

I was curious to try this: my main objective is to stop oil from coating the belly of my airplane and creating a modest vacuum in the crankcase. I'm not interested in returning anything back to the engine, life some systems do.

There are plenty of oil catch cans and separators out there and they would all do the job, but where to get the suction? Manifold vacuum isn't very effective at the power levels we run our engines. There are systems that use reed valves plumbed into the exhaust that take advantage of the positive/negative pulses and that provides directly to the breather. Fairly simple but if the valve chokes up, it could cause problems. But I like how they would simple burn up anything that comes out of the breather.

How about using a simple small Venturi tube sticking out from the bottom of cowl? A short hose to the catch can, and it should provide a modest amount of vacuum. Question is how to design one the right size and how to make it simply and cheaply. An off the shelf small Venturi from Aircraft Spruce is about $70 and it's supposed to provide about 2-3" vacuum which seems adequate for this purpose. As far as I can tell from my research, there is no such thing as too much suction as far as crankcase vacuum goes. Racers often run separate vacuum pumps pulling up 15"!


P.S. During my research I came across some threads in aviation forums where many people (including many certified mechanics!) were adamant that breather tubes should face forward into the slipstream! Good grief ...