created Sept 17th, 2006
For about two years now, I've been trying to think of a good way to vent crankcase pressure without letting it take my oil with it. Street cars don't have much of a problem with it, but with an airplane engine running almost wide open for hours at a time, it's a real issue. What I used on my 3100cc engine worked OK, but two pinholes in the weld leaked a little oil, and I'll admit it was fairly hideous. It consisted of a 3" aluminum can welded onto a 1/8" plate that covered the top of the engine, and filled with filter material (OK, it was Scotchbrite, but I never said it). It was vented to an AN-6 fitting which dumped a few drops per flight overboard.
I kept looking for a "big hole" in the case that I could tap into without having to do anything extra, such as welding fittings onto the valve covers. Since I'm not using the factory's mechanical fuel pump, that .925" diameter hole was mighty enticing. It occured to me that if I hogged out the fuel pump's innards and filled it with something to trap the oil droplets while also venting the top to the atmosphere, that might allow the oil to cool and collect on the stuff on the way to allow the pressure to vent to atmosphere.
One problem with aftermarket Corvair fuel pumps is they are heavy, at over two pounds. I knew that the OEM pumps were magnesium and much lighter, because I'd tried to weld up a similar breather back when I built my first CorvAircraft engine in 2000, and I watched it shrivel up before my eyes under the arc of the TIG welder. No fire resulted because of the argon shielding that TIG uses, but it looked a bad trip to me. The other problem is they are made of steel, so machining with my primitive tools was impossible. The OEM magnesium pump is much lighter and machines as easy as aluminum when using woodworking tools.
So I got an OEM magnesium pump (thanks, Mike Moyer), disassembled it, soaked it in carb cleaner for a few days, and then drilled the base through with a 5/8" drill bit. This opened up the base to as large as I dared. This gave me the biggest hole to the outside world, so the air/oil mix velocity would be as slow as possible.
Next I hogged out most of the guts with an end mill bit chucked up in my drill press. I left the top few millimeters of the center hole, along with some of the web that holds it there.
Then I cut out a thin sheet of aluminum to fit over the circular boss on the upper side, and used JB Weld to epoxy it in place. Then two of the diaphragms were hacked up to resemble gaskets for above and below this "center section", and a piece of stainless steel screen perched on top.
The last addition was one of the brass looking Brillo pads. I tried my best to crumble some of it, but it remained intact, passing my test for whether or not I wanted this thing on my engine. I drilled a few holes in the top and plugged the former "inlet" with a 1/8" pipe plug, and added an AN-6 to the outlet. Because I wanted to run the outlet pipe off to the side, I also drilled a new chamfered hole similar to the original one that is used to hold it in place. I learned this last trick because at first I was just pressing up against the base with the pointy bolt, but that forced the shaft up against one side of the hole, leaving a gap in the worst place (the o-ring couldn't fill the gap). Oil dripped down on the pulley, and a little oil was slung all over the cowling in a nice little ring. Drilling a quarter inch hole in the shaft for the indexing bolt to hold it in place without exerting any side force on it ended that problem completely. An oring in the original groove is required, of course.
So far it seems to work great. The theory is that the round plate knocks down any liquid oil that's headed out, where it drips back down into the bottom. The Brillo "Choreboy" pad catches smaller particles that can't traverse the maze to get out, and the air leaves with no liquid oil attached. The oil puddles in the bottom and runs back down into the case. If for some reason it was unable to return to the case, the overboard tube gives it a place to go. That tube hasn't seen any oil yet though.
In an effort to lighten the heavy steel cover, the next step will be to weld up a proper "chamber" that will consist of a plate that can be screwed in to the base, with a similar setup in the top. This will save several ounces over what I have presently. I have no idea why the top of this thing is something like .090" thick. It's like they expected it to deal with 50,000 psi or so...which is kind of funny since it has a vent hole in the top!
After flying it for about 15 hours without leaking a single drop, I can now proclaim this thing to be a total sucess. I did discover that under negative g's oil will pour right out of this thing, so I've now replaced the Brillo pad with some open pore foam to keep the oil in the engine even if it's thrown up against the breather. This particular foam is from a BugPack VW aftermarket air filter "sock". I also learned that if I don't put thread sealer on the pointed bolt that holds the "former pump" in place, oil will leak out around the bolt threads and leak a drop at a time onto the belt, which then gets flung in a fine mist that leaves a ring around the cowling. Not much, just enough to be annoying. I've put 30 hours on it and the oil level still looks like exactly like it was when I filled it up, if that gives you an idea of the amount I'm talking about. The next incarnation of this thing may be a big blob of expoxy at the back, sloping torwards the front, to eliminate the area where oil puddles, and giving the oil a way to run back into the case when it's parked
It's not just that it's cheap...it's that it fits perfectly and looks like it came from the factory...which it did...
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