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Discover
Joined: 26 Feb 2007
Posts: 429
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 Posted: Mon Jan 28, 2008 7:37 am Post subject: Evacuating crankcase? |
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Just curious about your thoughts on evacuating the crankcase via the vent tube. Some racers believe they get more hp by keeping the windage down through evacuating the crankcase. I've seen some race planes with a vacuum pump evacuating the crankcase. Could placing the vent tube in the slip stream such that a low pressure/vacuum is generated help get some of the mist and psi out of the case and increase hp? The whistle slot may add to the low psi thru sphinon effect... What do you think?
---- flyingtiger0747(at)aol.com wrote:
Given that the oil temp must remain between 180 and 200 degrees for optimum
performance (i.e., keeping the acids and water out of the oil) and the fact
that the engine is quite happy with 6.5 qts, I don't see any benefit in
spending the time or the money on an air-oil separator.
Regarding the 6.5 qts: Not all engines like 6.5 qts. Mine likes 6 qts. I
can go 15 to 20 hours at 6 qts. I have about 300 hrs since top overhaul.
Some engines like 7 qts. I've yet to see one, with or without an air-oil
separator that likes the oil level above 7 qts.
Regarding blow-by: (or, what causes an engine to prefer an oil level below 8
qts) There are a lot of rotating parts inside the engine. All of those
parts are covered (ideally) with oil. As a result, there is a LOT of oil
mist inside the engine. Any blow-by, past the rings, takes the oily mist
with it. The greater the blow-by, the higher the oil level (allowing more
oil to reside closer to the rotating parts), the higher the percentage of oil
in the oily mist leaving the engine through the breather tube. In the
absence of a dry sump oiling system (which would be worth a few horsepower)
and in the absence of positive crankcase ventilation, the only alternative is
to live with blow-by evacuating some of your oil out the breather.
The purpose of an air-oil separator is induce a vortex into the blow-by air
which causes the heavier oil to move to the outside and to a return line.
The air goes down the middle and out the breather tube (attached to the
air-oil separator). Since the M-20 is so small, by design, it is not very
efficient in separating the oil since the vortex doesn't have a lot of
residence time to add energy to the heavier oil mist. The larger Walker type
air-oil separator is a superior design and a lot more efficient.
That being said, save your money and put it toward that Garmin 530 you have
your eye on.
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Mon Jan 28, 2008 11:07 am Post subject: Evacuating crankcase? |
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On race planes with up to 14:1 compression ratio, Ken, at LyCon, puts in a much larger diameter fitting for the crankcase ventilation. On an 8.5:1, it's hardly worth the aggravation.
--
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Gary
AuCountry Aviation
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