Venting of Lycoming 0-290 D Engine

Dear Phil,

Your friend has the physics backwards.

Lycoming's use of a vent diameter of 5/8" should be taken as the
minimum needed to properly vent blow-by from the crankcase. Anything
attached to that outlet must then INCREASE in diameter according to
length... or be provided with some active means of pressure reduction
that is independent of throttle position. A venturi would work (and
has been used for this purpose in the past) but they tend to clog-up
or freeze. A metal funnel (!) has also been used on a small vent line
but slow speeds calls for a pretty big funnel (ie, about a 6" funnel
for a 1/2"x15' vent-line at about 60 mph) but it looks weird as
hell. If you have sufficient vertical height you can use a passive
oil separator... but keep in mind they make dandy condensers of water
as well as oil. Ditto for the active types, which generally use some
form of rotary motion to condense/capture the vapor, plus a scavenge
pump to get it back to the sump or oil tank.

Unstated -- but implied by the aircraft-type -- is the need to install
a flop-valve on the vent line to deal the occasional negative-g
maneuver. Without it, he's going to blow his oil overboard. If he
insists on using a small-diameter vent line, he'll probably blow his
seals at the same time.

-R.S.Hoover
-EAA 58400

PS -- There is also the tray type of oil separator/catchment that fits
under the firewall. It can hold a quart or more of oil which you
discretely drain between shows.