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During initial testing, I also had problems with cooling the oil.  A left side
mounted NACA duct (about 5 inches high with the opening about 1.5 inches deep)
with an exit from a squarish plenum into a top-plenum on the firewall mounted
7-vane oil cooler.

I could not use the Lancair recommended 9-vane oil cooler because of space and
the position of the NACA duct.  My oil temps were 240-260 and rising during
climb or cruise.  During testing, I frequently fitted an external aluminum
scoop to force air thru the cooler.  I fabricated oil cooler air exit plenums
which routed air out the bottom rear, out thru a "cowl flap" type exit or out
thru a "shark gill" type exit.  Some of these attempts were to "suck" the air
thru the system.  None of these changes were effective enough to provide a
final solution.  Finally, I smoothed the plenum in the NACA duct to flow the
air into the 3" SCAT tube that led to the oil cooler.  The smoothing was done
with micro and is the shape of a sharply curved funnel transitioning from the
rectangular NACA duct entry port to the 3" diameter tube.

VOILA!  My oil temp was now reasonably normal.  The original plenum was boxy
and trapped enough air to stop flow thru the system.  I also continued to
experiment with the various air exit modalities and found any forcing plenum
or exit externally to the lower portion of the cowling RAISED the oil temps.
The best solution was to just dump the air into the lower cowling and let it
exit with the rest of the cooling air.  Now, during climb at 120 Kts I see a
temp of 190-200, during cruise 170-180, during descent and post flight taxi
200-210.

By the way, the reason low air exits are bad is that they are in high pressure
areas.  If used on the left side however, they must be low so that the hot air
does not come up the side of the fuselage and enter the side-mounted cockpit
cool air NACA ducts.

So....

I agree that the pressure on the top of the cowling is, at best, neutral if
not actually a low pressure area.  I do not believe that separate air exit
plenums and ports are effective if they are placed low enough and end up in a
high pressure area.  

You could have a problem with the internal shape of the NACA duct plenum.

I have a friend with a 360 that used an automotive duct (racing) which is
shaped like an off-center funnel placed on the side of his cowl.  It worked so
well that he had to build an adjustable flapper valve to shut down the air
flow during cold weather.  His duct was obtained from an automotive racing
catalog.

Scott Krueger
N92EX