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Notice the radiused surface on the aft side of the outlet. NACA did studies of cooling inlets and outlets during WWII for piston aircraft, using the best methods found on both our and German/Japanese aircraft. Outlets are as important as inlets for good aerodynamics. Actually, you need matched inlets, internal ducting, and outlets to get the lowest drag. This BMW has some comparativelygood outlet geometry, but could be better.
Google for "AR-5 aircraft" by Mike Arnold, who studied these methods when building his fine little plane.
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