Steve,
There are some very good articles on cooling drag on the
CAFE Foundation web site. One thing they mention is that the
most efficient exit is a 'bluff body'. I believe the info is
in 'Local Flow 2' on the site:
There was some work done here in the USA many years ago
that showed inlet size can be relatively non-critical to drag,
as long as the shape of the inlet lip is done correctly. The
technique was to regulate total flow through the system with
an exit flap. The concept was to use 'external diffusion',
where the air was slowed at the opening, instead of inside the
cooling duct. Testing showed that there was little difference
in total drag, but duct construction was simplified since
shape/surface finish were not as critical (air was already
moving slowly when it entered the duct). I think Tracy played
with the technique a bit on the RV-8; perhaps he can add some
info.
Charlie