<<Max pressure above the engine is a help, but not the primary concern! Any
increase in the DeltaP from top to bottom is helpful, but as it turns out,
not the major factor.  The cowl opening on most airplanes, as it turns out,
are larger than they should be!
....
This is also helpful, but believe it or not, if the engine is well baffled
by design, you can put a 1" hole in the back plate and not significantly
affect cooling.  Yes, I know that seems counter-intuitive and it is, but
it's a true statement--by data collection.
...
Walter Atkinson>>

Comments very much appreciated, Walter.  Regarding the first comment snipped
from your reply, I was mixing two objectives - cooling effectiveness and
cooling drag (efficiency).  I believe being able to maintain maximum
pressure above the engine will allow a higher pressure below the engine and
hence more velocity recovery.  The balance between the two is all about
compromise.  I agree that if other things are done well a significant leak
won't effect cooling effectiveness, but it must have an effect on cooling
drag as "wasted" air is going through the engine compartment instead of
around it.  Again, excellent comments.

Gary Casey