[aerohead]

For the 1976-78 CNR 'banana' car,Morelli and Pininfarina did both.
Four extractors,2-up,2-down,were positioned,oriented,and sized,such that the air escaped into lower pressure air,blending at exactly the same velocity as the adjoining air stream,for a 'seamless' merge,and lowest drag.
It's the best cooling system I've ever seen for a 'passenger' car.
For racing cars,the Olds AEROTECH would represent a good illustration of ideally-ducted,airtight design.(it's a ground-effect car,so all aspirations occur on the upper body).