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I dunno about the NASCAR 4 square inch rule, but NACA did considerable cooling duct work in the 1940s, indicating that diffuser inlet ducts are essential, as well as proper outlet ducts that smoothly rejoin the cooling air with the slipstream.
Probably, a round or elliptical inlet at the stagnation point, with radiused edge and diffuser duct that grows in diameter to 2 or 3 times the inlet size, coupled with a constricting outlet duct that rejoins the slipstream in almost parallel flow, would be best.
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