[aerohead]

If your local library can get you an inter-library loan of Sighard Hoerner's book, 'AERODYNAMIC DRAG,' he's got quite a bit of detailed aerodynamic results for a variety of scoops, complete with Cds for each kind.
In modern times, for raw harvesting power, the Baumann scoop, which you'll find on the P-38 Lightning aircraft, or NHRA Pro-Comp Top-Fuel funny cars are preferred. Since they're 'external,' their Cd and frontal area must be considered together.
The lowest drag inlet is the NACA submerged inlet, which requires special geometric considerations 'below the skin.'
Between the two, you may be able the calculate parasitic loads, or the lack thereof.
NASCAR uses NACA inlets behind the B-pillars, of which flex ducts are attached to route the flow inside the cabin.