My fluid mechanics class was quite a few years ago, but... Not sure I entirely agree with his claim that air is incompressible below .5 mach. Maybe from a practical standpoint, but from a technical standpoint, water in incompressible, air is compressible. There were really two benefits of hood scoops on 70s cars, one was the ram-air effect, it can be argued how on effective it was. The second was cooler air fed directly into the carburator(s) (remember them?) since it didn't mix with hot air under the hood. Cool air is denser, so it helps in burning more fuel. Chrysler really had the best ram air systems. Most articles of the time claim the most effective set-up was the Dodge Challenger T/A and AAR Cuda. The scoop was about one inch off the hood. Scoops flush with the hood didn't function as well, since there is a "boundary layer" of air that slows down as it comes in contact with the hood (friction). The cool air probably contributed more to small hp gains than the ram air effect, but it did look cool.
My roomate in college borrowed a piezometer from the mech lab to try it out on his '74 Trans Am. It did show different pressures, and that Pontiac really did put the hood scoop in the right place.
Jack
'97 RT-10