On Sat, 19 Apr 2008 09:00:57 -0700 (PDT), Bob
wrote:

Sorta like all the hood scoops
and spoilers on 70s muscle cars. The advantage really doesnt start to
work till your over 140 mph.

Scoops work somewhat even when the vehicle is stopped. The outside air
is generally significantly cooler and denser than the underhood air.
Once the machine is moving you get the benefit of ram air as well.
That does not work much at slow speeds, as you say.