Look at the Le Mans GT1s from a few years back. The factory placed the scoop atop the green house for the ram air effect.
On a 911 there
is a high pressure region around the air intake in the lid. There is also a virtual hurricane of air moving through the engine compartment, ~1500 liters/sec at 6000 rpm. As such, cool air can be found just about anywhere in the rear portion of the engine bay. This is not the case where water cooling is used. In a water cooled car there is relatively little air moving though the compartment and what air there is has been heated by its passage through the radiator. Ducting cool air in this situation is much more profitable than in the air cooled situation.
In a front engine car there is a convienient relatively high pressure area at the base of the windshield.
Ruf placed NACA ducts in the quarter panels to collect cool pressurized air.
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Bill Verburg
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[This message has been edited by Bill Verburg (edited 10-25-2001).]