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Always ran a fan on our circle track car. Our approach was, tight ducting to the front of the radiator, no ducts to the rear, large UNshrouded fan with fixed blades. Fan was on a switch on the dash. Our rationale, create high pressure in front of the radiator, allow the air to exit the radiator freely and do NOT allow the fan to block the flow. (we ran a flex fan at one point, thought it was the way to go, found the blades flattened out and actually blocked the air exiting, hence the unshrouded fixed blade fan.) The switch, a bit more complicated, but two thoughts. The first with a fan spinning, it will act as a wall at race speed and actually block air. Air coming through the radiator will spin the fan and offer less resistance to the air if it is not on. The fans resistance to air flow should be less if the magnets are not charged. The second, even though we ran an alternator, with the fan running, it drew enough voltage that the alternator was 'on' all the time, which of course put an addition load on the engine. Aside from cooling, most of this we felt was insignificant, but our theory is, enough insignificants, you can see a significant gain.

With these cars, and the aero involved, I am not sure which is less detrimental. Seems to me free flowing air under the body work will create a higher pressure area, as well as increased heat around the engine bay, which in turn would be worse then ducting it out. The question then becomes were to duct it out without messing up the aero where ever it exits? Considering the race speeds, I doubt back pressure from an exit duct will effect temps significantly unless you're way way to restricted. By ducting the exit, you control were the heat is, you control flow under the body, now where do i dump the air?

Mark