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Chillsworld,
Now that I think about it the area that I drew is probably wrong.
Everything I have said has been about efficiently ducting air through the radiator. I think the time attack cars have exhausts that are too big because the air that goes in has to go out, and I don't think there are any large pressure differentials on a car that can account for serious compression/expansion of the air.
Whatever location you want to exhaust the radiator, you want to match the velocity of the exhaust stream with the air going around the car. The full opening in the hood is not a cross section of the flow in a good design because if the air were traveling straight up you'd get some serious turbulence there.
That however doesn't really change the argument; taking this into account, it's now plausible that the P1's "effective" opening is around the 115% NACA figure (which is also a reasonable amount for the air to be expanding due to heat). If you are to have any chance of matching the velocity, you have to match the speed of the flow first, and the inlet to outlet ratio + the temperature increase across the radiator is what determines the ratio of the speed of the flow going into the inlet and leaving the outlet.
With those time attack cars then, there are a couple possible problems (in my amateur eyes):
1. The rectangular exit is not the appropriate shape because air doesn't flow straight over the hood, a lot of it "spills" to the sides, and you'll get some vorticity or something like that at the edges.
2. The radiator is nicely ducted and the air may be attached along the duct, but the inlet is WAY smaller than the outlet, and the radiator is not going to heat the air up enough to let it leave at a high enough velocity. This would create lift, although it would probably increase flow through the radiator. My guess is that it's better to enlarge the inlet and match the outlet rather than blindly enlarge the outlet.
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