I have no sheepskin, certificate, or tenure with an aero company. But, I have made observations to various car designs over the years that capitalized on the airflow under the car.

Many ALMS, JGTC, and F-1 cars use flat surfaces, tunnels, and diffusers under the car to "accelerate" the air traveling under the car. This stands to reason that they believe airflow is best when it's faster under the car than over the car. Many of these same cars also feature very low air dams. If you limit the amount of air going into an area, but accelerate the amount of air leaving it, you essentially create a mild vacuum under the car, sucking it down to the ground.

Think of the roof on a house during a hurricane... the air underneath the roof is still. The air on top is moving at 120mph. The roof eventually flies off, not because the wind "ripped" the roof off, but instead the roof gave way to the massive pressure buildup. The air under the roof was trying to equalize with the high-speed air moving above it, and when the structure "moves" to meet it, bye-bye roof.

On our cars, we have the suspension to offset this. But the same forces are still at play. If you force the air under the car to excavate faster, you force the car to "squat" as it tries to equalize the pressure. This is exactly what Doc was referring to with the paper trick.

Lowering a car, limiting the flow that enters underneath, and accelerating the existing air underneath all work together to accentuate this effect of decreasing CoD while increasing downforce. The higher the speeds, the greater the effect. At our (legal) highway speeds, we would barely notice the effect. But, the effect is still there. That's why it takes an extra 100hp for the SUVs to cruise comfortably alongside the coupes at 80pmh... to overcome the drag handicaps.

Ferrari Enzo and the SLR-Mclaren are just two examples of how auto manufacturers that employed the concepts of diffusers, flat bellies, and low chins. There are many more marques that are now on the bandwagon, and for a good reason: the concepts work.

If we wish to achieve a more slippery shape, we should follow.