The air under the car is a pressure wave. It is so powerful the Russians actually built a transport ship using the concept.
Russian Tank-Ship-Plane | Yurock

Perhaps your Indy guys are aware of this force. Perhaps not. But simple logic suggests that removal of this Pressure wave, by diversion or spoilers, would reduce the lifting effect it has on the chassis.

More to the point, it takes HP to create this pressure wave. Diversion would allow this parasitic power loss to return to use at the wheels.

My 75 Starfire came from the factory with a 2" X 24" diverter wing under the front bumper. I presume it's purpose was to increase air flow into the engine compartment. Using a flat aluminum strip for a stiffiner, I extended the diverter with a 2 1/2" vinyl base cove under the car from front wheel to front wheel. Thereby creating an angled air dam under the car.
I also had two angle steel rails under the quarter panels added for stiffness and to transfer the rear wheel loads better to the chassis. They also had the black vinyl draped during my wild days.

While the whole rig looked amatuerish and funky it seemed to function well.
The improvement in top speed, handling and MPG were very noticeable.

Does that qualify as a test?


Later versions of the car and many others came equipped with similar ground effects. The Chevy Monza version had a factory kit. I still have the parts hanging in my garage.

 

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.

 

The '78 Brabham used a similar design.

 

My statement applied to regular cars and then I added that race car designers had worked to reverse the pressures.

 

I stand corrected.....it is just a very interesting thing they are doing...I was fascinated by it...but, with the type of driving I do, it just wouldn't be worth the $$$$$ to work on it.