I only offer a different way of looking at existing ideas......Waldo

    Underside Ogive Air-dam for Rear Wheel Drive Cars
I designed this set-up for Ron Preutt's Pretty Woman C-Gas Bonneville Racer. Ron was a courageous racer in the mid 1990's and would let me experiment on his racer. He set many records in the 250 + mile per hour range. His creation was sort- off a stock bodied 1988 Thunderbird running a bored and stroked, highly modified, Ford engine, running gasoline and unsurpercharged in any way. The engine made I think 1,160 horse power at 6,800 rpm this was quite an achievement at that time.

Basically the "device " was a steel reinforced subsonic shaped ogive clear Lexan skirt under the car. Starting at the rear of the transmission and ending in front of the rear wheels at the sides of the body. Behind the rear wheels were two steel rod reinforced flat skirts of clear Lexan (brand name for polycarbonate plastic sheet). The car was only an inch and a half off the salt at the front. Most of the air encountered by the car during a record run was directed over the car. The air that went under the car was forced (with as little drag as possible by the Ogive shape of the skirt/dam under the car) out in front of the rear wheels. It was obstructed from re-entering the underside of the car by the skirts behind the rear wheels. The aerodynamic result was an extremely low pressure area between the rear wheels. The result of which would be a vacuum at the rear of the car. The car was supposed to stabilize and suck down eliminating wheel spin at the rear and it should track straighter because of the pull.

At the speeds (250+ m.p.h.) Pretty Woman was running the suction force created was so great that it blew out the special custom rear shocks built for the car. Pruett was shaken and disapointed with the results. What we needed to do was strut the rear of the car and eliminate the shocks and it would have worked fine.
Stock cars such as NASCAR cars can use this concept for stability and traction. Street cars since they need a good all around suspension, would need alot of "development" which translated means money and guts!