I have seen this article: http://insideracingtechnology.com/phnxtrk2.htm
about the nascar aerodynamic, and they are remained hit from this image:
The author write: " A closer look shows an adjustable support for that fender flare. On superspeedways the fenders and wheel cutouts are fitted to the tire very carefully to minimize aerodynamic drag. On shorter tracks like Phoenix the fenders are flared out to produce downforce. If the bodywork behind the tire can be curved so the trailing edge of the tire sticks out away from the sheet metal behind it, the tire becomes a pump pushing air out from under the car increasing downforce on the front of the car. I'd guess the right-front tire, when turned into a corner would pump enough air to make a difference."
This fender flare, if used in an original car modified with an total underbody front cover to pull out the hot air from engine space, with aerodynamic advantage, or it is better an other solution?