Thanks aerohead for your opinion! i was waiting for you : D
but i still not conviced about some-things.
I did some Flow ilustrator jobs (i know that the program isnt a guide at all, but its something)
the setup of the program is stock but with 200 on velocity. it was tested with and without floor with similar results on the upper part of the cars.
in the image you can see that the lowest drag of the ''CARS' is the kammback with airfoil one ,and looks like it have just a little drag on the airfoil.
Repeat: i know that the program isnt a guide at all,it have bugs and other unreal things but its ''something'' to see what i mean a little better.