i think that hennesy and those kind of cars have it cause they have engine in the back and have to get rid of the heat trough that..
True, but like I said earlier, theres a lot of air that get trapped in the rear end.
It flows from under the car and pass thru suspension and end up in the back of the tail lights area.
The rear end of the fibreglass body acts like a parachute.
Heres a great example integrating mesh in the rear end of the car I saw a while ago.
That would decrease the pressure created in the back of the car at high speed.