The reason for locating them at the front edge of a wing on a plane is that they are there to help the boundary layer stay attached to the top of the wing when it is at a high angle of attack. So it has to be at the front end of the wing.

For a car you want the boundary layer to stay attached as it rounds the top edge of the hatchback. That can reduce the drag by reducing the size of your wake. But I think it is sort of gimmicky on cars. The vortex generators on planes are very narrow and sharp. They are basically scaled down wings. You can't safely have rows of knife blades on the top of a car, so on cars they tend to be sort of blocky things and a lots less like little airfoils. I doubt it works very well.

 

Any break in curvature greater than 8 degrees will cause the boundary layer to fracture and break away leading to increased drag, or force pulling back on the surface/edge from which it breaks. A long time ago we tested some of those 'vortex' devices that supposedly reduced drag but our many high speed trials showed no statistically measureably difference whether on or off for a CRX or Hyundai Sonata so we guessed any advantage was disadvantaged by increased drag of the devices themselves. actually, they may have hurt.
My aero engineers say only more rounded edges or even knife edges of cars would be useful. If you notice the latest Ferrari, Porshe or F1 vehicles seem to follow that trend. Like most things nowdays increased aero isn't cheap; just look at those awful looking F1 cars with little fins and sticky things everywhere that they spend millions to develop just for 1% gains in slipperiness. They sure don't look it but to the wind tunnel ...

Our advice is use a good quality airplane wax; we did see increased top speed by a mph or 2 with that. And tape all door/hood/trunk gaps with tape; that works too if a bit impractical for street cars.
Oh and under car trays work too but I wish we had also tried channeling as todays super cars all seem to have in their under trays but if they have it ...
PS with duPont supporting Jeffy, I don't know why 24 doesn't have a wax that has no drag with their experince in wax/coatings. Maybe some carnuba/silicone/teflon mix ...
And some more thoughts about having more curvature on the driver side than the passenger side for Daytona and Talledega would impart some additional force toward the inside of the turns ... Ever tried looking down one side compared to the other? Remember, while there are templates there are tolerances and only a small change would matter a lot at 190 mph. Think of it as a vertical wing with the 'top' of the wing toward the inside ... Keep turning til the 'top' is down and you have the modern race car; downforce without the penalty of centripetal force downside. Only weight, not downforce is the enemy in centripetal force that causes tires tio slide sideways.
Don't ever kid yourself that NASCAR isn't as crazy tech as F1; their budgets just not as large.

 

<quote>Any break in curvature greater than 8 degrees will cause the boundary layer to fracture and break away leading to increased drag,...</quote><BR>
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8 degrees? It's a lot more complicated than that. It would be heavily influenced by the air speed and would also be influenced by temperature, pressure, etc.

 

absolutely but the aero for the vehicle speed range up to 120 mph 8 degrees is pretty much the change that allows air to smoothly disengage compared to snapping away. I;m not sure about the effect of temperature other than the effect on pressure and density but yes they are part. Still, if you design a tapered tail you need to stay inside 8 degrees and that only applies if you bring both sides of the air stream together. Complicated? You bet your sweet ....