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I'm going to go out on a limb and say they are not necessary and dont provide any additional benefit. I say that only because the GT2RS keeps setting track records without them.....
This video does make it look like they are not needed. At the same time. Why does water stay on the back window at speed? This vid makes it look like waters should be blown off.
No the fins that sit on the roof that keep the air flowing against the car
Phew... you had me worried for a minute. I will stick to my original position. Vortex Generators help with airflow in a way that I dont think would help fuel economy. Unless there is something specific you are trying to achieve that the vortex generators would help with then maybe. Otherwise it's just a cosmetic thing on a daily\weekend car....
I have used vortex generators on some RC stunt planes. They are effective at modifying air flow. But, you have to know how and where to position them and what it is you are specifically attempting to accomplish by using them. I used them to hold the plane at angles that would be impossible without them and to facilitate enough air flow to maintain control at very low speed. I know they were used on the EVO sedans, but never got a close enough look to tell what they were doing. I doubt they would accomplish anything useful on a 911.