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So I have a number of threads out there on spoilers, increasing compressor efficiency, intercoolers, intercooler sprayers...... all in search for lower IAT's after I pulled the data logs from a few HPDE days to find that I had IAT's in the 180°F range and was pulling power. So while brain storming I was looking at the rear deck grill that reminded me a of a vortex generator, then I thought yes the spoiler does increase the AOA with the sunroof up so maybe this is all to help send air down to the intercoolers. That being said I thought about Vortex generators on a wing and how much more air by using them on the top of the rear window if that would help prevent air separation thus sending more air to the intercoolers.
I've thought about this too. But for the 911, I don't think there's any significant flow separation from the rear windshield because the transition from the roof is very gentle. Where we saw vortex generators used back in the day were on Evos where there was a sharp angle going from the roof of the sedan body to the rear window which created massive flow separation.