We went to Rusholme for a curry last night. One of the chaps who worked there had a black Impreza WRX. Nice car, better in black than other colours anyway.
He'd fitted a roof scoop though, and I told him it looked like a black-cab thing.
Just like this:
http://www.toucantraveller.com/taxi.jpg
He said that this scoop was very beneficial because at speed it caused the rammed air to pressurise, and directed this pressurised air at the rear spoiler, resulting in much improved downforce.
I didn't want to appear to facetious, I'm already bad enough after a few drinks, so I just said I thought he should think about testing that theory in a wind tunnel.
Anyhoo, was he right after all? I'm not good with physics, but my instinctive thoughts are that it doesn't work like that at all, and simply causes a major drag and slows the car down. I'd have thought angling the entire roof of the car in the reverse angle of his scoop would work better. His scoop might provide some downforce if he was driving in reverse though.
Here's an illustration I made
http://www2.css-networks.com/downforce.jpg
What do you more intelligent folks reckon?