SReid,Btrueblood,
Thanks guy's for the links good reading and the pictures are worth a thousand words. I have only had access to an early type tunnel open wheeler that uses skirts (ex maclaren designer). The sports cars in the link are more advanced.
I would like to add the following comments...
This duct is more than big enough without aero calculations remember the size of your throttlebodies? The worst case area sizing is in first gear at full throttle (no speed) as long as your duct losses (straight and smooth not corrigated) and inlet area are bigger than the throttle body area It...
I am involved with a few cars that run at these speeds and slower and downforce is a significant advantage in high speed sweepers. Sorry no info on diffuser design thats why I am reading but if you can test with tuffts and a video car and a manometer strake. Check paul vandenbergs book racecar...