Most manufacturers have wind tunnels or rent local ones. BMW has their own, not sure when it was built initially or upgraded. BMW Sauber's wind tunnel is in Switzerland, and I doubt they ever put a road car in it. Ever.
Cars really need a flat bottom/sealed underbody to get significant aero gains from a splitter. Good road legal examples are the R34 GTR, and even better, any modern Ferrari like the F360.
|