What kind of car? I did some testing for this on a 68 Camaro in my build thread here:
https://lateral-g.net/forums/show....php4?p=613328 Scroll down to post 485.
The more extreme builds I've seen are venting 100% of the radiator air through the hood and bottom feeding the radiator inflow (check out the Vorshlag build or DuSold's Camaro).