[lunarhighway]

i think the main advantage of ducting is that one would have control of where and how post radiator air exits and how it merges with the existing flow around the vehicle.

who can really say what the air does when it exits the radiator? with so many subtle differences in engine bay layouts, and all the different pressures in and around it i think it's hard to come up with a generic picture. so in real world imperfect design i think it's better to have some sort of duct that guides the exit flow than nothing at all. the only danger i think is that a poorly designed duct (and very few existing cars will allow a mathematical correct duct to be retrofitted) might restrict airflow, especially at slow speeds when the airflow trough the radiator might not be so well defined, and hot air might remain trapped in the duct.