The reason that they use flaps instead of louvers is that the air takes the path of least resistance. If the louver path is easier than the air going thru the radiator, then the louvers will flow until the louver path equalizes with the radiator path. Therefore, the louvers can help at some times and hurt a tad at other times. The flaps only help since they are a diode in the system. Louvers are just like an air gap between the shroud and radiator....a leak.

Louvers look cooler than flaps...the cool look may help cool the car....???

mike in tucson