Originally Posted by
RacingManiac
A Spoiler will some times lowers drag by allowing the flow to reattached. But on a Evo or STi, that is an actual air foil, a wing. A diffuser does not generate downforce, but rather the throat of the under body section, a diffuser allows the flow to decelerate and rejoins the normal flow thus not to add more drag and allows for the entire underbody works more effectively.
Its not that hard to make a wing that works, but the problem being how well it works. But I think in general they do. The fact that a car's shape in general creates positive lift, any thing to break off the flow will kill some of that lift, but adding drag in process.
Interesting thing about that though, on the new Evo MR or whatever, with its vortex generator on the roof, which actually creates turbulent flow, which(according to my fluid class last week, on the subject of vortex generator, which has been around for a long time) can actually stays attach for longer length along the body's boundary layer and thus reduce drag.....and in Evo's case, reduce the drag that will probably be caused by the big rear wing, and since the flow will be better attached, allows it to work better too....