-Lower ride height, less frontal area. (Is the Evo lower?)
-Front end moves the air around the car instead of under it. This is good, sides are way smoother then the floor. Probably increase cooling as well as the underfloor pressure is lower -> bigger pressure diff. over the radiators.
-The Evo front is wider in front of the wheels, this is very important as the wheels are a big source for drag. To be able to move the air around the wheels in a controlled way is extremely important, it also create a nice low pressure area outside of the wheels, this is very good for brake cooling.
The same goes for the rear wheels minus the brake cooling naturally.
-The rear bumper of the Evo kit is probably not better then the std. though, it's wider behind the wheels and this catches the air, for strict racing I'd use the std unit or possibly make some cut outs to let the pressure out of the wheelhouse.
-Rear spoiler is great. maybe not for drag though...
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)
A friend here has a home made spoiler that looks like the IMSA one, he says it makes a huge difference in fast turns. A wing would be better for drag naturally.
The std "duck tail" on the 75 is terrible, REALLY heavy. the thin trims and a proper wing is lighter and much more efficient.
Time to re-read theat book... Race Car Aerodynamics by Joseph Katz. Very good book.