This is a project which possibility I studied for years in my own Z3 Coupe.
Issue: Rear Bumper of Z3 works like parachute. It creates massive amounts of drag on both sides. On inside it gathers oncoming air from under the car and collects swirling high pressure airmass under neath the car, creating lots of lift which makes Z3 unstable at high speeds and extremely prone to aquaplaning on super sport tyres. On the back of the car it creates again more drag. Having large rear area after which, air stream is trying to fill the larger low pressure cone behind the car, and this also sucks air from top of the car downwards, creating even more lift.
Why diffuser does not work: Rear bottom of the Z3 is uneven like plowed field. A rear diffuser would not work in the rear under the car, rear diffuser needs flat floor before it to work optimally, in Z3 it would not work even in sub optimally.
Solution: Cutting away rear bumper removes parachute effect from the car and makes rear wake area smaller, effectively removing drag both sides of the bumper. Instead of using rear diffuser, a rear low mounted wing works well without flat floor in front of it. Wing is simply in the airstream and creates downforce into car, it's effectivity is further aided by reasonable ground effect proximity.
In use: Car has more surefooted grip and positive predictability at any speeds. Aquaplaning is totally gone, a night and day difference! Both Roadster and Coupe has around 0.37 cD which is horrible. This helps to reduce lots of drag from the car, so your top speed might raise and your engine does not need to work nearly as hard at motorway speeds to slice through wind.
STL-files for 3D-printing all 6 parts and also 4 part mold for carbon fiber making and 1:1 printable sticker print file to help cutting into rear bumper.
