I have completed a set of three CFD "virtual wind tunnel" simulations of a Mini-Z MR03 Mazda 787b 3D model and I think the findings are quite interesting.
The table below shows drag and down forces for the three different rear wing configurations tested:
Down Force (gf)
No Rear Wing
Flat Rear Wing
45o Rear Wing
All simulations were done considering the car "racing" at 10 m/s (36 km/h).
The first interesting finding is that, according to the simulation results, a Typical Mini-Z body shell like the Mazda 787b ASC generates 7.8 gf of lift when ran with no rear wing. At 10 m/s (36km/h) this force can be significant, close to 5% of the car weight.
A second finding is that adding a simple flat rear wing can, somehow, reduce the lift force to 0.5 gf, practically neutralizing its effect. This benefit comes at no material penalty in drag, as the increase observed in the drag force was just 0.4 gf.
Finally, by using a 45 degree rear wing a down force of 8.5 gf can be achieved. This result however comes along with a 3.5 gf increase in drag.
Detailed results for each of the three simulations are posted in the following links:
Virtual Wind Tunnel: Flat Rear Wing
Virtual Wind Tunnel: 45 Degree Rear Wing