Functional Rear Diffuser

Minato

ecomodder.com has a lot of excellent aerodynamic readings. Their main goal is to reduce drag. For the most part, you do not want a sharp angle of attack, otherwise you are going to get highly turbulent flow. The airflow is most likely detached and turbulent at speed by the time it gets to the rear of the underside of the Z anyway since it has no flat paneling on the bottom. You might have luck getting partially reattached flow with a mild angle of attack, starting parallel and increasing to ~10 degrees. Essentially anything that decreases turbulence underneath the car is going to generate a downward force that will counteract any lifting.