Rear Diffuser Concepts

[bryson17]

I'm looking on d3cadillac.com. I'm just browsing basically, but like the rear diffuser shown here:

I did a little reading on wikipedia and a few other places about how they work.

A rear diffuser is basically used to prevent airflow from becoming turbulent when air from below the car meets higher pressure air behind the car. I'm wondering about the aerodynamic applications. Are these used to decrease drag (by decreasing turbulence) or to increase downforce (thereby increasing drag)? Or is it tunable depending on the particular shape of the diffuser?

Just curious...

[The Tony Show]

Rear diffusers are mainly designed for smoothing airflow leaving the rear of the vehicle to prevent lift at high speeds. Look at the rear of a Ferrari or Lamborghini for wild examples, or the mesh cutout on the back of an STS-V for a more subtle one.

That particular bumper is a little confusing to me- the cutouts are in the wrong place to be functional as a diffuser, and the exhaust outlets would be blowing right into the inside of the cover and melt it in no time. It's likely that they're routing the exhaust exit pipes through those holes for a unique look, in which case it would have no effect on aerodynamics.