The purpose of this thread is to discuss the BRZ/FRS's aero.
The JDM underpanels help smooth out the air under the vehicle by making the underbody smoother. A smoother underbody means that there's less turbulence, thus decreasing drag. Theoretically, this can reduce wind noise, improve gas mileage, and increase top speed.
For reference, Subaru stated the following coefficient of drag (CD) statistics:
no underpanel, no wing: 0.29cd
no underpanel, with wing: 0.28cd
with underpanel, with wing: 0.27cd
Here's a picture of the JDM underpanel.
While reading about aero diffusers, it appears that the underpanels
may have another benefit. That benefit is increased downforce. The reason I say may create additional downforce is because it depends on whether or not the rear diffuser is actually functional. In other words, the air at the back needs to increase in pressure/decrease in velocity in relation to the air underneath the vehicle.
When equipped with the underpanels, air can travel at a greater velocity because there's less turbulence. When the air reaches to the rear of the vehicle, a diffuser will can slowdown the air by "diffusing" it. Essentially, the vehicle is being pulled down due to the difference in air pressure. The video below explains how diffusers work.
[ame="http://www.youtube.com/watch?v=3DIJMLrGz8Q"]Diffuser - Explained - YouTube[/ame]
Here's a picture of the BRZ's rear diffuser.
From the picture above, it appears that the BRZ's rear diffuser has a small angle to increase air pressure/decrease in velocity. However, it's not clear whether or not it's functional in producing downforce.