2000 Audi R8

Images copyright Bob Nelson and Marc Urbano
Text copyright Michael J. Fuller

Compare the diffuser section above (Sebring) with below (Sears Point).  The Sears Point diffuser is smaller than maximum width (100 cm). 
Sears Point diffuser section.  Note that the diffuser's walls have moved in towards the car centerline, the walls being in the same place as the strakes in the Sebring diffuser.  The Sears Point diffuser does not have any strakes in the section.  The narrowed diffuser is now less effected by the negative interaction with the rear wheels therefore generating actually more downforce even with reduced area.
The bottom edge of the longtail sections is very sharp (yellow highlights).  A vortex sheds off the sharp edge producing downforce, creating what is, in essence, an additional diffuser section. 

©Copyright 2000, Michael J. Fuller