Phil,
I'm a little confused. What part of what Tim is saying are you disagreeing with? That the diffuser creates a downforce because of vacuum? Or where the highest point of downforce is in relation to the diffuser?
Also had a question from the link that jhnmdahl provided
Secrets of Underbody Tunnels, Rear Diffusers and Venturis | Symscape
"By installing an inverted wing close to the diffuser exit it is possible to create a low-pressure area, which essentially sucks the air from the diffuser. The diffuser and wing combination permits a higher air-mass-flow rate through the diffuser, thus resulting in higher downforce. Sharp edges on the vertical tunnel walls generate vortices from entrained air and help confine the air through the diffuser and reduce the chance it will separate."
I'm having a problem envisioning what an inverted wing close to the diffuser exit looks like. At least in reference to the diffuser on the Lotus. Is it a wing connected to the end of the diffuser?
Also are they saying that a 90° corner would be more effective than the curved transitions from vertical to horizontal that our diffusers have?