[browney]

Denis Bassom
I guess it would depend on the front spoiler as well. A front 'lip' can effectivly shelter some of the protrusions at the bottom of the engine bay also reducing underbody drag. I guess it is possible for flow to seperate in the underbody if you have protrusions, and that would reduce the effect of the diffuser as well as increase drag.
Although, as I have demonstrated, I'm not an expert.

Locost47
I guess that another advantage of a diffuser is that by raising the pressure at the back and 'boat tailing' the profile of the rear, you will propably decrease drag too. Does anyone ever use a 'nozzle' shape on the front of the car to try and get faster underbody flow, or will this increase front end lift too much?

Do you know where I would be able to get some info on diffusers? I've read Hucho's book on the 'Aerodynamics of road vehicles' but our library doesn't seem to have much more and I can't find any journals that I have full text access for on the subject. It seems like getting answers on bluff body aero is really difficult (unless you hang aroudn on ten-tenths ;-) ).I'd love to find out more about the difference between mixing the air at the rear of the car at high speed vs. bringing back down to closer to the free stream.

I hope you love your job, I dream of doing stuff like that!