I've just read this topic and found some answers to my questions on how diffusers work... I was confused more or less as tomspotley about the "contraddiction" of diffusers vs nozzles.
I found this post extremely helpful:viewtopic.php?p=359737#p359737
And this one too:viewtopic.php?p=359784#p359784
But now I have some further doubts:
1) if the car had no diffuser (as stated in the latter post) and only a flat floor, wouldn't an expansion occur all the same? It would be sudden and not gradual as in the diffuser, but wouldn't this pressure difference (between floor and environment behind the car) produce an acceleration of the underflow and thus downforce?
2) (opposite "extreme" case) If the car had the whole floor shaped as a diffuser, would it produce more downforce than a flat floor and a diffuser only at the end?
Maybe I'm still lacking some physics knowledge to understand perfectly diffusers... but a good explanation, maybe with the effect of the two "extreme" cases, would be helpful. Thanks in advance and nice topic!