[MTrenk]

freebeard:
--> This is why almost all racing series have height restrictions on exhaust exits, and have banned skirts that touch the ground.
--> Nowadays, I am studying the effects of placing exhaust above the diffuser on my team's car, trying to pull air under the car to get the diffuser to 'up to speed' faster, since underbodies can become effective at as low as 20 mph under normal conditions (our average speed in the Formula SAE competition is around 30-40 mph, so it's vital we make as much use of these things as possible).
--> It wouldn't be negative pressure, just less pressure (probably).

Frank Lee:
--> The underbody is very different from the rest of the car. Yes, the further you go along any surface, the thicker the boundary layer (because of air molecules getting 'stuck' on the surface molecules, resulting in a zero velocity at the surface), but if you look at a flat panel boundary layer vs. an airfoil boundary layer, I think you'll get an idea of why it matters so much how things are curved.

aerohead:
--> Dimpling a surface changes up Reynold's numbers. Size matters, but so does everything else.