Beyond the physical protection of the parts involved, the belly pan is there to provide negative pressure in the engine compartment, thereby improving airflow through the engine compartment. Notice the two "NACA" (triangular) ducts, they are there for air intake, causing the airflow to exit the engine compartment. This creates a negative pressure which aids airflow through the radiator and exiting to the underside of the vehicle. This improves cooling and lowers air resistance. It may be "technicalogical (sp?) wheelspin" but it is an Audi after all.

 

My A4's belly pan was missing when I bought the car. It was discarded by the original owner. I've experienced no adverse effects at all from its absence... the engine cooling is just fine even in triple-digit Texas summer heat.

 

I remove mine from the Audi allroad 2.7T each summer, here in the scorching Southern CA low desert - where daytime summer temps exceed 118F.

Ralf5k - your "theory" sounds plausible.....but the reality is my allroad is primarily designed for COLD WEATHER OPERATION, first and foremost. It NEEDS all the cooling it can get. With the belly pan on during summer - my car heat soaks so bad, the temp needle is one tick from full overheating....and that's normal! My A/C operation is completely inoperable, until the needle goes back down to 1.5 ticks above middle.

The belly pan in my car literally traps heat inside the engine bay. The only solution is to leave the hood UP after each drive....but that looks ghetto and doesn't give Audi a good reliability impression! I understand things flying up into the engine bay IS a reality....albeit a remote possibility, at best.

 

Z,
Your heat soak issue is probably due to a failure in the afterrun heat control system. Although I'm not familiar with the specifics, the idea is to deal with the heat soak condition.
Audi has been using such a system for quite a while now. The first I ran into it was on C4 turbos which included a temp sensor in the valley between the head cover and the intake manifold. This sytem also included an auxiliary coolant pump (leak) to circulate coolant and manage heat soak. It also included a fresh air fan drawing air from below the engine and distributing it over the injectors.
As for your theory that these vehicles are for cold weather use, I assure you that they sell quite a few in the middle east (so-called Saudi version). They have speciial adaptations to deal with the climate issues experienced there. Again I don't know the details. Long story short, you should check out the afterrun cooling system for your vehicle.

 

Ralf5k:

You're probably right - I NEVER heard that after-run system work, ever. This probably explains why I'm missing coolant...yet, never see a leak under the car.

 

on a side note along with aerodynamics, has anyone ever studied the design of the exhaust(stock)? if you follow with the mufflers they are angled in line with the rear bumper.
when i did straight pipe exhaust, i noticed a slight drop in MPG. after thinking over what it could be, i realized that the design helped keep air moving out to the rear and removing them not only caused inconsistant airflow, but the bumper would act as a parachute also..

this is the conclusion that i came to about the aerodynamics in the rear.

 

The pan is there for aerodynamics to some extent. Air flows over (actually under) the pan easier than it does across the irregular surface of the engine. Also keeps water, snow and dirt out of the engine compartment. However, Audi calls it a "sound absorber" when you look it up in ETKA (parts catalog) or ELSA (service manuals). It's primary purpose is to reduce engine noise.