I'm confused about aerodynamics.

http://en.wikipedia.org/wiki/Diffuser_(automotive)

 

Good observation ;-) The lift force can't happen on a 360 because the rounded shape is far of being perfect as a wing. The lift force of a wing is created only with one very specific shape. The body shape of a 360 "breaks" competely this principle and that's why it can't fly ;-) As Napolis wrote, the wikipedia link explains very well the exact opposite effect which is downforce. But remove the front diffuser and back diffuser of a car with a flat underbody, and your car may behave very dangeriously.

 

The shape of the top of the car is designed to slip through the air efficiently and to produce down force to help it stick to the road. The underside of the car with it's flat surface and diffuser is designed to accelerate the air under the car, creating lower pressure and thus "sucking" the car down to the pavement.

Disclaimer: The above is horribly over-simplified.

 

The diffuser on the 360 works to generate negative lift, or downforce. At high speed I have found it very stable..... At least in CS form. So did the F40. MyTR and 348 on the other hand got fairly "light" at speeds over165

 

Airflow over top of the car: at the end of the car this creates more vacuum which means less pressure. Actually it means the car will be 'sucked'.
But under the car: the bottomplate is pretty flat but the airflow is canalized from the beginning of the car till the end (the big diffusor). Bernouilli's Law (conservation of energy per volume); in the beginning the airflow accelerates and at the end (diffusor) the volume is getting bigger so the air molecules are going slower so pressure is less (less lift) so downforce. Because the diffusor is so big the effect of the downforce is much much bigger which results in downforce.
The diffusor of the 360 was the beginning of this with sportcars. So, it is really possible to let it 'fly'. When the car comes at the top of a hill where a lot of air can go under the car (refer to the angle of attack of an airplane wing) it is possible that the car really lifts (remember the flying Mercedes in the 24 hours of Le Mans something like 10-12 years ago, just flying over the trees).
Therefor they have improved the diffusor at this moment. Now they use a smaller diffusor (in volume) but steeper and with fins to break the airflow.

So, automotive aerodynamics is the opposite of aerodynamics of a wing; at a wing you have the effects of the airflow more at the beginning of the wing resulting in 'sucking' the airplane in the air. At a car; the results are at the end with a lot of downforce.

 

I don't even know where to begin addressing the replies to the initial question, most of it was just nonsense, so I'm just going to answer the original question. My background: I'm a NASA test director at the world's largest wind tunnel. Previously, I was an aerodynamicist at a couple NASCAR and Indycar teams.

Question: why is the top curved? Answer: most important reason, which applies to almost ANY car, you've got to put a person in the car and that person has to be able to see all around the car. How many wings have you seen with a passenger compartment? The only way to get two people in the car is to make the top curved. Secondly, and this is VERY important to Ferrari, it's got to look good. A flat, sloped top would be horrendously ugly.

But yes, the curved top does, in general, generate lift. So what do you do about it? On the top, they can do little things, like put little spoilers at the back, vent low pressure areas, etc. But the biggest thing they do is related to the second part of the question....

Question: why is the bottom flat? Answer: the biggest difference between wings and car is ground effects. The presence of the ground so significantly affects the aerodynamics around the car that on open wheel cars, the underbody generates approximately 50% of the total downforce. The best way to take advantage of ground effects is to use venturi tunnels, which is one of the reasons Group C race cars were so fast. Most cars don't have room for tunnels so the second best thing is to make the bottom flat so the air flows under the car as fast as possible, which lowers the pressure under the car.

 

I have changed the drawing a bit to explain the airflows under and above the car:
Top of the car: So you can see that the airflow becomes instable at the end (some vacuum resulting) in less pressure (lift). But because the back of the car is higher then the front this effect is reduced (cfr. angle of attack of a wing).
Under the car: airflow is guided from the middle of the front to the diffusors. As explained before, air (compressed between ground and car) goes faster under the car (Bernoulli; when you compress the air it goes faster and pressure goes down) then air comes in the diffusor, suddenly the volume is bigger, airspeed drops (normally more pressure) but important the airflow becomes instable and results in some vacuum which is an extra under pressure and 'sucks' the car to the road (especially at the rear wheels, traction, for more stability).

A 100% flat bottom can be faster but then you have the results of the airflow behind the car (vacuum) which can give instability (by braking, .....) A sportscar has to be fast and stable. When it is more stable it is faster overall.

(Sorry for some expressions; for me it is not so easy to explain it in English)
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Here's a nice little summary of aero in race cars. A little technical, but not much more than necessary.
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Welcome. Be sure and read the aviation section of FChat. It is amazing to read some of the posts from some guys that have had interesting careers such as yourself.

 

I've often wondered the same thing. All these wings, diffusers, etc, but overall these cars have an airfoil shape.

 

A great website on prototype (Group C, ALMS LMP1&2, etc) aerodynamics.

http://www.mulsannescorner.com/