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Good call on the underbody channels. They create vorticies (like minature tornados) that suck the back of the car down. The air spins around real fast and trades pressure energy for movement (kinetic) energy (faster means less pressure).
You can get downforce by kicking the rear end up, but it is highly dependent on geometry details and the speed you are traveling.
Tyson - Sorry to hear you don't believe the venturi effect.
"Island911" states that the problem is to simple to be explained by either simple wing theory or simple venturi effects. I entirely agree. Side-body spillage, the presence of the rear axle, the effect of under-body air friction and the rear tires all play a huge roll for a production car. These are all critical parts of how a factory like Porsche tunes the underbody aerodynamics.
The comment about compressibility changing the venturi effect is ultra-bogus though. Compressibility effects for a car traveling at 100mph are worth roughly 0.7% of the total number. This is why I noted that the air is "pretty much incompressible". Don't believe that? Check out Chapter 3 of "Modern Compressible Flow" by Anderson where he shows that increasing the area of a pipe increases the pressure for anything below Mach 1. You may not like the answer, but that's how it is.
Practically speaking, try it out and see how it goes. Just be careful because it could get more squirrely after the change, especially as the speed is increased. You can also tape yarn strings to the rear deck or underbody to visualize the flow and view it from a video camera or chase car. The fact that race teams tilt the front down suggests that it will do something good.
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