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Expanding a bit on Insite's explaination, the car is part of the lift. The air flow pattern is influenced by the shape of the body work. The lift created by an airfoil is heavily influenced by that flow. If you look at the airfoil on the boxster, it's basicaly just a nearly vertical wall which would have almost no lift if it weren't attached to a car.
There are countless tables of airfoil lift and drag coefficients developed for the aviation industry, relating to Reynolds number typically, but I'd be very surprised if you ever found coeficients that you could plug into the simplified form of the navier-stokes equation you've got there relating to a specific car.
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