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2002 996tt aftermarket exhaust system and underbody airflow


Vehicle Information: Year: 2002; Body Type: Coupe; Total Mileage: 8200;

I'm not sure if my question e-mail from yesterday actually was sent, so I'm trying again. If you did receive something like this question yesterday, please ignore this duplicate.My question has to do with underbody airflow, and whether downforce at the rear may be significantly compromised by some aftermarket exhaust systems. Paul Frere's book indicates that the engine/transmission unit in the 996 is tilted 1.7 degrees forward, raising the rear of the engine and creating a venturi effect for the underbody airflow as it exits the rear of the car. This creates downforce at the rear axle and improves handling at higher speeds. I have noticed that several aftermarket exhaust systems have muffler boxes that are considerably larger,and hence "taller" in cross-section, than those on the factory standard system. If you squat behind a car with one of these systems installed, or view the car from a distance, you can see a bit of the muffler boxes extending below the rear valence panel. You don't see the factory system. It seems to me that this location of the aftermarket system could potentially interfere with the airflow exiting the car, perhaps compromising or eliminating entirely the venturi effect mentioned by Frere. I assume you haven't personally tested this hypothesis in a wind tunnel, but would you have any annecdotal information -- perhaps from 996tts or GT2s that have been modified for track use -- about what the effect on downforce actually might be? I have no idea how much downforce is contributed by the venturi effect on a stock vehicle. If there is a significant loss of downforce with the modified exhaust, I imagine that the only way to compensate may be to add rear wing. I am extremely mindful, however, of your postings about not upsetting handling balance by adding too much rear downforce, and I would not want to venture into aerodynamic quicksand if the effect of the exhaust system is small or nonexistent.Thanks again for the always informative and entertaining reading.

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