Porsche
With the 911 GT3 I found the better car to use as a pltform (as opposed to the Suzuki) is the Toyota MKIV Supra(turbo).
Buy the Supra and any other car to get the Supra in slot 2. Soup that car up and tune it to the best of your abilities, save your game. Now you can input the code for the 911 and it should perform better than the MKIV. enjoy
More tricks, tips and cheats for this game are right here - Gran Turismo 3: A-Spec cheats
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