Screw up the ignition timing so that it runs like crap, set it up to run way too rich and loosen up all the clamps on the exhaust system so that it leaks and you might get backfires. Stupid Idea Dude.
First, if this is a '99 GTP you have a 3.8L supercharged V6. Back in the '50s this was a fun thing for young people to do. That was in the days when you could "fool around" with ignition timing, carburetor mixtures, etc. to get a car to backfire. With today's modern computer controlled engines it's not possible to "fool around" with these kinds of engine settings without a computer. You also have a catalytic converter to deal with and is required to pass emissions inspection. Backfiring will blow the catalytic converter apart in addition to the muffler. Making a car backfire is no longer fashionable or practical. I suggest you move out of the 1950s! HTH. -Jim
Search Pontiac Grand Prix Questions
Pontiac Grand Prix Experts