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The crackles and pops are actually small explosions in the exhaust. Because the fumes are quite hot, the tailpipes in a car rise significantly in temperature. Any leftover fuel that did not get ignited in the cylinder, can ignite in the exhaust, making the bangs and crackles enthusiasts love hearing.
The reason there is leftover fuel is because the car his a rich air : fuel ratio. To prevent the mixture from being too lean and causing problems, a car’s injectors can be programmed to shoot more fuel into the cylinders, causing the rich mixture. The unburied fuel can then crackle and pop in the exhaust. Additionally, on gearshifts and when one gets off the throttle, the cylinders stop firing, and for a brief second, all the fuel inside sent to the exhaust system, causing more bangs.
However, ever since we decided to care about the environment and started putting catalytic converters in cars to clean up the fumes, there are less backfires and exhaust crackles. Mostly only performance cars that run REALLY rich fuel mixtures or cars in which the owner has removed his or her catalytic converters and replaced them with test pipes have lots of crackles, pops, bangs, and exhaust flames.