If fuel passes through an engine without being burned for some reason, when the fuel reaches the hot exhaust it will go bang.
It is not dangerous. The explosion is loud but is contained within the exhaust system. I have never heard of a backfire injuring anyone – perhaps it can happen but I’ve never heard of it.
It is not good for the exhaust system because it causes an overpressure that can damage the catalytic converter or muffler.
There are a few causes. One is a problem with the ignition system such that the spark doesn’t occur or doesn’t occur with the correct timing so that fuel escapes before it is ignited. Another is poor fuel/air mixture so that fuel gets through unburned.
You need to make sure you distinguish afterfire and backfire. What you usually hear in sports cars (or tuned cars) is *afterfire* where extra fuel is burned in the exhaust and creates a loud pop/bang. This is not dangerous, just wasteful of fuel and could mean the car is running inefficiently if it’s not intentional.
Backfire is when combustion occurs in the intake portion (ignition travels backwards, hence the name). This is very dangerous to the engine’s longevity and can significantly reduce power, but it will not cause the car to explode or anything like that.
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