Burnt Bumper From Exhaust

[Whoa Black Boaty]

I see a lot of track cars that have a burnt stain just above the end of the exhaust on their rear bumper. Can someone tell me what it's from? I'm not sure if it's burnt and permanent or just dirty and can be cleaned off. (guessing it's from car backfiring and shooting flames)
cheers

[Sinista32]

Most of the time the melt is caused by the tip actually touching the rear bar.

Shooting flames is ignited unburnt fuel

As for the stains you can clean em off unless you leave them for too long.

Decats, straight through type mufflers, straight through exhaust and running rich cause the fuel/oil stain.

[AMR33]

It's from being super awesome