Ok, so not to sound so stupid here but I've always wondered that - why when you see a car with an aftermarket exhaust (specifically an import) when they get on the gas you see a plume of black coming out the back?

 

The soot has nothing to do with the type of exhaust the car has. The car may be running too rich, or may just be burning oil. Maybe it's a diesel??? There are a few reasons for it.