Rolling coal

Rolling coal is a term used to describe the result of intentionally removing the pollution controls on diesel trucks or cars. This causes the vehicle to produce black, sooty exhaust. Participants often "roll coal" on pedestrians, bicyclists and drivers of hybrid vehicles.
Vehicle modifications.
Pollution controls such as exhaust filters are removed. Additionally, the vehicle may be modified to deliver more fuel to the engine than usual. The turbocharger may also be disabled, although this is uncommon because it can reduce performance and cause engine problems.
Legality.
In most countries, rolling coal is prohibited by anti-pollution laws, for example, in the United States it is prohibited by the Clean Air Act.