WTF Is Rolling Coal?

Originally only found in truck-based motorsport, rolling coal has become a modification found in many road-going trucks in a stand against any form of eco-friendly motoring. These systems have now also made their way into the modified car scene, so here’s a quick explanation of what is going on behind the scenes of these smoke-billowing crowd-pleasers.

Rolling coal is the combination of throwing a large amount of fuel into the engine’s cylinders along with the bypassing or removal of the emission control devices further downstream. By flicking a switch rigged up on-board to the ECU, the air/fuel mixture can be made extremely rich, pushing it past the optimum combustion condition called the Stoichiometric ratio. This richer mixture (below 14.6:1) subsequently creates more particulates within the exhaust gases, otherwise known as soot.

The EGR (exhaust gas recirculation) valve controls the amount of exhaust gases that are recycled back to the combustion chamber to help absorb unwanted emissions from the post-combustion gases. However, in rolling coal setups, the EGR valve can be bypassed using aftermarket kits. Couple that with the removal of the catalytic converters within the exhaust system and you have virtually no control over the emissions coming from the engine, thus resulting in huge billows of soot emanating from the tail pipes.

To finish off potentially the most-obnoxious modification known to man, straight-piped ‘smoke stacks’ can be fabricated directly behind the truck’s cabin or even as straight-up chimneys protruding from the bonnet, making sure there is no dilution of the exhaust gases immediately after combustion.

Originating from the sport of truck pulls – where large trucks compete to see who can drag the heaviest load – rolling coal was designed as a grand show of horsepower and torque in these most American of gladiatorial battles. But the trend has swiftly made its way into road trucks.

Unfortunately, the US Government deems any form of emission-control tampering illegal under the Clean Air Act. If all the videos on YouTube are to be believed, it seems most rolling coal systems are normally only used in rare Prius-scaring situations, therefore switching off any bypass valves should see a truck successfully complete an emissions test. So just don’t get caught smoking out some poor hybrid driver by the local law enforcement and you’ll be just fine!