While environmentalists want to get the best fuel efficiency possible, coal rollers spend time and money to get the worst.
The engine has to be a diesel. No matter how modified a gasoline engine is, it isn't going to work. The smoke is the result of unburned fuel particles, and only diesel will do. Gasoline isn't dark or thick or offensive enough. But not just any diesel truck can roll coal. As the truck pulling crowd knows, such an achievement requires some modifications. The goal is to get extra fuel into the engine, which will be immediately burned off and puffed out as smoke. Witnesses then interpret the smoke as proof that the truck is indeed quite powerful. Quality matters as much as quantity. The blacker the smoke, the more unburned fuel it contains, and therefore, the better it is. To achieve this feat, a coal roller has to buy extra stuff. An aspiring coal roller may be able to get away with using the truck's stock exhaust, but that doesn't make much of a statement. Chimney-pipe-style exhaust stacks installed in the truck bed seem to be the way to go.
It's possible to spend a lot of money modifying a diesel engine to roll coal, anywhere from about $500 to $5,000. There are a couple approaches. The first and most expensive is to take a lesson from the truck pullers, and modify the engine for performance gains, of which excessive black smoke is a normal byproduct. For prospective rollers willing to crack open the engine, installing bigger injectors is the most likely route to producing the biggest clouds. This step often requires a lot of custom tuning and complementary modifications, because it'll significantly affect the way the truck runs, and that means there should be a thought-out, cohesive plan in place, preferably with the help of experts. Otherwise, the truck will run horribly. Upgrading the engine is the best way, but that's probably best left to the motorsports crowd.
It's easier and cheaper to achieve a coal-rolling truck that's all bark and no bite. An engine tuner (aka programmer) is the simplest and least expensive way to actually feed the engine more fuel. This will basically flood the engine with excess fuel that the engine has no choice but to burn off, and it doesn't accomplish much else on its own. Another option, if you're coal rolling just for attention, is a smoke switch, which is probably the way to go. Like the other methods, a smoke switch will trick your engine into a state of fuel gluttony for gluttony's sake. It's very much a DIY project, and a cheap one, at that ... which saves money for all that extra fuel.