Seeing a vehicle with open headers is far less common than the majority of vehicles equipped with a full exhaust system. Open headers are extremely loud, potentially illegal on public roads and most commonly used on a race track. Open headers are the optimal design for exhausting engine gasses.
A full exhaust system is made of multiple components, which release gasses exhausted from an engine's combustion chamber. Starting at the engine's cylinder ports, an exhaust manifold or headers direct exhaust into a front pipe that leads into a catalytic converter, which acts as a filter. Exhaust then heads into a muffler, where engine exhaust sound is muffled, and finally it releases through a tailpipe. Open headers directly exhaust gasses from the engine through piping pointed outside.
Open headers provide optimal exhaust flow and velocity while minimizing back-pressure, which smothers engine power because of the direct path from engine to the air.
The Department of Motor Vehicles sets vehicle requirements that vary by state. Open headers are often illegal because harmful exhaust gasses are not filtered and delivered straight into the air. Open headers will fail emission testing. There are exceptions. For example, Delaware requires every vehicle originally equipped with a catalytic converter to have one.
