Dual exhaust refers to the exhaust system in vehicles, especially passenger cars or pickup trucks, which have two distinct exhaust systems to carry exhaust gases away from their internal combustion engine. This is most common on vehicles with V-shaped engines. It is both a common aftermarket performance enhancement and is often found on stock vehicles. It provides better performance than badly-implemented single exhaust systems because it allows for less resistance to exhaust flow, both due to the size of the piping and also because an exhaust system which has passages of inequal length before two pipes come together will have a resonant effect that results in uneven exhaust backpressure which changes at different RPMs.
Typically the exhaust system consists of a manifold or header which combines the output of several exhaust ports on the cylinder head into one pipe, a catalytic converter which consumes unburned fuel to improve emissions, some additional piping, and a muffler which is typically a metal can with baffles in it to reduce exhaust noise. Exhaust systems may also contain a resonator box which changes the sound of the system, usually making it quieter and of a consistent volume at assorted RPMs. A dual exhaust system has two of each of these things. On computer-controlled vehicles it also has at least two oxygen sensors in order to monitor emissions; OBD-II vehicles will at least have two additional oxygen sensors to measure catalyst efficiency.
Dual exhaust systems may not provide peak performance in all conditions. Some V-shaped two-stroke engines, which are dependent on exhaust scavenging to pull the air-fuel mixture into the combustion cylinder, use an X-shaped pipe to maximize the scavenging effect. Variations on this theme are also used with four stroke engines to improve exhaust flow rate. Many vehicles have mock dual-exhaust systems, for example the third generation Chevrolet Camaro IROC-Z28 which has an H-pipe in between the two exhaust pipes, and whose two pipes both go into the same muffler which has two outlet pipes, or some straight-six vehicles which have a single pipe from the manifold to the back of the vehicle, where it branches off into two mufflers, or goes through a muffler and then to two outlet pipes.