An exhaust is a device that allows the engine's hot, noxious gas (exhaust) to escape from an automobile. The sound of the escaping gases can be heard as they leave through some kind of muffler or another noise-reduction system. The function and design of this part depend on the type of vehicle. For example, some vehicles are very simple in their construction, while others have more complicated systems for releasing gases, such as by chemical catalytic conversion into less toxic substances. The advantages of having an exhaust include reduction of backpressure, which boosts performance and fuel economy; also reduce turbo lag since there will be less retained heat in the intake charge and lower air density due to greater expansion ratio(denser airs slow.