A true dual-exhaust system helps to improve the performance of a V-6 or V-8 engine by splitting the exhaust from the two banks of cylinders. However, it is less effective on smaller vehicles that use an inline four-cylinder engine. The system reduces the pressure to expel the exhaust fumes, allowing the engine to send more energy to the transmission.
The faster a car engine is able to move air through the system, the easier it is for it to combine fuel and air for a higher performance. Some drivers claim an increase of up to 12 horsepower just by adding the exhaust. In order to provide the full benefit, the exhaust must feed the muffler directly from the exhaust manifold. Systems that combine the exhaust using a "Y" configuration that splits later defeat the purpose. Similarly, splitting the exhaust after the muffler gives the appearance of higher performance but does nothing at all for the vehicle.
A properly fitted dual exhaust helps to improve the fuel efficiency of a vehicle. However, this improvement is slight and could be lost if the driver presses the car for higher performance.
While drivers also enjoy the throatier exhaust sound of a dual exhaust, the law in many places requires vehicles operate below certain noise levels. Dual exhaust systems are often expensive, and there are additional costs involved with installation. To get the most from the system, it is necessary to improve the air intake, which also adds to the cost.