Asked by: Lawrence Cuthbert, Jarrow
An ordinary car’s exhaust is designed to duct toxic gases away from the engine and through a catalytic converter (to remove some of the pollutants) and a silencer (to reduce engine noise). The diameter of an exhaust pipe, and the number of twists and turns it takes, affects the friction on the escaping gases, which increases the back pressure at the cylinders. This increases the energy required to push the gas out the exhaust pipe and thus reduces the engine’s torque.
Using wider pipes or dual exhausts reduces the wall friction and back pressure, so torque increases. But if the pipes are too big, the gas expands, cools and slows down, which makes it harder to push out, so the back pressure increases and torque drops again. Most ‘showy’ exhaust pipes are too large for their engine and reduce torque at all but the highest rev ranges.