Scavenging is the process of pushing exhauste gas-charge out of the cylinder and drawing in a fresh draught of air ready for the next cycle in automotive usage. This process is essential in having a smooth-running internal combustion engine. Modifying the exhaust system, (i.e. modifying the exhaust gas velocity by changing exhaust tube diameters) can detract from the "ideal" scavenging effects, and reduce fuel efficiency and power.