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we should also talk about header clocking by firing order into the collector - for scavenging goodness. Scavenging matters because ideally we would have a reversible and adiabatic expansion of the exhaust gases in the cylinder from the maximum cycle pressure to the ambient atmospheric pressure during the power stroke. If the gas is leaving the cylinder at some elevated pressure because of the pressure drop throughout the exhaust system, the potential expansion work relative to the ambient pressure is wasted and the engine power is reduced.