The Nature of the flow in the exhaust pipe of a turbo-charged two- or four-stroke cycle engine is in general so complex that practical tests on running machinery are necessary to compare the merits of one exhaust system with another. The analysis of the results of these tests is rendered somewhat difficult by the lack of reliable methods for measuring accurately the transient temperatures in the exhaust pipe. Indicator diagrams have therefore to be used in conjunction with empirical methods for computing the exhaust energy, these methods being associated with the measured exhaust pyrometer temperature. In calculating the theoretical possibilities of different exhaust pipe configurations for four-stroke cycle engines Jenny has suggested that the total energy was the sum of the internal and kinetic energies of the gas entering the pipe. In the present treatment it is suggested that a better criterion is the enthalpy available if the gas expands isentropically from stagnation conditions to atmospheric pressure, the justification being that this would be the energy utilised in the turbine.