The practical performance, both the efficiency and durability, of a high-pressure (HP) turbine depends on many interrelated factors, including both the steady and unsteady aerodynamics and the heat transfer characteristics. The aerodynamic performance of new turbine designs has traditionally been tested in large scale steady flow rigs, but the testing is adiabatic, and the measurement of heat transfer is very difficult. Transient facilities allow fully scaled testing with simultaneous heat transfer and aerodynamic performance measurements. The engine matched gas-to-wall temperature ratio simulates more closely the boundary layer and secondary flow development of the engine case. The short duration of the testing means that the blades are effectively isothermal with a rise of only a few degrees during a test. To isolate the aerodynamic losses, and thus the entropy generation due to the viscous losses, the entropy reduction due to heat transfer during the expansion needs to be determined. This entropy reduction is path dependent and requires knowledge of the full temperature and heat flux fields. This paper demonstrates a simple methodology for estimation of this entropy reduction, which allows the calculation of the adiabatic efficiency from the results of engine representative nonadiabatic testing. The methodology is demonstrated using a computational fluid dynamics (CFD) prediction which is validated against experimental heat flux data. Details of the other corrections required for transient test techniques such as unsteady leakage flows are also discussed.