The performance of a centrifugal impeller is predicted under steady ideal axial inlet conditions. The operational range is given in terms of load and flow coefficient and polytropic efficiency. The flow field at the exit of the impeller is analyzed to show how it deteriorates while moving close to the stall limit of the characteristic curve. The effect of a strong inlet distortion obtained by intentionally altering the geometry of an inlet volute on the impeller is analyzed. The investigation is carried out for two different distortions, without and with pre swirl, and for two pressure ratios, close to the design point and close to the stall limit of the operational range. The effect of the distortion is analyzed by both a quasi-steady and unsteady approaches. The first method, while faster, is found to give results in partial disagreement with the fully unsteady analysis and is therefore abandoned. The impeller under unsteady distorted inlet is found to have degraded performances in terms of efficiency and load factor, but the overall operational range seems not be significantly affected by the fluctuations of inlet total pressure and angles.