studied the tip clearance of centrifugal compressors. The clearance was altered by shimming the shroud wall, i.e., the axial clearance was changed. The effect of the tip clearance was examined using six different compressors and the clearance ratio (the clearance divided by the impeller exit width, t/b2) was changed from 0.01 to 0.1. The variation in efficiency drop versus the clearance ratio was different for the cases analyzed. The slope of the efficiency drop curve varied from 0.03 to 0.1 efficiency drop per 1% change in the clearance ratio