A computer submodel, predicting the twin-turbine discharge coefficients is presented. Gas flow through axial separated spiral volute casing into the interspace is solved with divided expansion in two convergent nozzles. Mixing taking place in the interspace is assumed to be isobaric and the gas expansion through the rotor is simulated with rotor characteristics derived from conventional turbine characteristics. The results of computation are turbine discharge coefficients and efficiency map, determining the twin-turbine boundary conditions under partial admission. The method has been validated using experimental data of 6- and 12-cylinder turbocharged diesel engines and the predictions are in good agreement.