The article presents the analysis of a commonly used performance modification method for turboprop and turboshaft engines with a free power turbine. The description of analytical and numerical methods of evaluation for a change of parameters and geometry of turbine assemblies are presented. There are also given the basic methods of changing throat area of turbine nozzle guide vanes and its influence on engine performance. The calculation methods are verified by experimental tests run by WSK “PZL-Rzeszów” S.A.