In a typical air turbine starter (ATS) engine testing application, compressed air is supplied to the turbine by means of an inlet duct usually with a 90 degree bend and discharged from the turbine into the exhaust chimney through a combination of two duct passages. The primary duct is integral to the engine for connecting to the containment ring. The secondary duct is a transition piece for connecting to the exhaust chimney. As these ducts consume additional pressure and adversely affect the performance of the ATS under test. The design of pressure-efficient outlet ducts is therefore essential, and is the topic of present study. The aerodynamic performance of the overall passage depends on the (i) angle of bend, (ii) the shape of the connecting bolt, (iii) the outlet area and shape of the exhaust duct transiting between the bend and the chimney. Combinations of different angular bends, different shaped bolts and varying size of transition pieces are analyzed using the enterprise version of CFD tool, ANSYS. Three dimensional mesh independent simulations using k-epsilon turbulence model are carried out for a combined geometry of inlet duct, rotor-stator combination, outlet ducts together with the bolts. A combination of the duct passages that has resulted in lowest possible pressure drop is suggested as result of the study i.e. the 90 degree bend duct gives 9% pressure difference between inlet and outlet and this might slightly affect the efficiency of the air turbine stator, however the mass flow rate values remains similar to the stator inlet mass flow rate. Hence the 90 degree bend duct is suitable for the test rig. The static pressure loss and total pressure gain is about 0.04% and −0.004% respectively for baseline and aggressive duct of stator and rotor, hence the baseline duct profile is better than aggressive duct. Among different shapes of connecting bolt, the baseline geometry gives slightly lower efficiency of 85.6% when compared to all other models. But due to manufacturing feasibility the baseline geometry is preferred. Exhaust duct model 7 gives pressure drop as 0.062 bar twice the amount of pressure drop in model 6, but it does not affect the efficiency of air turbine starter. The shapes and sizes of the bend, bolts and the transition piece are recommended.