This study describes new results of the performance evaluations of an air powered vane type rotary novel air turbine/engine. The mathematical model with different parametric values such as; different rotor to casing diameter (d/D) ratios at optimum vane angle of 45º and injection angle of 45º, have been considered and analyzed. The optimum power output is obtained at some typical values of rotor/casing diameter ratios without consumptions of excessive air. The study shows that the optimum power developed under such conditions would be 4.3-5.5 kW (5.84-7.47 HP) at linear expansion (without excessive air consumption) when d/D ratios are between 0.85 to 0.80 and casing diameter is kept 150 mm, injection pressure as 6 bar (90 psi) and speed of rotation as 2500 rpm. This power output is enough to drive any motorbike or light vehicle.