Abstract This paper reports on an experimental and numerical investigation of a modifiable Tesla turbine driven by compressed air. To study the impact of geometrical parameters like the rotor’s diameter and space between the rotor and the housing, two Tesla turbine prototypes with diameters of 11.25 and 15 cm were designed and built. For both, a 3 mm space between rotor and housing is considered. In particular, for each turbine the effect of disc spacing and pressure flow in the range of real operating conditions is analyzed. Experimental characterization based on a thermodynamic analysis allows estimating optimum values of 0.9 mm and 103.89 kPa for the disc spacing and pressure flow, respectively. Additionally, values for the efficiency and the stall torque are calculated when the pressure flow is varied in the range of 50 to 327 kPa and of 59.34 to 272.46 kPa, maximum efficiencies of 33% and 50% are obtained, and stall torque measurements reach up to 0.304 N · m and 0.448 N · m, for the turbines with small and large diameters, respectively. 3D simulations performed with the open source CFD library OpenFOAM ®, well reproduced the stall torque measurements. Numerical results show that the loss of isentropic power in the space between the rotor and the housing is up to 36.17% and that the largest wall shear stress values are located on the periphery of the rotor’s disc.