Conventional Savonius or modified forms of the conventional Savonius rotors are being investigated in an effort to improve the coefficient of power and to obtain uniform coefficient of static torque. To achieve these objectives, the rotors are being studied with and without central shaft between the end plates. Tests in a closed jet wind tunnel on modified form of the conventional Savonius rotor with the central shaft is reported to have a coefficient of power of 0.32. In this study, modified Savonius rotor without central shaft between the two end plates is tested in an open jet wind tunnel. investigation is undertaken to study the effect of geometrical parameters on the performance of the rotors in terms of coefficient of static torque, coefficient of torque and coefficient of power. The parameters studied are overlap ratio, blade arc angle, aspect ratio and Reynolds number. The modified Savonius rotor with an overlap ratio of 0.0, blade arc angle of 124 degrees and an aspect ratio of 0.7 has a maximum coefficient of power of 0.21 at a Reynolds number of 1,50,000, which is higher than that of conventional Savonius rotor (0.19). Correlation is developed for a single stage modified Savonius rotor for a range of Reynolds numbers studied. (C) 2008 Elsevier Ltd. All rights reserved.