The present invention provides means to extend the flow rate range over which a downhole turbine 70 will return a power output sufficient to meet the minimum downhole power requirements. In one embodiment, the present invention relates to an arrangement of axial vanes 77 that are situated such that the rotation of the rotor 76 generates an increasing drag force, thereby extending the upper limit of the flow rate range. In another embodiment, the present invention relates to an arrangement of restriction assemblies 75 that can be used to maximize the fluid velocity relative to the fluid flow rate past the stator 74 to achieve the necessary speed and power to rotate rotor 76, thereby extending the lower limit of the flow rate range. In another embodiment, the axial vanes 77 and restriction assemblies 75 are used in combination to further extend both the upper and lower limits of the flow rate range of the downhole turbine 70.