Abstract: A bypass configured butterfly type rotary valve particularly suitable for controlling engine coolant flow to a passenger compartment heater core outlet. A baffle is employed to increase linearity of flow control when the butterfly is close to shutting off flow to the heater core outlet. An additional baffle increases linearity as the butterfly nears the fully open position for shutting off flow to the supply return or bypass. In one embodiment of the baffle is formed integrally with the valve body. In another embodiment, the baffle is formed as a flapper mounted on the shaft with the butterfly, but remaining stationary as the butterfly is rotated. The additional baffle is formed as a scalloped thickened edge of the butterfly in one embodiment, or as a separate insert in another embodiment. The baffles increase the linearity of the valve throughout the full range of movement of the butterfly.