The bypass allows the coolant in the engine to circulate while the thermostat is closed. The bypass lets the engine coolant heat up uniformly so no hot spots are created. Once the thermostat opens, it does bypass some water that could be going through the radiator and cooled. I've sometimes wondered how big the bypass really needs to be in order to work the way it was designed to. Could a restrictor be added that would maintain the bypass function yet allow more coolant to be cooled by the radiator? Has anyone done it?
Not much to add other than that for size big blocks use a 7/16-1/2" bypass internal to the water pump housing. There is no or minimal cooling circulation with the stat closed, and I always thought the bypass was to protect the heater core from excessive pressure.
