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?
I found this out the hard way. When I built my 434 the Indy intake wasn't drilled for the bypass nipple. The engine would get hot quickly then the thermostat would open and it would cool off then repeat the process. I drilled and tapped the intake for the nipple then installed the bypass hose and it's been fine since.
