I have heard it several times from a few older mechanics I know, just thought I would throw it in here. They said the water flows fast enough to not be able transfer the heat to the cores in the radiator and then go back into the motor hotter than it should. They would reinstall a thermostat and no more overheating problems.
They were talking about older engines, Flatheads, and Y blocks. I figured it couldnt hurt to try installing a thermostat, since one should be there anyway.
