someone i know recently demonstrated that their modern function is apparently poorly understood.
back in the day, a thermostat just sat on the high side of an engine, and opened when the coolant temp convecting around it raised enough. if there was a problem, like the thermostat failing closed, you could take it out and the engine would simply run cold because it was like a faucet that controlled the total coolant flow. and thus easy to understand.
but then came emissions regulation, and the need for closer engine temperature management, so "bypass" cooling was introduced by some manufacturers. i.e. there is a separate coolant circuit that flows in parallel to the radiator which allows a much narrower band of engine operating temp, and thus better control of emissions. it works by introducing cooled fluid into what is actually the main circuit, so the pump always has full flow, the engine has no hot spots, and management is close. but, and this is apparently the part that's not so easy to understand, if you run such a motor /without/ its thermostat [which typically has a different configuration also btw*], you completely unbalance the fluid dynamics and you can end up with only bypass flow and consequently have no cooling.
this person i know has just blown their third [expensive] race engine not understanding this so i repeat it here for the record. and if this person is reading, and you're still hellbent on running without the thermostat [which doesn't achieve what you want anyway] *block the freakin' "bypass" circuit*!!!
*vs.