you can run a restrictor instead of a thermostat but with most street cars your better off with a thermostat. but when the thermostat opens it's good to get the water moved threw fast.

when you run a restrictor you want the water to flow threw the motor slowly so the water has time to cool in the radiator. the water is always running threw the motor and radiator.

with a thermostat the water stops flowing thew the radiator and cools in the radiator. but water is still moving threw the block threw the bypass hose. when the thermostat opens you want to get the cold water into the block and close the thermostat fast so the radiator has time to cool the water before the thermostat opens again.

this is how i understand how it works.