Thermostats and Restrictors

The secret of the thermostat lies in the small cylinder located on the engine-side of the device. This cylinder is filled with a wax that begins to melt at around 180 F (different thermostats open at different temperatures, but 180 F is a common one). A rod connected to the valve presses into this wax. When the wax melts, it expands significantly, pushing the rod out of the cylinder and opening the valve.

It's amazing how many people think the thermostat's main function is to keep the engine cool by regulating the amount of coolant through the radiator. More importantly it prevents the engine from running too cold. It allows the engine to heat up quickly after start up and to keep it at constant operating temperature minimizing cold running where accelerated engine wear occurs. Another lesser known fact is that it also acts as a restrictor to bring up the boiling point of the coolant inside the engine. But not so that it restricts flow enough to induce hot spots. It's a poor practice to remove the thermostat on a street car when fighting overheating problems.

A common misconception is that if coolant flows too quickly through the system, that it will not have time to cool properly. However the cooling system is a closed loop, so if you are keeping the coolant in the radiator longer to allow it to cool, you are also allowing it to stay in the engine longer, which increases coolant temperatures. Coolant in the engine will actually boil away from critical heat areas within the cooling system if not forced through the cooling system at a sufficiently high velocity. This situation is a common cause of so-called "hot spots", which can lead to failures.

Years ago, cars used low pressure radiator caps with upright-style radiators. At high RPM, the water pump pressure would overcome the radiator cap's rating and force coolant out, resulting in an overheated engine. Many enthusiasts mistakenly believed that these situations were caused because the coolant was flowing through the radiator so quickly, that it did not have time to cool. Using restrictors or slowing water pump speed prevented the coolant from being forced out, and allowed the engine to run cooler. However, cars built in the past thirty years have used cross flow radiators that position the radiator cap on the low pressure (suction) side of the system. This type of system does not subject the radiator cap to pressure from the water pump, so it benefits from maximizing coolant flow, not restricting it.

Nucleate boiling is the boiling that takes place when liquid coolant comes in direct contact with the hot metal of the engine block or cylinder heads which has reached or exceeded the boiling point of the coolant. Under these conditions, the liquid turns to a vapor as it boils at the hot metal surface, and will then finally break away to be replaced by additional liquid coolant. Very large amounts of heat are absorbed into this layer of liquid coolant from the metal surfaces of the engine, yet those surface temperatures in a controlled nucleate condition never rise much beyond the boiling point of the coolant, and thermal control is maintained.

Nucleate boiling is reduced or eliminated when the coolant itself becomes continuously displaced from the metal surface by a layer of vapor called a "film blanketing" (surface) layer. When blanketing occurs, the metal surface becomes insulated from the surrounding liquid coolant, and an ever - increasing localized high temperature spike develops.

In order to prevent this blanketing effect you need to continuously flow as much coolant as possible. A restrictor does just the opposite, it restricts flow to the water pump and causes local hot spots to develop and eventually warp a head WITHOUT the violent gushing of coolant and steam. By the time overheating is noticeable it's already too late and the damage done. It may help overheating caused by the water pump pressure overcoming the radiator cap's rating but a higher pressure cap within the radiators pressure rating would be a more sensible approach in this case rather than decreasing the pump output.

For applications requiring a thermostat, NEVER use a restrictor!