Automotive electric fan thermostat

Closed-type cooling system.  The closed-type system doesn’t vent excess coolant to the outside when the heat causes expansion of the coolant.  It has a coolant reservoir that collects coolant that has expanded with the rising temperature of the coolant and would otherwise overflow from the system.  When the system temperature drops, the coolant is drawn from the coolant reservoir back into the radiator by suction created by coolant contraction thus maintaining a completely filled radiator.

This cycle occurs when the coolant reaches a certain pressure in the radiator regulated by the spring holding the overflow cap closed in the filler neck.  The pressure of the expanding coolant overcomes the tension spring in the cap and allows the excess coolant under pressure to overflow into the expansion tank, or recovery tank through the overflow port in the filler neck.

In a closed-type system care should be taken when checking your coolant level in the radiator.  After initial fill up, the radiator is full and the cap is installed, all level checking should be done at the expansion tank.  As the cooling system goes through it’s heating and cooling cycles, coolant needed to replace air trapped in the system will be drawn from the expansion tank.  This is where you add coolant if the system is low.  Removing the radiator cap will result in loss of system balance and it will take another heating/cooling cycle to reestablish this balance.

Open-type cooling system.  Early vehicles with narrow radiators placed the tanks at the top and bottom of the core.  The filler cap was placed on the top of the top tank.  The radiator was filled with coolant to within one to two inches from the top of the tank to allow for expansion of the coolant.  When these systems were pressurized by adding a pressure release cap to the radiator, excess coolant created from expansion was vented to the outside.  An over flow catch can was installed instead of dumping this excess coolant onto the ground.  However, the coolant is not drawn back into the system as it is with the closed system.  In street applications, an upright radiator (top and bottom tanks, with the cap on the top tank) represents a compromise that will work, as long as the car is not operated at sustained high RPM, like those seen in racing.

Radiator Caps
The radiator cap is often overlooked in common engine overheating problems.  It is designed to add pressure to the cooling system and increase the boiling point of the coolant.  That is accomplished through the use of an internal spring-loaded pressure relief valve.  Which one to choose depends on the type of system your vehicle is equipped with.

Top Mounted Radiator Cap Location
In a cooling system, a higher pressure equates to a higher boiling point for the coolant.  Higher coolant pressures also transfer heat from the cylinder heads more efficiently.  It is recommended that a radiator cap be used with the highest pressure rating that the radiator is designed to accept.  The coolant will typically only build to 16-18 PSI, due to expansion up to 200°F.  However, if the engine does overheat due to external factors, the pressure inside the cooling system could reach as high as 28 PSI.  Once the radiator cap has opened and vented coolant, the engine will not cool down until it has been turned off and more coolant added.  The radiator cap is basically a "safety valve", so always use the highest-pressure radiator cap that the radiator will tolerate.  If you are unsure of the pressure rating for your radiator, check with the manufacturer for the maximum recommended operating pressure.
Cross flow Radiator Cap Location
Crossflow radiators with the tanks located on the sides of the core are used in closed systems.  The radiator cap should always be located at the highest point of the cooling system, and on the low-pressure side (after the radiator core).  Cross flow radiators mounted higher than the engine are ideal because the cap is on the tank that is connected to the water pump inlet.  This configuration offers three advantages:  1—The cap is at the highest point of the system, allowing any air to migrate to the area just below the cap.  In the event the cap vents due to excessive pressure, the air will escape first.  2— This area has the lowest velocity within the system, allowing air to separate from coolant even at high engine RPM.  3— The cap is located on the low-pressure (suction) side of the system, so it is unaffected by the pressure generated by the water pump.

Water Flow Thermostats
The thermostat is a temperature-controlled device installed into the cooling system that opens and closes to regulate the flow of coolant into the radiator.  It opens the valve in the thermostat allowing coolant flow at a predetermined temperature and closes halting flow when the coolant temperature drops.  Thermostats are available in a range from about 160° to over 200°.  Older vehicles with open-style cooling systems and classic engines usually use a lower temperature thermostat.  These engines operate at lower temperatures and coolant loss can occur in an open system if the system overheats.  Newer vehicles equipped with closed systems are operated at higher system temperatures to aid in reducing emissions and do not lose coolant because it is drawn back into the system when it cools.  Any aftermarket thermostat housing that mounts the radiator cap directly above the thermostat location, or that mount the radiator cap in the top coolant hose, are not recommended.  Both of those housing styles are poorly designed, and will push coolant out of the cap at high RPM.

Coolant Flow Through The System
Older vehicles 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 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.  Some added restrictors or slowed water pump speed that prevented the coolant from being forced out, and allowed the engine to run cooler.  However, vehicles 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.

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.

Hoses
Standard full-size hoses should be used to ensure maximum coolant flow.  Using smaller "AN style" hoses can decrease flow that could inhibit proper cooling.

Pulley Ratio
For street applications, the water pump speed must at least match the crankshaft RPM, to a maximum of 10% faster than the crankshaft speed.

The Choice for our Application
Our application, a ’32 Ford highboy roadster, equipped with an upright-style open-type cooling system has to cool a big Buick Nailhead V8 with a small grille opening.  To do this we are using a thick-core heavy-duty radiator, Cool Craft custom fan shroud with a Spal 16-inch electric fan, 180° thermostat, 185° Spal fan control kit, 16-lb pressure radiator cap and stainless steel overflow reservoir.

We settled on this combination after considering all of the previously mentioned factors.  The key player in keeping our system operating within the proper range is the fan control.  Ours engages the fan at 185°F and shuts off at 165°F.  This allows the cooling system to operate within the proper range without overheating at slow speeds or when idling and while avoiding any significant loss of coolant.