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1957: two steps forward...

1967! (A half century later)

About Chrysler cooling systems: Heat and thermostats

Why do we have automotive cooling systems?

As engines burns gasoline, expanding gases push the piston, driving the crankshaft; but only about one quarter of the chemical energy in the gasoline becomes kinetic energy (motion). Most of the rest is waste heat, some of which is thrown off via the exhaust system. The rest heats up the engine block and head.

cooling system operation

If there were no cooling system to carry this heat away, the metals would eventually melt, and the engine would “seize.” In nearly all cars, liquid coolant (antifreeze) moves heat away from the engine metal. If it’s cold outside, some of this heat might be used on the air passing across the heater core, warming up your frozen toes or defogging your windshield, but the coolant moves most of the waste heat into the radiator, where it is shed to the air passing across the surface. Heat that escapes either via the exhaust or via the radiator does no useful work; it is lost and gone forever.

Thermostats: cooler and hotter, and why they changed

A cooling system in good condition can get rid of heat faster than the engine can produce it, but it needs to be regulated, so the engine can warm up and operate efficiently. Thus, the thermostat, which starts out closed (so that heat is kept within the engine and coolant) and later opens, allowing hot antifreeze into the radiator, where it can be cooled down.

The thermostat controls the minimum temperature of the engine. It cannot control the maximum temperature. The only way the thermostat can cause the engine to run too hot is if it is faulty and doesn’t open completely (or at all).

stant superstat thermostatIt is commonly, but wrongly, thought that a cooler-running engine is better. It is not! An engine prevented from reaching proper operating temperature because the thermostat is “too cold” (opens at too low a temperature), or missing entirely, wastes fuel, wears faster, and dirties its oil faster. If you are removing the thermostat or running a 160° unit to prevent or fix overheating, you’re just masking the symptom of whatever is wrong with your cooling system. Find it, fix it, and run the correct thermostat.

The specified thermostat temperature is 180° (meaning the thermostat opens and allows the antifreeze into the radiator when the fluid reaches 180°) for Mopar vehicles through 1967 without CAP (“Clean Air Package,” Chrysler’s first emission control system). That’s true for Imperial, Chrysler, Dodge, and Plymouth.

Starting in the late 1960s, higher-temperature thermostats—185°, 190°, 195°, and even 200°—were installed at the factory to help clean up emissions. Hotter coolant means more complete combustion because of less quench-out, so there would be fewer unburned hydrocarbons (HC) and carbon monoxide (CO) in the exhaust. But higher combustion chamber temperatures also mean more oxides of nitrogen (NOx) in the exhaust, so starting in 1971 the factory began backing down on thermostat temp—from 190° to 185°, for example; specifics depended on engine and configuration. But then in 1973, valve-regulated exhaust gas recirculation came along as an effective NOx control strategy, so thermostat temps started climbing again; the spec was 195° pretty much across the board by 1974.

Cool air versus hot engines: which is better for performance?

Generally, and within reason, hotter coolant is better. A hotter engine is more efficient, and there’s less engine wear and longer engine life, because there’s less unburned fuel to wash the oil down off the cylinder walls. The exhaust contains less toxic unburned fuel and carbon monoxide.

oemMaking more power by raising the heat may seem wrong to those who know that cooler air inducted into the engine is denser (contains more oxygen), and so makes a bigger bang in the combustion chamber, therefore more power. That effect is real, so it’s worth ducting the engine’s air intake so as to allow the engine to breathe air that hasn’t been heated up under the hood (which is why so-called “performance” open-element air cleaners that breathe hot engine-compartment air are a step in the wrong direction, but that’s another topic for another day).

Cooler intake air tends to have much less impact on power than a hotter engine. Energy is “lazy” — it follows the easiest path available. The greater the difference in temperature at any given junction, the more heat transfer takes place. For engines, the larger the difference between the temperature of the just-burned fuel/air mixture and the engine metal it’s directly exposed to, the more easily the (heat) energy will flow into the engine metal. Likewise, if we reduce the difference in the temperature of the cylinder gas and the temperature of the engine metal, less (heat) energy will flow from the one to the other.

Imagine a bowl of soup at 180°F. If you leave it on the table in a 70°F room, the soup will gradually shed heat until it’s cooled down to room temperature, a difference of 110 degrees. If you put the bowl of hot soup in the refrigerator instead of leaving it on the table, it will shed more heat faster, cooling more rapidly down to refrigerator temperature, 40°F. That’s a difference of 140 degrees. (All temperatures on this page are on the Farenheit scale.)

In an engine, by reducing the difference in temperature between the combustion gas and the engine metal, you cut the loss (waste) of heat from the combustion gas, so more energy stays in the combustion gas, which is doing the work of pushing the piston. Smokey Yunick wrote in his highly-respected book Power Secrets: “Some guys go to great lengths to keep the engine temperature down … though the engine doesn’t overheat, they don’t realize that they’re putting energy (heat) into the cooling system that could be used to produce power at the crankshaft. Running the engine at 180 degrees will drop the overall horsepower by 2%-3%. For max power the cooling temp should be at least 200 degrees.”

stant thermostat for moparsMost of us aren’t building race cars, so other factors come into play. Higher-temp thermostats cause higher underhood temperatures at idle and after shutdown. This doesn’t matter with fuel injection, but old cars with carburetors have open low-pressure fuel systems which don’t get along well with elevated underhood temperatures. The results are percolation (gasoline boiling in the fuel lines and carburetor) and vapor lock, bringing poor idle and hard hot restarts. That needs to be part of choosing a thermostat temperature.

You can use a 195° thermostat for efficiency, but it may cause or worsen starting and idling problems. While you can back down to a 180° thermostat, you can also adjust the carburetor float adjustment, add heat shielding, insulate or reroute underhood fuel lines, insulate the headers (if you have them), and make sure the manifold heat control valve works correctly. That might buy you enough heat tolerance to go with the 195° thermostat. Find the setup that works best for your car with your local gasoline.

Parts of this series: Intro and Thermostats, Radiator caps, Radiators, Coolant, Fans. Thanks to Stant USA Corp. for use of their photography.

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