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The Basic Theory - of cooling operations regarding thermostats is as follows. With the radiator, engine block, and overflow tank we have app. 17.2 quarts of a 50/50 mixture of antifreeze and water to cool our engines. A few quarts reside in the engine block the rest outside the engine. A thermostat, by closing, separates these two circulation areas whereas the block will heat it's own water first until it reaches the thermostat opening temperature. Then the stat will open thereby increasing the volume of coolant that is being circulated by the water pump. This "dual" system allows us to reach operating temps faster to heat the oil, fluids, and to render the interior heater usable in a quicker time frame.

The Stock Thermostat - in our cars is a 195° thermostat. This is by GM design (although a poor one) as GM decided the cheapest way to reduce emissions to meet EPA standards was to increase the heat range our cars operate in. Good for them - bad for us. For us this results in a loss of power, overheating cars, and increased wear to our engines, not counting our nerves.

Normal Operating Temp - on our cars is in fact 195°. Every calibration in the GM tech manual that calls for normal temp at 1000 RPM idle is set to 90° C which converted is 194° F. Exactly why we have a 195° thermostat.

Thermostat Myth - Many assume a 195° thermostat is open at 195°. This is not correct. This is when the thermostat is calibrated to "begin" to open. It will be totally open at approximately 206° and close at 188° in order to try and maintain a 195° "middle of the road" temp. This is known as the operating range.

Ideal Situation - would be for our engines to be cooled totally by the opening and closing of the thermostat. But when this doesn't happen because of increased load (towing, A/C use), underhood mods, or an intense environment our cooling fan kicks in at 234°.

Cooling Fans - on an electric fanned car should not be considered "normal operation". As I said the "ideal" would be totally controlled by the thermostat. Our cooling fan is our "danger zone" or "early warning" or "last resort" or "safety measure". Our fan is saying "your engine is about to overheat I'm going to cool it down for you". Please keep in mind, before anyone gets in a great debate on this issue with me the purpose of this bold and brash statement is to later explain why some cooling mods don't work. Of course our cooling fans ARE a part of our "normal operation" because we DON'T live in an ideal world....... see where I'm going with this?

Be Sure - that if your car is running 260° - don't think that since your fans don't even come on until 234° that you are OK. Antifreeze will boil at 262° at sea level with 15 PSI radiator pressure. You are about to cook something. That is what I mean, your fans kick on at a very late stage.....almost too late.

Running the Fans - all of the time is not a great crime. Yes your warmup time will be slightly increase...... big deal. A belt driven fan runs all the time. What will be the most affected is that your electric fan motor is good for X number of hours. By design your fans don't run all the time. In some situations they are OFF most of the time. So running the fans all the time will consume your X number of hours of fan life at a more rapid rate. If you can live with that then there is really no problem......unless of course that last hour of life is in the middle of the Arizona desert 100 miles from nowhere.

Environmental Theory - A 195° thermostatically controlled engine will act differently in Phoenix, Arizona on a 120° day in rush hour traffic, with the A/C on, than say a cold blistery day up North somewhere. In traffic driving will have a different effect than a highway cruise. A/C On vs. A/C Off. Underhood modifications which increase horsepower usually also increase the heat for example headers. Which leads us to the conclusion that the 195° stock thermostat is a "one size fits all". One part - every car. This is where the problems begin and the misunderstandings start. This is why a 160 works better for one person while another gets better results with a 180. But for all practical purposes most owners I have encountered agree - "We have to lower operating temps".

Temperature Momentum - is the last theory I have before getting down to solutions. If you point an airplane nose down at 200 MPH and pull the nose up right before you hit the ground - you will still smash into the ground - belly up. If your boat is headed to shore at 50MPH and you make a sharp left turn, right before the shore, you're still going to hit the shore - you'll just do it pointing left. AND if your temperature has an upward momentum, especially due to abnormal conditions, it is going to peak REGARDLESS of what thermostat you have installed. This is a very important theory to answer questions like "I put in a 160° thermostat and my car still overheats" or someone who says "160 is too cold, your car won't run right"...... not necessarily true.

Which is Why - there is such a great debate on which stat is right for you. The installation of a lower thermostat and an adjustable fan switch that lowers the temps that your fans come on is an "attempt" to lower the operating range of your car. Just because you put in a 160° stat doesn't mean your car will operate at 160°, but what's important is that it will try to. It will "attack" a temperature momentum rise at an earlier stage and in many cases that is enough to solve serious overheating problems. Temperature Momentum at it's late stage is sometimes impossible to overcome. It just has "too much" momentum, and dealing with it early or before it happens is the key to success. And this is exactly why a 160° thermostat may be too cold for the rest of the world but it may be just right for you. There are those that recommend a 160° for summer and a 180° for winter which is why I run a 170° year round.

I am coming to the conclusion ~ that this would be the best guideline I could offer for which thermostat to use:
195° ~ If you live in the Arctic Circle
180° ~ If you live in a predominantly cool climate.
170° ~ If you live in a predominantly warm climate.
160° ~ If you live in a "hotter than hell" climate.


Acceptable operating temps are the result of:

1. The tuning of your engine.
2. Your driving habits.
3. The increased loads you have implemented. (towing, A/C, headers, etc.)
4. The thermostatic and fan turn on temps you have selected.


What do we do?

You may be here because you have:
1. Added mods or increased the strain on your cars cooling system.
2. Gone through the basic troubleshooting of your cooling system and still overheat.
3. Are just generally concerned about your cars operating temp.
4. Live in a harsh temperature environment whereas the stock system just doesn't cut it.

Modern automotive internal combustion engines generate a tremendous amount of heat. This heat is created when the gasoline and air mixture is ignited in the combustion chamber. This ignition (explosion) causes the piston to be forced down inside the engine, levering the connecting rods, and turning the crankshaft, creating power. Metal temperatures around the combustion chamber can exceed 1000° F. In order to prevent the overheating of the engine oil, cylinder walls, pistons, valves, and other components by these extreme temperatures, it is necessary to effectively dispose of the heat.

It has been stated that a typical average-sized vehicle can generate enough heat to keep a 5-room house comfortably warm during zero degree weather (and I'm not talking about using the exhaust pipe). Approximately 1/3 of the heat in combustion is converted into power to drive the vehicle and it's accessories.

It is our goal as racers to bring that 1/3 to a much higher standard, making use of every bit of energy to accelerate our vehicles. But, this higher standard also causes even greater heat generation, requiring even better cooling system components and design.

Another 1/3 of the heat is carried off into the atmosphere through the exhaust system. The remaining 1/3 must be removed from the engine by the cooling system. Modern automotive engines have basically dumped the Air Cooled System for the more effective Liquid Cooled System to handle the job.

In a liquid cooled system, heat is carried away by the use of a heat absorbing coolant that circulates through the engine, especially around the combustion chamber in the cylinder head area of the engine block. The coolant is pumped through the engine, then after absorbing the heat of combustion is circulated to the radiator where the heat is transferred to the atmosphere. The cooled liquid is then transferred back into the engine to repeat the process.

Excessive cooling system capacity can also be harmful, and may effect engine life and performance. You must understand that coolant temperatures also affect oil temperatures and more engine wear occurs when the engine oil is below 190° F. An effective cooling system controls the engine temperature within a specific range so that the engine stays within peak performance.

In this system, the coolant is circulated by the water pump, and the thermostat controls the temperature. The thermostat is closed when the engine is cold, allowing coolant to circulate ONLY in the engine block, bypassing the thermostat and radiator. This allows the engine to warm up faster and uniformly so that "hot spots" are eliminated. When the warming coolant reaches the thermostat, the thermostat will begin to open and allow coolant to pass to the radiator. The hotter the coolant gets, the more the thermostat opens, allowing more volume of water to pass to the radiator. The thermostat also controls the length of time that the coolant remains in the radiator so that the heat is dissipated effectively. On a racing application you can get by without a thermostat by using the correct restrictor disc installed where the thermostat would normally be installed. Selecting the correct restrictor is imperative to system efficiency.

Important Warning:
Removing the thermostat to increase water flow because your vehicle is overheating is dangerous to your engine and is NOT what you want to do. Not only does the engine take longer to warm up, causing excessive metal-to-metal wear, but once the engine does warm up it can get too hot because the thermostat also controls the length of time that the water is in the radiator so as to dissipate the heat to the atmosphere.

kwalsh wrote:Thank you HenryJ.

Now I feel much more comfortable with my 180 Jet thermostat.

The graph below illustrates the importance of how critical optimum coolant temperature is to the longevity and performance of an internal combustion engine. Cool water makes good horsepower, to a point. Warm water minimizes engine cylinder wear, to a point also.

However, there is a "middle ground" where both optimum performance as well as minimal wear share similar characteristics. That "magic" number lies in the 175-180 degree range, which requires a 180 degree thermostat..



The all too commonly used 160 degree thermostat is way too low to be considered for performance or engine longevity. As the chart illustrates, engine wear increased by double at 160, than at 185 degrees. So then, why do the 160's exist in the first place? The 160's were commonly used in older, open loop cooling systems where only 6 pound radiator caps were used, and low 212 degree boiling points were experienced. In contrast, modern cooling systems can see upwards of 260 degrees in coolant temperature with radiator pressures exceeding 45 pounds. Many early hot rodders found the 160's to be better performing than the 190's, however, the in between "180" appears to satisfy both ends of the spectrum. The correct water temperature is required for the cylinders to achieve a minimum specific temperature in order to allow a fully homogenized Air/Fuel mixture to combust efficiently. Guess what the minimum number is… right! 180 degrees. Even so, you might see some still recommending the lower 160's, for no other reason than to possibly get that last drop of horsepower out, at the high price of dramatically reducing the life of the engine and it's internal components.