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Old 07-25-2021, 07:26 PM
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Default Thermostats - Rated Temperature and Fully-Open Temperature

I was reading about thermostat rated temperature and fully-open temperature. The references claim that the rated temperature is the temperature at which the thermostat WILL START to open, not be fully open, and that the fully-open temperature is 15 - 20 degrees above the rated temperature.

As an example, a 180 degree thermostat should start to open at 177 - 183 degrees, and will not be fully open until the coolant around it is 195 - 200 degrees. Are we considering this when selecting thermostats? I have a 180 in my GTO and it typically gets to 190 or 195, wondering how much I'd gain by going with a 160 degree thermostat instead.

It also makes me about claims when people say they have a 180 degree thermostat and their gauges show that they're on 180 all the time. How would the system operate in a way that keeps the gauge at the "start to open" temp? The thermostat is designed so that the coolant keeps getting hotter until it's fully open, it shouldn't be possible to keep the engine at the "starts to open" temp.

Interested in any thoughts on this one!

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Old 07-25-2021, 08:02 PM
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The information in your post is correct except for the "quality factor" in the current thermostats. I assume you know how they are constructed. The thermostat "Bulb" is the pellet that rests in the coolant. It contains a wax with a specific melting point. As it heats, it exerts increasing pressure on the pin sticking out that is trapped in the tower of the thermostat body. Much like a single piston brake caliper, as force is applied, an equal and opposite force is applied to the sealing disc of the thermostat, compressing the spring and opening the disc, allowing small initial flow through and more as it continues to heat. The spring rate, effects the closing speed, and to a small extent the closing temperature.
Now enter the Chinese and their thermostats. Low quality, Chinese made thermostats, may not have thermowax that melts at the exact correct temperature, Also they don't always get a full fill when the wax is installed, In addition, the wax often leaks out at the pin due to a poor seal, so they work for a few weeks or months, then fail. Finally, the closing spring may have the wrong spring rate and not close fully, or close too fast and cause "bounce-back" you can watch on your gauge as the vehicle warms. You can see the needle make pretty wide sweeps until the engine finally warms up all the way and stabilizes. My advice is to avoid ALL cheap thermostats, and if possible buy a USA made unit. Stant Super Stat, and Robert Shaw are the only two that seem to be consistently accurate. The cheap Stant thermostats are marginal as are Gates, and many others. A good thermostat will keep operating temps typically 15-20 degrees above the rating as you stated. Good luck.

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Old 07-26-2021, 11:28 AM
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Makes sense. My 160 deg stat runs around 180 on a hot day or at the track, but runs 160-170 on a cold day, where I assume the system is not requiring much coolant flow to keep the Temps down.

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Old 07-26-2021, 12:46 PM
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My 180 start runs at 195. Could be something to that.

Considering operation also occurs in cold temperatures it makes sense. The thermostat should be minimum temp. In 0 degree temps even a small amount of movement from the stat prior to its rated temp my produce temps lower than the stated minimum.

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Old 07-26-2021, 01:02 PM
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Yep. It is a regulating valve, not just open or closed. So one working correctly will always operate the engine at the rated temp, never lower. As soon at rated temp is reached within reason, it will begin sending some fluid through the radiator. As mentioned, some are smoother and more accurate than others.

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Old 07-26-2021, 03:50 PM
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None of that really matters. The tstat rating is the set-point or regulation point. It is the system operating design point. It is the temperature at which the cooling system water is designed to operate at, given adequate cooling system capacity. Running above that temp is an indication of inadequate cooling capacity.

To prove it, take the car out on a freezing azz winter day, like -10 deg F. Observe what the temp gauge reads. It will be no more than the t-stat rating, within measurement accuracy. And why is that? On such a cold day, there is excess cooling capacity and the system will run as designed. It should NOT be any different on a boiling hot day, as long as the cooling system is up to the task.

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Old 07-28-2021, 01:56 PM
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The -10 degrees hypothesis doesn't hold up. You could run a car without a fan at -10 and it would likely stay at the rated temp on the thermostat. A 100 degree change in ambient air temperature is going to affect the performance of a cooling system on any car.

The thermostat manufacturers state that the range from "begin to open" to "fully opened" is 15 - 20 degrees. They're designed that way. I'm assuming that if you have a 180 degree thermostat installed and you run 195 - 200 that's OK; if you're running 220 you may have an issue.

I have a 180 in mine now, I'll run up to 195 on a hot day, it may get to 210 on a long highway drive on a hot day. I ordered a 160 to try it out, we'll see how the temps change with no other changes to the cooling system once it's installed. If I'm still at 190 - 195 with normal driving, we'll know that the thermostat isn't the determinant in my car. My car's an A/C car, I have a Desert Cooler radiator and the factory clutch fan setup, shroud, etc.

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Old 07-28-2021, 04:03 PM
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Quote:
Originally Posted by Longs View Post
The -10 degrees hypothesis doesn't hold up. You could run a car without a fan at -10 and it would likely stay at the rated temp on the thermostat. A 100 degree change in ambient air temperature is going to affect the performance of a cooling system on any car.

The thermostat manufacturers state that the range from "begin to open" to "fully opened" is 15 - 20 degrees. They're designed that way. I'm assuming that if you have a 180 degree thermostat installed and you run 195 - 200 that's OK; if you're running 220 you may have an issue.

I have a 180 in mine now, I'll run up to 195 on a hot day, it may get to 210 on a long highway drive on a hot day. I ordered a 160 to try it out, we'll see how the temps change with no other changes to the cooling system once it's installed. If I'm still at 190 - 195 with normal driving, we'll know that the thermostat isn't the determinant in my car. My car's an A/C car, I have a Desert Cooler radiator and the factory clutch fan setup, shroud, etc.
Actually it does hold up. At that cold of a day, you have almost infinite cooling capacity. It should be NO different on a hot day if the cooling system is adequate or sufficient. Granted nothing is perfect and there are limitations, but i mean within reason.

What does your system with the 180 deg stat run at on the coldest day you've driven it? And why does it?

My car had a 180 stat but would still run way over 200 on a warm day, until I improved the cooling system sufficiently. Now it holds less than 5 deg over even on a hot day with the A.C. on.

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Old 07-29-2021, 04:03 AM
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I've never had a vehicle with a proper cooling system that would run over the rated temp of the thermostat except for momentarily exceeding it by about 5 degrees during the normal cycling of the thermostat.
ANY time that an engine exceeds the rated thermostat temp for an extended time (assuming the T-stat is working) indicates the engine is capable of producing more heat than the cooling system is capable of removing.
Ambient air temps should only come into play when it exceeds the design parameters of the cooling system, our old cars were probably not designed to work with ambient temps above about 120 degrees. Which can happen at road level in the southwest on a sunny day. That is nether a cooling system "problem" or a T-stat problem, it's operating the vehicle outside it's design parameters, would doubly apply to a modified engine, more HP generates more heat.

Many, many thousands of people over the decades have removed their 180 T-stats that were running at 200 degrees and replaced them with 160 T-stats to "make their engine run cooler" ... doesn't work of course, if the cooling system can not remove enough heat to maintain 180 degrees it certainly can't maintain 160.

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Old 08-03-2021, 07:41 PM
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Default Temps so far

My car's an A/C car, original 335 HP 400 4 barrel, I have a Desert Cooler radiator and the factory clutch fan setup, shroud, etc.

As I mentioned in post #1, I had a 180 degree thermostat installed. It would run up to 195 or a little higher on a hot day driving around town, it may get to 210 on a long highway drive on a hot day.

I ordered a 160 degree high flow thermostat from Amazon. It was listed as a Stant Super Stat part number 45356, but I received a MotoRad 2000-160 high flow thermostat. MotoRad purchased Stant some time back, so is this really the right part number, or did the supplier just substitute the equivalent? I don't know but I decided to keep it.

I removed the 180 and tested it and the 160 in a pot of water on the stove. The 180 opened right around 180 and seemed to be fully open by about 195. The 160 opened right around 160 and was fully open by about 175. The 160 high flow definitely had a larger opening than the 180 that was installed. Both were functioning as expected relative to rated and fully-open temperatures.

I installed the 160, nothing else changed from the 180. I took the car for about a 10 mile in-town ride with a brief stop in the middle. The outside temperature was 78 degrees so not real hot but not cold. The car never got above 180 degrees, so I DID realize a 12 - 15 degree reduction in running temperature compared to the previous thermostat. Even if the car runs 10 or 15 degrees hotter on a really hot day or on a long highway drive, it will still be a clear win.

In my case, the thermostat appears to be the determinant of the operating temperature, at least going from a 180 or a 160, the thermostat was the limiting factor.

Relative to the other responses, do I believe that on a 90 degree day, a typical car with a 180 degree thermostat will run at 180, rather than closer to the "fully open" temperature for the thermostat? No. Do I believe that's much more likely on a 10 degree day in the middle of winter? Yes.

I'll report back when we get up around 90 degrees, which should be coming later this week.


Last edited by Longs; 08-03-2021 at 07:47 PM.
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Old 08-04-2021, 08:20 AM
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I'd say the "high flow" nature of the new t-stat had more of an effect than the new temp rating. And bear in mind that with a 160 it will begin to open sooner, so it takes longer for the engine to reach a higher temp, so a longer drive in that hotter weather will give you a better idea. Keep in mind an OEM type replacement t-stat was probably not designed with a high flowing Desert Cooler in mind. Nice radiator like a Desert Cooler needs as much flow as it can get to really shine, whereas an OEM radiator uses much smaller passages and is probably designed around a lower flow rate.

Car T-stats are pretty much just like the one in your house, either system it's attached to has the capability to do what the t-stat asks it to do, or it doesn't. If the AC in a house won't cool the house to 68 degrees, turning it to 60 won't make a difference.

But of course the T-stat in a house doesn't alter flow like it can in a car

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Old 08-04-2021, 12:01 PM
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I would agree the better results are from the increased flow of the larger t-stat opening.

And as far as water temps running within a few degrees of the t-stat rating or not, whether or a hot or cold day, I'll say that if you could envision an enormous (like 10 ft x 10 ft) and super efficient radiator, it would run within a few degrees of the t-stat rating in Phoenix on a 114 deg day. I know that is not practical, but you get the point. Just like the case on a 10 deg winter day with unlimited supply of cold air, with almost unlimited cooling capacity of this uber radiator it should be no different in the heat of summer.

Vehicles are designed with limitations and have to be practical and reasonable so there is some trade off. But the goal is to regulate temps that are maintained within bounds. Mine were solved with an Optima core radiator and electric helper fan inside the shroud, along with the clutch fan and careful attention to details. Even in 99 deg Alabama heat, with the A.C. on temps were almost always within 5 or 6 deg and never more than 10 deg of the t-stat (180).

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Old 08-04-2021, 07:14 PM
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Good analogy, just the point I was trying to make.

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Old 08-08-2021, 02:39 PM
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Yesterday I took the car on a pretty long drive, mostly 40 - 50 MPH, up and down hills, not on the highway, to a car cruise. The outside temp was 89 degrees. The last part of that drive was idling the 3/4 mile cruise route (starting and stopping, people walking out in front of the cars, the usual stuff). I got up to just under 200 degrees. I previously would have been at between 210 - 215 degrees with the 180 degree thermostat, with no other changes. I'll take it, glad to be running 10 - 15 degrees cooler just by changing the thermostat. I'm assuming that it's a combination of the thermostat opening temperature and the high flow, that the thermostat is less of a restriction to overall cooling system function.

To all the theoretical discussion above, the purpose of the cooling system is to prevent the car from overheating. My GTO cooling system capacity was 19.4 quarts according to the manual, could be slightly more with the Desert Cooler radiator. That's the volume that's available to circulate to dissipate heat. If you could run a huge 10' x 10' radiator, of course it would allow the system to run cooler because the volume NOT in the engine block at any point relative to the coolant volume in the block would be much greater. It's the same idea that if the ambient air temperature is 10 degrees, the system's going to be able to dissipate more heat and run cooler than when the temperature is 90 degrees. Ambient air temperature matters, unless you have a cooling system with much more capacity than is required.

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Old 08-15-2021, 11:08 PM
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Static is whats on the t stat, and its Start to open temp.
Dynamic is its full open temp, which is dependent on the cars cooling system.
Typically the Dynamic is 10 -12 degrees warmer than the Static rating at operating temps.
Verified in a pot on the stove as well.
Putting in a Lower temp T stat often HELPS cars as they get a head start on keeping the temps
down and the stat is wide open at 10-12 degrees above its rating and is below 195, which is what you want.
Only in an ideal world with low octane fuel, 8.5 or under CR and very very efficient cooling
will a Pontiac V-8 run at the static rating of a T stat.
Of course if you don't have a factory Fan shroud, you are in a league all your own with money and amps and defying actual science.

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Old 08-15-2021, 11:14 PM
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Static and dynamic temperatures lmao. Who comes up with this? Lol

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Old 08-16-2021, 12:08 AM
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For what it’s worth, my 400 in my 77 Trans Am (8.8:1) had a 160 in it from the time I bought it 11 years ago until last week. It wasn’t until a few weeks ago, when I installed a temporary mechanical gauge, that I realized the engine was only running 160ish in town and 170ish or under on the highway. I swapped in a 180 last week and it runs right around 180 in town and 190ish on the highway.

I did test the gauge with a pot of boiling water and compared it against crossover temps with infrared thermometer.

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Old 08-16-2021, 01:58 AM
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Quote:
Originally Posted by 70GS455 View Post
Static and dynamic temperatures lmao. Who comes up with this? Lol

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GM does. I went to Class on Heating and cooling in the '70's
Among other classes they sent me to.
My Instructor spent about 3 hours going over how fan shrouds were developed
in that class.
Engineers in Product durability probably came up with the terms which are used for other things besides just Thermostats. like Compression ratio, Static and
Dynamic.
The Temp isn't Dynamic, the function of the thermostat at operating temperature is.
Ever heard of "Thermodynamics "?

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Old 08-16-2021, 12:53 PM
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Always need to keep in mind a t-stat in an engine is NOT like a T-stat on your heating system. The auto t-stat is a temperature controlled throttling valve, the t-stat in your house is an on/off switch.

The hard truth a lot of people don't want to admit is that if your engine temp continues to rise after the t-stat is fully open then your cooling system is not up to the task of cooling your engine in the conditions it is currently experiencing.

You must remember a cooling system is designed to operate under a set of conditions determined by the engineers. When the factory specifies a specific t-stat rating they do so understanding how it effects the whole system.

One important consideration is the delta-t between ambient air and the coolant temp. The size and capacity of a radiator is designed around a particular range of delta-T between coolant temp and ambient air temp. "Delta-T" is an actual part of the formula for determining how much heat a radiator can dissipate. Just like cooler ambient air temp can remove more heat from coolant because it increases the delta-T between air and coolant .... higher coolant temps can do the same thing.

Simplified example is ... if coolant is 100 degrees, and ambient air temp is 100 degrees ... NO heat is removed from the coolant. If ambient air temp is 70 degrees and coolant is 100 degrees you have a Delta-T of 30 degrees. BUT, you can also get the same Delta-T by having 130 degree coolant and 100 degree ambient.

Point I'm trying to make is that the factory sizes radiators based on their predictions of normal operating conditions. If they use a 190 degree T-stat, then they size and design the radiator for about a 90 degree delta-T. The larger the normal Delta-T, the smaller the radiator can be since higher delta-T creates more efficient heat exchange.

A lower rated t-stat will only lower engine temps IF the system has excess cooling capacity over and above what the engine conditions require. And .... as you lower the T-stat temp, you lower the delta-t between coolant and ambient, so the radiator sheds heat less efficiently .... fine, as long as the radiator is large enough to compensate for this reduced efficiency it's a win/win .... you get lower engine temps.

BUT, if the system will not maintain engine temps with a 190 t-stat and it's ideal delta-T, then it certainly won't at 160 with reduced delta-T. IF it DOES reduce temps, then it was probably another reason like removing a flow restriction .... and of course what's the other major factor in a thermodynamic equation .... flow rate.

An auto's cooling system is a far more complex system in theory than people think. They kind of take it for granted because most people interact with it so easily. But the stock systems are very carefully thought out, and very carefully designed to be a compromise between cost and efficiency.

Non stock systems are a whole new ball game. Change radiator, water pump flow rate, fan type, shrouding efficiency, frontal area, engine HP, etc. etc. ... well then, who knows what t-stat rating will work for you and will provide the best balance. Probably 90% of aftermarket T-stats are designed to operate in STOCK cooling systems, who knows how they will respond to a modified system without experimentation.

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Old 08-16-2021, 01:06 PM
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Quote:
Originally Posted by Formulabruce View Post
GM does. I went to Class on Heating and cooling in the '70's

Among other classes they sent me to.

My Instructor spent about 3 hours going over how fan shrouds were developed

in that class.

Engineers in Product durability probably came up with the terms which are used for other things besides just Thermostats. like Compression ratio, Static and

Dynamic.

The Temp isn't Dynamic, the function of the thermostat at operating temperature is.

Ever heard of "Thermodynamics "?
Actually I have heard of thermodynamics, in my sophomore year of engineering in my thermo course. But the professor didn't call the opening point of a thermostat static and the fully open point dynamic. We covered topics like entropy, enthalpy, steam tables, carnot cycle, isothermal, adiabatic, heat transfer equation etc.

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