Hello all!

I went out and found this chart.
https://durathermfluids.com/pdf/techpapers/pressure-boiling-point.pdf


It discusses temperature of plain water @ atmospheric pressure, like an open kettle) and the same under a pressure cap of a sealed vessel.
There is a chart for water with antifreeze too under both conditions.
Going with the idea from another posting that the cooling system will be under 11 psi so that it does not lose Coolant the 8 PSI is very realistic.

After looking into those temperatures you will ask why so much more heat over a thermostat setting that opens in the 195 degrees area.
Well, this is when the thermostat will just "crack open" not fully open. The design of the system is not up to its operational temperature as it should be higher. This is only the first flow temperature or minimum. If you go into a long downhill coast or turn on a heater you want a buffer reserve in the engine.

In the paragraph above the charts it explains what is happening down inside the engine next to the cylinder walls or what we don't want to happen throughout the cooling system.

Of course it took evolution to get this idea proved.
After years of having what works keep on working like in the days of steam engine trains that Constantly was adding water at every fuel stop not so well for cars.

Somewhere with the advent of increasing the number and their size of cylinders pressure caps came a long. Some of you may remember we use to leave the radiator purposefully low in Coolant so not to add water unnecessarily. This always created an air space that put air in the Coolant when circulated.

Somewhere in the "muscle sixties" the hot rodders group figure out that a bigger radiator and water was heavy!
Now some students, with slicked back hair or maybe a curly haired chick that had not fallen asleep quite yet were listening. He or a she, told their friends and began to network, as we call it today, but under hoods in front of multiple carburetors and thought about that wasted air space.
A recovery bottle was added for a catch basin and it was about then, the bubble gum went flat under the desk!
A radiator cap was made to allow the Coolant back in after the radiator Coolant cooled back down.
The air space was gone and and amazingly, a gain of cooling happened to boot! Say what?
Now was it an accident that the air bubbles got figured out to be an insulation or was it some Einstein event. We will never know as NASA might have taken credit for it. It was the Sixties you know?

Anyhow, it comes down to antifreeze becomes a thermal fluid additive that enhances water density up,to,a point. That's why I chose this sites charts. Still Water is best refrigerant know to man, except for Hydrogen. It works better pound for pound moving BTU's over any of DuPont's inventions.

As far as where to mount a gauge the head is probably the best place. It's the highest point in the system and heat rises. Of all the areas that you think get hot this is where the water is less dense and we need to be concerned with heat transfer as those parts in the head are getting flamed!

The thermostat is on the outlet for the closest response. All the water is under the same pressure through but the last of the final heat transfer is all done by the point. From that point on outward we need to lose it. As a "rule of thumb" all of the heat transfer across the radiator is a 30+degree drop by design. The quantity of mass and speed flowing through a cooling medium's size, is a design factor. Water and air have fixed factors.

So, once the thermostat cracks there is a lag time before you reach a design temperature of stabilized operation with the design to be sufficiently large enough to cover in excess of expected temperature with a margin left for some aging and lack of maintenance.
Having the temperature of the Coolant higher than the crack open setting is not uncommon to avoid over cooling. So I agree with other posts of 5 or 10 degrees being normal.

Using mechanical gauges or infrared you have to allow for variances, i.e. Aluminum, rubber or brass conduct differently. In refrigeration, I allow approximately 10 degrees from the pressure reading of refrigerant converted into degrees inside versus the outside of copper tubing of heat exchangers.

If you measure a thermostat outlet you are probably under in temperature of up to 10 degrees of the actual center of the Coolant flow being slightly hotter.

On the car radiator you can measure the inlet to the outlet as a comparison and see an overall drop of at least 30+ degrees. I can usually read it in idling mode to be 150 degrees on the lower hose.
I would not want to see it less than that. If it is, the thermostat is broken in the open position and is not closing. Most likely causing slower warmups and richer running.

You can measure down the radiators face and "you should see" the most drop in the first upper half, if the radiator is flowing and breathing through correctly. It should not be nearly as hot at the bottom as the top.
If not and you feel cold spots or read cold spots in the radiator tubes it has an issue of being clogged, missing fins or something. A radiator in this shape the engineered design is going to fail near it's "top allowance" for that hot summer day. The radiator should alway be able to throw off mor heat than the engine can make.
Turning on the heater valve adds about half a radiator of size with its fan on. If the temp then drops from being to hot, the radiator has a problem.

You have to play with your thermometers and takes notes. Compare what is normal on the inside gauge and expect repeat performance within a minor range or you have a problem starting.
Just like a voltmeter know the where from where and let that be its normal.

Hope this "tunes you in" a little better. (:)
Phil