Thermostats start to open at their stated temp and open all the way 20 degrees later.

Just for grins, I was having issues with t-stats. I had a 180 deg. but the car would never get to that number. I thought it was the factory gauge. I experimented with different coolant levels in the radiator and of course it went above the 180 so I knew it wasn't the gauge.

No real reason to be afraid of a 160 stat. An engine will only run at that temp if the cooling system and everything else about the engine is up to snuff and capable of holding that temp.
In most cases, with stock cooling systems and warm climates, the engine will still run well above that thermostat temp rating. It will just settle in with what your system is capable of.

I have one car here with a 160 stat, with it's original 50 year old radiator, stock cooling system, clutch fan, etc... It generally runs around the 175 mark most of the time, and on the hottest days here in AZ 185 is the hottest I've seen it.
I've since put a Cold Case radiator in it, left everything else the same, and it now does run closer to the stat rating and has a more consistent temp on the hotter days. Hasn't seen 185 since.

Most of our other cars are also 160 stats and efforts made to keep them at or around that temp for various reasons. The biggest being that we push the compression envelope with our engines, and have to run this crappy 91 octane pump gas. Tunes have to be spot on but also just as important to maintain and have good control of engine temps. Especially here in AZ where our summer temps are over 100 degrees. We don't care about emissions, I'm not looking to burn off all the hydrocarbons, I'm looking to control combustion chamber temps and detonation, with sharp tuning, tight quench, proper camshaft, and controlling the engine temps. There is no real downside to it that I've found after doing this more than 30 years.

Actually, not to be nit picky, but interesting none the less and fitting for this discussion, from my testing, I've found that stats actually start to crack open roughly about 5 degrees before their rated temp. Pretty easy to experiment on the stove top with a temp probe.

Then when put in service, if you have an excellent cooling system, you can actually see and maintain that crack open temp.

For instance my chevelle has an excellent cooling system, Griffiin rad etc... and the 160 stat in that car actually cracks open at 155, and is fully open around 165. However, as soon as that stat cracks open, on cooler days of 80 or less, it will maintain 155 degrees and is rock steady. Higher rpm on the highway can make the temp rise to 165 but as soon as I slow down it will pull the engine back to 155. Never any colder than that. So most of the time cruising around, this thermostat never even really opens completely, but it does make the engine run at a minimum of 155, and with a good cooling system, the thermostat just barely cracking open is enough to keep the engine at that temp, and will open more if the temp rises. It's doing what it's supposed to do. It's pretty interesting how this stuff acts with a good cooling system.

Yep, a properly sized and well maintained cooling system should be as spot on and invisible in operation as a central air system in a home. Typical modern auto cooling system will get to the proper temp and stay within a couple of degrees of that in pretty much any outside conditions or load.

I second the suggestion of a mechanical gauge in the intake manifold. I'm not buying that the car is running 140 with a new 180 thermostat.

I must have done that demonstration 100s of times when I taught auto mechanics at a high school.. (And it was in the book.) But what do I know?

Good grief dude, are you seriously referencing STEAM ENGINES to try to justify your nonsense?

The heat being created by combustion goes out your exhaust. Power isn't measured in exhaust gas temp, or exhaust manifold temperature, nor is it related to the cooling system's inlet temperature. These things aren't related in any manner what-so-ever.

If what you said was correct, bigger motors would need smaller cooling systems to run as hot as possible TO MAKE BIG POWER. That's exactly opposite of how builds go. Sorry you don't understand the relation between heat, complete combustion, and emissions. I don't have time to teach it.

Steam engines...LMFAO. I've heard it all now.

It's important to remember that what you get on your gauge is only reading the temperature at one point in the entire cooling system. It should be on your water crossover which is essentially your water inlet temperature. That tells you how hot the water is when it goes back into the motor. If your cooling system is being overwhelmed, it will be really hot at this point.

So you are going to have a pretty big split between your inlet and outlet temp, which is why all these comments about needing to boil water out of your oil are nonsense. Your water temp reading isn't your oil temp, or your exhaust manifold temp, and darn sure isn't some kind of "power" equivalent. A well cooled engine runs better, lasts longer, and makes more power. I run a 160 degree thermostat, and it never gets above 175 unless I'm sitting in traffic in the summer (in Texas).

Before I got my Cold Case radiator, I got stuck in the staging lanes and did a pass with the motor around 205, and was .2 slower than the previous run. Go to a drag strip and ask people if they run their engines as hot as possible or as cool as possible.

Yep, even my sons fuel injected mustang doesn't like heat. We were at the track a few weeks ago and I tried hot lapping it, the car slowed down. It ran it's best times with a full cool off. I keep a 180 stat in that one with a cold case radiator. Preferably I don't like to see the temps above 190 in it because it will ping and complain on our 91 pump gas when the temps get that high, and it's just a bone stock 9:1 engine assembled by Ford. I don't dare run cheaper gas in this one during the summer months, especially with the AC on. Matter of fact when I was hot lapping it, it had some slight ping during the longer pull in 3rd gear. The slow drive back to the pits cooled the car down, a testament to the efficiency of that Cold Case radiator. Then a 20 minute cool down in the pits with the hood up was the ticket for best times. If I had ice I would have been icing down the intake.

While thermostats technically open and close they actually modulate.

OP, did you drill a hole in the thermostat to bleed air?

No. Both thermostats I tried were left stock.

Mine stays at 150 with an aluminium radiator and a stock (no clutch) five blade fan.. car seems happy enough at this temp. Outside temp was around 65-70 degrees..

First thing I'd try is a new gauge.

True.

Clearly NOT. The water crossover is the last point before the thermostat, where the coolant LEAVES the engine for the radiator. The water INLET temperature would be measured at the lower radiator hose area in the water pump.

Because the coolant has already picked-up heat from the engine, and is about to exit the engine to be cooled in the radiator.

The mistake in logic you're making is largely the result of having no practical way to keep the inlet air/fuel as cool as possible while running the cylinder block nice 'n' hot.

Cold intake air/fuel = high-density, good power, reduced detonation
Hot "short-block" = lower friction, less wear.

Folks over-cool the block trying to keep the intake manifold and cylinder head intake ports (and therefore the intake air/fuel stream) cool. As far as the intake air/fuel mix, cooler is better until you have fuel vaporization problems. It needs to be "just" warm enough to deliver a proper mix of fuel with the air to each cylinder; if it's too cold the fuel falls out of suspension in the air stream, doesn't vaporize very well, and too much goes out the exhaust unburned. Welcome to needing a choke and intake manifold heating methods in cold weather--you have to put so much fuel in the air because lots of it is not vaporized and is wasted. This is hardly a problem once the engine warms up.

Good. Drilling holes in thermostats is nuts unless you've disabled the OEM coolant-bypass system.

There's a reason thermostats don't come from the factory with bigass holes in them. True enough, some do have tiny stamped vents, or a "hole" that gets plugged with a "jiggle valve" when the water pump flow increases.

Any air in the system is moved to the radiator as soon as the thermostat opens the first time after cooling system service. At that point, it's seen as a "low coolant level", and the radiator is topped-off. In a properly-functioning cooling system, there is no other source of air that needs to be blown out of the engine.

You are correct, I had the flow backward in my head for some reason. Still, the temperature of the head and combustion chamber being cool is an important aspect that you are overlooking. It isn't just the temp of the air/fuel mix as it goes into the chamber..the temperature of the combustion chamber is important as well.

EngineMasters has done extensive testing on this, and they showed that the temperature of the fuel and air, and the temp of the head itself are what make the difference. The air/fuel mixture is going to fast between the carb and head to have significant heat transfer.

By having a cooler engine, you have cooler fuel, and avoid all the problems that you end up with by having an over-heated cylinder head. It's the whole reason aluminum heads exist (other than weight).

While the car is 52 y.o. this month it's still "new" to me. Before I change anything else I going to order a "heat gun " this Friday. I'll post my findings here after i get some readings.

I've always believe you want a cold(condensed) fuel/air charge to the "hot" combustion chamber. The heat causes rapid expansion of the condensed charge, increasing compression and power.

That is not correct. The spark plug provides the ignition source. Hot combustion chambers can cause pre-ignition and other problems. Hot chambers make less power. The rapid expansion of the burning air/fuel mixture pushing against the enclosed space is what makes the power. Every interaction the particles have prior to the ignition of the mixture favor COLD to make power. Heat is managed and dissipated, it's never desirable. If it was, you can put chemicals that burn WAY hotter than gasoline in your tank. They will just destroy your motor.