Just as a side note, I always test a thermostat in a pot for opening temp and fully open temp with thermometer before installing. Both figures should be available. Too many stories of faulty units out of the box to take a chance.
The pressurised system will mostly come into effect when water temp reaches unpressurised boiling temperature. I do believe it might help to supress boiling near hot spots within the engine.

_40  at idel will take some time before opening happens,we all know that the therm will  open at some point

I don't know what the pressure cap story is.

It is important to understand that the thermostat is not simply an on or off device.  It opens gradually, starting around the thermostat rating, being fully open at some point after the temperature rating.  So your observations are correct.  When the system has cooled sufficiently. the thermostat will return close to a closed state, preventing the flow of coolant, at which point it begins to heat again and open, allowing coolant through the radiator to cool.  The fact being that if the thermostat does not open, the engine will not stay cool, no matter the weather conditions.
You acknowledge that yours opens at about 170.  It would not maintain a temperature around 170 if it did not open.  The temperature would only climb.  I acknowledge that in colder weather, it could take longer to reach that 170 degree mark, but it most certainly will and must.


Lawrenceville, GA

for what its worth, charley in correct, i don't by the pressurcap storey,pressure in the system stops water from boiling,boiling water causes cavitation hot spots in the engine,so some pressure is important. cavitation will give false readings on gauge.(that is why bypass hose is critical so pump keeps flow in engine when stat is closed. my ranchero runs 160 at ford temp location and170 at therm housing.little to no flow to rad till 170 at stat when it opens. yes i need a new stat to keep temp close to 180.

My trusty old '56 Ford manual (right or wrong) shows the OEM thermostat spec - "standard" = 157-162. So a new NOS could be just that.

The engine tuning - standard equipment - had a number of features that facilitate warm up. One is the "no vacuum advance" at idle and retarded initial timing settings. When using other tuning schemes - the engines may run quite a bit cooler at idle than the OEM - but warmed up and running, it should get to the t-stat rating as a minimum.

NAPA used to have a conventional 170 degree, 2 inch thermostat with the larger size (1-1/2"?) opening but I've lost track of the part number. Is it still available?

 
 Steve Metzger       Tucson, Arizona

The thermo works. I tested in in a pan of water before I put it in service. It is possible that it's a 160° instead of a 180°. It was a NOS unit that did not have the original box. I'm sure my gauge is reading correctly. It is within 10° of the readings I get with the infared gun. I just had the car out tonight and the temp never rose above 160. I think I'll try to find a 180 and swap them out and see what happens.

Durham Missouri

The 2 I tried were Mr. Gasket. Did not realize there was a difference. Maybe thats why they failed.

  

If the thermostat stays closed, the engine will not stay cool, no matter the weather.  The bypass only recirculates coolant through the engine, bypassing the radiator.  This means that the coolant can only absorb heat.  If it cannot transfer that heat to the air via the radiator, it will overheat.  The engine block and heads will not be able to transfer that heat effectively enough to stay cool without the radiator.

I haven't had any problem with American made Robert Shaw style thermostats.  I did have a Mr Gasket (Chinese) fail prematurely.  I never thought either took longer to warm up.



Lawrenceville, GA

My experience with the Robertshaw Thermostat was similar to Yours. I tried 2 of them thinking the first was defective. Neither One would get the Engine warmed up in reasonable Time. Went back to a standard 160 Stat and it worked as its supposed to. Never figured out why the Robertshaw Stat didn't work properly. maybe both were defective? I also installed a mechanical Coolant Temp.
Gauge. Drilled and tapped the Intake Manifold just behind Stat Housing  to the left on the flat area of Manifold. I also have the original Temp Gauge operating.  

  

http://www.hotrod.com/articles/davies-craig-reinvents-the-electric-water-pump/

It's a little modern but seems perfect.