[Schurkey]

Originally Posted by nUcLeArEnVoY

A thermostat's purpose is to maintain cooling system pressure,

Originally Posted by chuckies76ta

A thermostat's purpose is to maintain cooling system pressure, The thermostat does not maintain system pressure. The Radiator cap is doing that job.

Originally Posted by nUcLeArEnVoY

Lemme reword that. It helps act as a RESTRICTION, which helps aid in maintaining pressure.

Chuckie, and your second reply is closer to the truth. There are TWO sources of pressure in the cooling system:
1. Static pressure set by the rad cap, and
2. Dynamic pressure created by the water pump moving fluid against "a" restriction. The other side of the main restriction (generally the thermostat, but it could be the radiator) is going to be the suction side of the water pump, with reduced dynamic pressure. What makes this more complex is that there's a hundred tiny restrictions in the cooling system, including but not limited to the small holes in the head gasket, the heater core, the narrow passages of the water jacket as it goes around the cylinder bores, and so forth.

#2 is why designers moved away from vertical-flow radiators to cross-flow radiators starting in the '60s. A vertical-flow radiator has the pressure cap on the upper, HOT tank, the entry to the radiator core. If the radiator core gets restricted--scaled-up--the radiator becomes more of a restriction that the water pump is pushing against. This overloads the rad cap especially at high rpm, releasing water into the overflow tank at best, or to the pavement at worst. Either way, there's now less coolant in the system and greater chance of overheating.

A cross-flow radiator has the pressure cap on the COOL tank, on the other side of the core. It is effectively on the suction side of the water pump, so the suction side runs at rad-cap pressure and everywhere else has higher pressure due to water pump motion. The cross-flow radiator is generally better at "everything".

The thermostat SHOULD be the "main" restriction in a cooling system, even when the thermostat is fully-open. Whether it is, or not, depends somewhat on the condition of the radiator, and sometimes, whether the correct head gasket has been installed.

Originally Posted by nUcLeArEnVoY

and to set the minimum operating temperature. The cooling system's capability and efficiency is ultimately what determines the temperature the car runs at. If your cooling system is bulky and well-designed enough to actually open and close a 160 thermostat in closed loop operation, then great, If not, it'll operate at a higher temp and eventually stop at what the cooling system can support. The only issue with using a thermostat at too low of a temp that your cooling system can't maintain, is that said thermostat will always be open and you lose some of the system's ability to make pressure.

The real problem is that if the thermostat is already fully-open at idle or cruise, there's no reserve cooling capacity for when the engine is run harder. The only cooling system efficiencies at that point are faster water pump speed, faster fan speed, and--maybe--faster vehicle speed leading to some "ram air" effect at the radiator.

Automobile radiators are always undersized compared to the horsepower the engine is capable of. Compare a 400 HP automotive radiator to a big-truck (semi, or bus, or bigass farm truck) with a similar HP rating--the automotive radiator will be tiny. This is because a 400 HP auto engine is rarely run at 400 hp, and when it is, it's generally for short bursts. The big truck 400 hp engine may run at or near full throttle for hours on end. Automobile radiators are sized to easily handle idle and cruise with a little additional capacity, truck and bus radiators are sized to handle WFO throttle.

Take a 427 Chevy. The car engines were rated up to 435 hp, they had the usual thermostat and radiator-hose. The 427 tall-deck truck engines had maybe half that horsepower, they had governor-carburetors to limit RPM, and they had TWO thermostats of the same size and capacity as the single thermostat in the car engine, and a monster water pump. Look at the "cock and balls" thermostat housing on one of them--two rounded thermostat areas, plus the GIANT snout for the huge-diameter upper radiator hose. (Photos shamelessly stolen from the Internet. https://www.tradecraftspecialties.com/bbckb#Topic4

Originally Posted by P@blo

If a thermostat is rated to start opening at 180*, are we suggesting that the cooling maintain that temperature even when the valve is not fully open?

An automobile or light truck with a 180 thermostat should idle and cruise within a few degrees of 180. Let's say 185-ish. Thermostat open, but not very much. Then when the throttle is opened further--to go up a hill, or to beat the guy in the next lane--horsepower goes up, heat goes up, and the thermostat can open farther to allow the cooling system to reject more heat into the airstream going through the radiator.

Originally Posted by P@blo

it seems nornal to have the operating temperatures 15-20* above the thermostat rated opening point.

That's exactly what happens when something is wrong with the cooling system, or something is wrong with the engine tune so the engine creates more heat than it should. There may be an undersized radiator, a faulty radiator, not enough ignition timing, faulty fan/fan clutch, incorrect pulley ratio between crank and water pump, or any one--or combination--of additional faults too numerous to mention.

Originally Posted by P@blo

Some guys have probably over engineered their cooling system and can maintain coolant temperatures when the thermostat valve is only partially open

Nope. That's the way it's supposed to work; assuming a water-pump driven cooling fan.

You toss an electric fan into the mix, and now the coolant temperature is dependent on both the coolant thermostat rating, AND the cooling-fan thermostat.

My '92 and '93 Luminas with electric fans don't have the fan come on until 210 degrees. I think the second fan comes on at 220.

Originally Posted by P@blo

Seems to be a bit of stretch to say a cooling system has "lost control" not maintaining the thermostat rated starting opening temperature but never overheats the engine. My cars run 180* thermostats and typically run 195-200* no matter what I throw at them, driving style wise, and see almost zero ROI chasing something like this down.

The thermostat is fully open at about 20 degrees above it's rating. I suppose this might vary a little depending on thermostat design. But fully-open at +20 degrees from it's rating is a fairly safe bet. Anything beyond +20 degrees, and the thermostat is no longer in control of the temperature.

You have a 180 thermostat, so at 200 degrees, that thermostat is almost certainly fully-open and no longer controls the engine temperature. You have NO way to increase coolant flow through the radiator except to add RPM (fan and water pump speed) and/or vehicle speed. Your cooling system needs improvement, or your engine needs tuning/friction reduction. (Often a matter of more spark advance, (vacuum advance) richer cruise air/fuel mix, lower-tension/thinner piston rings to reduce friction, verify piston-to-bore clearance for friction reduction, lower exhaust restriction/block exhaust crossover in intake manifold, and so forth.)

Originally Posted by nUcLeArEnVoY

A cooling system that is efficient and has enough capacity to actually maintain the thermostat's temperature [rating, within a few degrees at idle or cruise] will maintain it in closed loop operation (the modulating thing you're talking about). If it's not up to snuff, then the engine temp will be [20-plus degrees] higher than the thermostat rating [at idle or cruise], meaning the thermostat will always be [fully] open as it tries to lower the temperature, but it cannot. This is like not having a thermostat.

Very close. I've added some corrections to your statement.