1971 Thermostat: 180 or 195?

[wbdeford]

While I am flushing the cooling system, I am going to replace the thermostat....Which temp should I buy?

[Scot Minesinger]

Fairly certain that it is the 180'F thermostat, should say in engine cooling section of shop manual.  My 1970 requires a 180'F thermostat.

[wbdeford]

Manual says starts to open 177-182 and fully open at 202.  But when I search auto parts sites, they list both as valid for this car.

[James Landi]

I have read some posts that indicate ethanol creates issues in hot intake manifolds causing hard starts when engines are shut down, but need to be restarted shortly thereafter... With that in mind, I am running a 180, my temp gauge reads 180-205  -- i think 207 is the upper limit?

[wbdeford]

Thanks...then it is clear the manual specifies 180....unless someone posts some advantages to using a 195, I will go with 180.

[EAM 17806]

"STANT" web-site states the original equipment for your 71 thermostat is 195' and the alternative is 180'. Check it out so you use the one the factory says is the proper one. The shop manual for 71 will verify which one is recommended. If you don't have one try the library.  EAM

[Scot Minesinger]

go with the 180'F, my temp gauge also reads 180-205'F

[James Landi]

Gasoline has been reformulated, and the results are hard starting when hot, and remember, lower octane, creates higher heat effects-- perhaps not ping and knocking, but less power and more heat, fiction and wear... Remember low octane means that the gasoline is ignited more quickly (pre ignition) and creates more heat and fiction-- a by product of lower octane than was appropriate and specified for our engines.  We folks need to figure out how to keep these big block engines cool.  Thermostat heat range choice is important, but old coolant, partially blocked, old radiators, and restrictions in air flow are potentially deadly during summer, high temperature driving. 

["Cadillac Kid" Greg Surfas 15364]

William,
With all due respects to the previous, There are a couple of reasons that Cadillac originally designed the motor to run with a 195 degree thermostat.  The first is at the operating temperature resulting from the 195 degree stat the engine is most efficient and produces peak power.  The second is that if the engine runs cooler an appreciable amount of sludge can and does build up throughout the motor.
Coolant temperature in the radiator vs air temperature (temperature difference) is what "drives" heat transfer. The greater the difference the greater the heat transferred from the radiator to the air passing over it. 
Coolant temperature is not engine (block, heads, etc.) temperature.  Combustion temperatures range round 1000 degrees F, and some of that heat is transferred to the heads, block and intake manifold.  The purpose of the cooling system is to transfer that heat from the head and block so the engine can operate at the desired ()by the engineers who spend decades perfecting it) temperature.  If you lower the coolant temperature you are actually reducing the heat transfer capabilities of the cooling system.
Of course, when the outdoor temperature is above say 90 degrees F, the heat transfer is reduced, but unless you never drive your car when the outdoor temperature is cooler, keeping the 195 degree stat will result in greater longevity and greater performance.
A foot note is that coolant temperature varies throughout the cooling system, and where you place the sensor as well as the sensor's accuracy and resistance to contact conduction can result in a wide variation in sensed temperatures.
Greg Surfas

[wbdeford]

Greg....thanks very much for your thoughts.  The manual says the thermostat starts opening at 177-182 and is fully open at 202.....when I first read that, I interpreted that to mean 180....but maybe that really means 195?

["Cadillac Kid" Greg Surfas 15364]

Yes,
That is what the motor "wants".
Greg Surfas

[wbdeford]

Greg,

I understand what you are saying about the higher temp being better for reducing sludge, corrosion, etc.  Those are the main reasons I will go with the 195 (since my cars rarely operate in very hot weather).  But what you said about a 180 thermostat reducing the heat transfer capabilities is a little puzzling.  True, the amount of heat leaving the car when the coolant is 195 in, say, 90 degree atmosphere is > than the amount leaving the car when it is 180 in 90 degree atmosphere.  So far, it all makes sense.  But if the engine starts to get TOO hot, the coolant will go over 195 anyway, regardless of which thermostat is in it, and the difference will become irrelevant.  IOW, I can see how a 180 thermostat can cause the engine to be too cool, but I can't see how it could cause the engine to be too hot.

Putting it slightly differently....I don't know the temp difference between the top of the radiator and the bottom, but let's say we have 2 cars under identical conditions, one with 180 and one with 195.  Let's say the 195 car is able to drop 45 degrees to 150 by the time the coolant moves to the bottom.  The 180 car may not be able to drop as many degrees, but the temp at the bottom would still have to be less than what it was for the 195 car.  Let's say it only drops 35 degrees, to 145.  It isn't removing as much heat AT THE MOMENT, but if that starts to become an issue, the coolant temp will start to rise, and as soon as it reaches the same temperature as the 195 therm car, we will essentially have identical systems.  Make sense?

William,
With all due respects to the previous, There are a couple of reasons that Cadillac originally designed the motor to run with a 195 degree thermostat.  The first is at the operating temperature resulting from the 195 degree stat the engine is most efficient and produces peak power.  The second is that if the engine runs cooler an appreciable amount of sludge can and does build up throughout the motor.
Coolant temperature in the radiator vs air temperature (temperature difference) is what "drives" heat transfer. The greater the difference the greater the heat transferred from the radiator to the air passing over it. 
Coolant temperature is not engine (block, heads, etc.) temperature.  Combustion temperatures range round 1000 degrees F, and some of that heat is transferred to the heads, block and intake manifold.  The purpose of the cooling system is to transfer that heat from the head and block so the engine can operate at the desired ()by the engineers who spend decades perfecting it) temperature.  If you lower the coolant temperature you are actually reducing the heat transfer capabilities of the cooling system.
Of course, when the outdoor temperature is above say 90 degrees F, the heat transfer is reduced, but unless you never drive your car when the outdoor temperature is cooler, keeping the 195 degree stat will result in greater longevity and greater performance.
A foot note is that coolant temperature varies throughout the cooling system, and where you place the sensor as well as the sensor's accuracy and resistance to contact conduction can result in a wide variation in sensed temperatures.
Greg Surfas