Always need to keep in mind a t-stat in an engine is NOT like a T-stat on your heating system. The auto t-stat is a temperature controlled throttling valve, the t-stat in your house is an on/off switch.
The hard truth a lot of people don't want to admit is that if your engine temp continues to rise after the t-stat is fully open then your cooling system is not up to the task of cooling your engine in the conditions it is currently experiencing.
You must remember a cooling system is designed to operate under a set of conditions determined by the engineers. When the factory specifies a specific t-stat rating they do so understanding how it effects the whole system.
One important consideration is the delta-t between ambient air and the coolant temp. The size and capacity of a radiator is designed around a particular range of delta-T between coolant temp and ambient air temp. "Delta-T" is an actual part of the formula for determining how much heat a radiator can dissipate. Just like cooler ambient air temp can remove more heat from coolant because it increases the delta-T between air and coolant .... higher coolant temps can do the same thing.
Simplified example is ... if coolant is 100 degrees, and ambient air temp is 100 degrees ... NO heat is removed from the coolant. If ambient air temp is 70 degrees and coolant is 100 degrees you have a Delta-T of 30 degrees. BUT, you can also get the same Delta-T by having 130 degree coolant and 100 degree ambient.
Point I'm trying to make is that the factory sizes radiators based on their predictions of normal operating conditions. If they use a 190 degree T-stat, then they size and design the radiator for about a 90 degree delta-T. The larger the normal Delta-T, the smaller the radiator can be since higher delta-T creates more efficient heat exchange.
A lower rated t-stat will only lower engine temps IF the system has excess cooling capacity over and above what the engine conditions require. And .... as you lower the T-stat temp, you lower the delta-t between coolant and ambient, so the radiator sheds heat less efficiently .... fine, as long as the radiator is large enough to compensate for this reduced efficiency it's a win/win .... you get lower engine temps.
BUT, if the system will not maintain engine temps with a 190 t-stat and it's ideal delta-T, then it certainly won't at 160 with reduced delta-T. IF it DOES reduce temps, then it was probably another reason like removing a flow restriction .... and of course what's the other major factor in a thermodynamic equation .... flow rate.
An auto's cooling system is a far more complex system in theory than people think. They kind of take it for granted because most people interact with it so easily. But the stock systems are very carefully thought out, and very carefully designed to be a compromise between cost and efficiency.
Non stock systems are a whole new ball game. Change radiator, water pump flow rate, fan type, shrouding efficiency, frontal area, engine HP, etc. etc. ... well then, who knows what t-stat rating will work for you and will provide the best balance. Probably 90% of aftermarket T-stats are designed to operate in STOCK cooling systems, who knows how they will respond to a modified system without experimentation.
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