ST1300 thermostats almost always fail open or nearly open resulting in coolant not reaching optimum temperature when going down the road. The air flowing through the radiator does not allow the coolant to fully warm up and the rider sees the 1 to 3 bar "dance".
I am sitting here wondering how a lower temp thermostat would help with the heat issue. For a given volume of fuel burned, there will be a fixed amount of heat generated. A lower thermostat would mean that more heat would be dissipated by the radiator leading to more heat issues. Seems to me that a lower water temp means a higher air temp. That does not even address the issue of what temperature is optimum for the bike to operate at.
Just my two cents from car work again!
It is kind of backwards thinking, but a stuck open thermostat could actually cause a hotter engine on newer vehicles, although the water temperature will remain cooler than ideal, which is what drrod was pondering several posts previous. Older vehicles (pre electronics) were "dumb" in that they did not advance and retard the timing, so a stuck open thermostat would just normally never achieve the correct operating temperature for the vehicle, which has its own problems. But the fuel and air was set.
Newer engines retard the timing slightly and increases fuel to bring the coolant to optimum running temperature (normal cold start up procedure). Essentially burning additional fuel to heat things up and retarding the timing to allow a less complete burn in the cylinder. Once the engine reaches the correct temperature it advances the timing and trims the fuel back, relying on its "knock" sensors to make sure it does not go too far. From that point it is up to the thermostat to regulate the temperature and keep it within set parameters. If it is always dropping below the set point the ECU will put the engine back into "cold start" mode to get back to that set parameter. You can actually watch this on cars with OBD2 readers if you access the timing info and water temperatures.
As several have mentioned the laws of the thermodynamics are set, more fuel = more heat. With retarded timing you also get additional heat from that burning fuel since you do not get an efficient burn. Retarding causes fuel to not be fully burnt by the end of the compression stroke, which causes more heat as it completely burns on the down stroke before the exhaust valve opens. I assume this is why people will notice a difference in idle, heat, power, and mpg when changing a thermostat that is not functioning correctly.
If any of you have experienced a knock sensor failure on your vehicles, you will notice the vehicle has poor performance and terrible mpg. Essentially the engine is "stuck" in cold start mode because it can't tell where the engine is knocking at, so it will default to this condition to protect the motor instead of blindly advancing timing. The same thing is happening on a stuck open thermostat, it is just cycling in and out instead of constant.
Now, about if one oil weight/brand is cooler than another - I am not touching that...
Anyway! Most minds are made up by experience and/or research. I am not telling everyone to go change their thermostat, but I could see why it would cause excessive heat, especially at the headers if the engine was stuck in "cold start" cycle. One of my first cars was a supercharged Cobra Mustang 5.0. I couldn't get that car to run without overheating until I figured out the previous owner put in a 165 degree thermostat, the oem was 182. Once I changed it I no longer had issues keeping it cool.
-Ryan
Unlike most laws, the laws of thermodynamics cannot be broken. Ignored, yes at your peril, broken, no.
