That is exactly the balancing act. Most motors run best at around 170-180. There are exceptions of course. The thermostat stays closed until the water comes up to temperature, whatever the therm is rated at. So If you have a 160 thermostat, that is not the temp the motor will stay at, that specification is the tempreature the therm opens at. At that point, the speed of the water through the engine, fan efficienty and radiator capacity now take over to keep the temperature at or about operating tempreature. If you have an over efficient system, when the therm opens, the system is so efficeint that the motor looses too much heat and cannot maintain operating tempreature. If the system is underefficient, it does not matter when the therm opens, 160, 180, 120, lol, if the system is undersized or mismatched, it can't maintain operating temperature, and runs hot to some degree. The trick is to get the system matched up so that it maintains a good operating temperature at all speeds, but especially at idle.
So your water pump has to run at an appropriate speed, like goldilocks, not too fast, not too slow. The radiator has to be of sufficient capacity and the fan has to be able to move an appropriate amount of air through the radiator, especially at idle.
Two sort of examples, big rig trucks with diesel motors tend to have a very, if not over efficient, cooling system. In the wintertime, this leads to some trucks not being able to maintain temperature which is why you sometimes see a truck with a big piece of cardboard in front of the grill to cut down on air flow. The other, at one time I was invlved with a super comp NHRA dragster that ran on a pair of alcohol fueled holley carbs. The alcohol caused the motor to run cool, really cool. So in order to get the motor to180 at the starting line, we had to let a race motor, with a teeny tiny radiator, sit and idle in the staging lanes just to get sufficient heat in the motor. What we learned was that the motor did not run as well at 160, 170 as it did at 180-190.
So maybe try installing a 180 thermostat and a 4 lb cap and see where you are at. You will need an accurate guage. I chuckle over the number of folks that use the original guages as the ultimate arbitar of accuracy. Don't fall for that. Install a good name brand electric or mechanical temp guage and get some accurate readings. If the motor is running hot, you can start to fiddle with fan shrouds, 5 and 6 blade fans, larger capacity radiator. Last year I got my hands on a five blade 348 Chevy with AC fan and installed it on my 261 motor. Finally, the motor ran at 190 in 90 degree plus heat. The extra blade and more aggresive pitch made a huge difference with air movement through the radiator at idle, as an example.
You should also confirm that you are using the pullys for the year motor. If not, you should consider doing a little math and see if that may be part of the issue, too slow or too fast on the water pump.
Always exceptions, you might get lucky and be one fo the few that can regulate operating temperature by changing the thermostat, but if you are not, you may have to tinker a bit to dial it in.
Last edited by Dragsix; 03/04/2022 4:54 PM.
