Here's something I posted elsewhere last year, hope it helps .
Yesterday I received an email from someone in Arizona asking about lowering the fan switch temp to help his bike run at lower temps. I think he was surprised by the answer, and I wanted to share it with everyone.
I am a professional mechanic in Florida. I specialized for decades in rebuilding automotive engines, and almost all of the engines I rebuilt were originally victims of overheating. As such, I studied up on how cooling systems work. I found along the way that most folks (read :nobody except engineers) really don't understand how the cooling systems work. I learned a very effective modification a couple decades ago, that I currently use in every vehicle I have, and I never have heat issues. Ever.
Let me also say that what I'm going to share is not new to this forum. I shared this back in '05 with the c-10 guys, and if you ask the c-10 guys who have been around, this has been nicknamed "Steve's coolant cocktail". So this isn't new, just maybe new to you
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First, let's get a few things out of the way with some education.
1) All a cooling system is doing is transferring BTU's from the engine to the ambient air. The more efficient that transfer is, the better the system will shed heat.
2) Coolant is very poor at heat transfer. Water is the most effective.
3) Every cooling system has a finite ability to shed heat, and that ability depends on the ambient air temps. So a cooling system in 70* ambient air may be able cool the coolant down to the thermostat cycling temp, but at 100* ambient it probably won't be able to do so unless it has a lot of surface area and flow volume in the radiator.
4) lowering a thermostat temp isn't going to help the cooling system shed heat, or lower the operating temps unless the radiator has the ability to cool down to that temp. In many cases raising the thermostat temp is the thing to do as it provides the most linear and consistent operating temps for the engine.
5) lowering coolant temps by thermostat opening temp will result in a less efficient engine. Heat in an engine expands the metal producing better cylinder sealing, helps the oil circulate with less pumping loss, and aids combustion. All within controlled parameters, of course.
6) Radiators shed heat by airflow through them, as they transfer the heat from the cooling medium to the air. Changing electric fan cycle temps is largely an exercise in futility, as at about 25 mph the forward movement of the bike meets the airflow ability of the fans. Any higher speed than that produces more airflow than the fans provide, so lowering fan "on" temps has no effect on the coolant temps.
7) lowering fan cycle temp settings really only effects the cooling system when in traffic, with low available airflow. Typically fans will come on around 210* and off around 190. the temp will never go below the thermostat setting. Lowering the "on" temp will only result in the heat blowing on the rider for a longer duration as the fan comes on sooner and stays on til the preset off temp is reached. Remember that once on, the fan will stay on till the setting temps drops to the pre-set off temp, which HAS to higher than the thermostat temp, or the fan will run all the time once the thermostat opens. If the original ON temp was 210*, and the new temp is 195* the max temp difference the rider would feel is 15* BUT with the lower setting the fan will cycle more and blow that 195* heated air onto you for a longer time. Pick your poison, there's no free rides... as long as the engine is running it's releasing and shedding the same BTU's.
I'm going to stop here and pick up on the next post.
Steve
