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2005 KLR650
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I approve!

Indeed, 165F is too cold for a gas engine. For optimal combustion, you actually want the engine to be as hot as possible without causing damage. Practically speaking, this is generally ~195-210F, depending on the cooling capacity of the cooling system, engine component compositions, etc. ~230F is where a lot of engines start risking damage due to warping components. Obviously you want some overhead between normal running temp and "danger zone", hence 195F thermostats being very common.

165F thermostats were an old hot rodder solution to inadequate cooling systems that couldn't keep up with heat output at full load; the extra overhead between running temp and harmful temps allowed you to run flat out without overheating the engine, and to shut down without heat soak doing the same. The downside to running this cold is very poor combustion resulting in reduced power and lots of carbon buildup in the cylinders.

I need to rig up a block plate for my radiator now that it's nice and cool outside. Thermobob is on the to-do list, but I'll probably wait until spring rains and valve adjustment time.
 

· Registered
2005 KLR650
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239 Posts
I'll disagree with that statement.
The engine metals are perfectly safe up to 300-350F+ as long as the engine oil stays below about 250-275F in the sump.
Most air-cooled motorcycle engines operate in the 275-350F cylinder head temp range very frequently.
Liquid-cooled engines need pressurized Coolant/Antifreeze to stay below the boiling point of the coolant, to prevent hot spots from developing. Once any hot spot develops the whole system can spike quite quickly.


I'll disagree with that statement also.
A 165F thermostat can not compensate for too small of radiator or too slow (or too fast) of coolant flow or too slow of air flow, be it in a KLR or any hot-rodded engine!
The engine metals are fine. The issue is when you have different chunks of metal at different temperatures. This is particularly problematic when the chunks of metal have different thermal expansion properties, like an aluminum head on a cast iron block. The two chunks of metal expand at different rates, and either one of them warps, or the gasket can't handle the slip between them.

I'm not as familiar with air cooled engines. Not my specialty :) But I could see them running hotter by default and caring less because there aren't coolant passages in the head gasket.

I didn't say 165F thermostats would fix a inadequate cooling situation. I just said it was an old hot rodder solution which gives some extra overhead before the engine overheats. It won't cure a constant overheat issue under normal running conditions.
 

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2005 KLR650
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239 Posts
Neither a 195F nor a 165F thermostat will give any "extra overhead before the engine overheats"! 165F isn't even hot yet.
195F is still not up to the boiling point of water only, even without a pressure cap!

Only a larger radiator or more efficient radiator or more cooling air flow can give "extra cooling capacity" to prevent overheating.

The Gen 1 radiator was almost too small, the Gen 2 & Gen 3 radiators are larger, much more efficient & more easily damaged.
You’re confusing dynamic cooling capacity of a cooling system, which is a function of how fast the system can move heat from the engine to the air, with thermal capacity of the coolant in the system, which is a function of the physical properties of the coolant and the volume of coolant.

It takes X energy to heat coolant by 1deg. If you have a cooling system that is running at 165F, it will take more thermal energy to get it to 230F than it will take the same system running at 195F to get to 230F.

If the cooling system is adequate to regulate temperature under mild load conditions, but inadequate under heavy load or high ambient heat conditions (this describes most pre-70s vehicles), you might be able to buy yourself a couple more minute of high load operation before the engine overheats by running a 165F thermostat. It won't change the fact that the engine will overheat, because that's up to the cooling capacity of the system, but that's often enough to get you back from the end of a drag strip, or up to the top of the grade you're trying to climb.
 
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