There really isn't a magic number like 212°F that the motor oil must attain to drive off water and fuel in the oil. You don't have to boil a fluid to cause it to evaporate, it just speeds up the process.
Harley designed the Twin Cam to run at 230°F oil temp under standard conditions. You will find that actually specified by Harley in various manuals. Assuming they actually know what they're doing when they determine design clearances, oil passage sizes, oil pump capacity, and then determine the optimum viscosity oil for those standard conditions. then I'd say 230°F shouldn't be cause for alarm at all. At some point beyond 250°F I'd be starting to worry, but not due to oil failure. Modern motor oils, especially synthetics, are capable of handling much higher temps. What would worry me more than oil failure would be mechanical failure due to excessive heat buildup in critical areas like the exhaust valves and guides. And sure enough, those items have had a large increase in failure rate since H-D repeatedly increased the displacement of the engines while also running them extremely lean to meet emission standards. Add to that the cost reductions in materials and the sloppy fit of parts, and high temps can accelerate the failures. Case in point, all the head gasket failures and valve guide failures on the rear cylinders of '07 and '08 models.
I'm a proponent of trying to run the oil temperature in the range the engineers supposedly designed the engine to work in. Having excess cooler capacity and fans to keep the air flowing at idle or slow traffic conditions is something I've recommended often. But I'm also a proponent of using thermostats on both the cooler and the fan to let the oil get up to the designed operating temp quickly, and then using that excess cooling capacity to just keep the oil temp at that specified range automatically. I've seen no evidence that running the oil temperatures 30 to 60 degrees cooler than the Harley spec will reduce or eliminate failures in otherwise properly manufactured engines. If 170°F is supposedly better than 230°F, then is 120°F twice as good as 170°F? At what point exactly does this improvement stop? If this theory of lower oil temps being better was true, don't you suppose all the engine manufacturers would provide much more oil cooling in their basic designs? Just some food for thought.
JMHO - Jerry
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Topic: Oil Temps (Read 6299 times)