An engine is thermally more efficient at higher temps, if it can stand it. Cars from the 1980's to present are set to run at about 220 degrees coolant temp for greater fuel economy and lower emissions. Our old dinosaurs are more prone to detonation at that temp, so 195-ish would be ideal. All things being equal, an engine running at 195 will be more efficient than one running at 160-180. I also agree that what was said earlier about installing a lower temp T-stat to raise operation temps is simply not true. Just the opposite.
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Jeff 
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