Coolant flow

......yeah and every single one of these antiquated books had ZERO clue about how ethanol laced gas would affect these engines. Leaning them out, causing hot spots and so on. You have to compensate for "whatever" fuel you run in the tune, or it'll never run right. Some of these guys just want to be right, no matter how wrong they are.
That book I showed is from 1970. The Mopar Performance book (when was the thermostat tech written, not sure) shows this:


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All these articles, books show is how the acceptable range below/above a thermostats rating can be. There’s Wide variation. I think it’s safe to say they are all referring to Standard production style thermostats allowing for Production tolerances at the time.
The closer to the thermostats rated temp they start to and are fully open the better.

I think it’s a good thing to read articles regardless of when they were written, not to win (yeah, maybe for some) as it shows how it was, any progress up to where things are now. And it might provide context