"I'll have a car that ducts virtually all the hot cylinder air and exhaust pipe heat down and out and behind the vehicle, leaving little or none to recirculate through the fan."
Well, that's what I have, too, and it seems to work.
I have the VW thermostat bellows under the 1-2 heads linked to the air vanes inside of the (1971-ish, flat-top) shroud. The air coming from the oil cooling tower exits out the back of the engine, below the heat shields, down and then caught in the air stream. The engine tins direct the hot air down and then to the rear and out the back (I have the sled tins, too).
My engine compartment is completely sealed to insulate the exhaust heat from the fan inlet/carbs. I also have an oil flow switch that shuts off oil flow to the external cooler under 180F coming from the filter.
All that said, the time it takes to get to 180F depends greatly on the outside ambient temp. 70+F ambient and it warms up in about ten minutes of neighborhood driving. Take it on a turnpike and it'll rise up to 200-ish and sit there with the external cooler fan running all the time.
Outside temps below 40F means it'll take twice that long to warm up (20-30 minutes) and turnpike speeds bring it up to 190-ish and it sits there. The external cooler fan may or may not come on, depending on how low the outside temp is.
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