Originally Posted by rich weyand
OK, here's my take.
I think the guy with post #2 is the most correct.
Basically, if you have a system that is a loop, and you put a valve in it (in this case, the thermostat), then when the valve is closed, nothing circulates in the loop.
With me so far?
OK, so that's not a good thing. So the General built a second loop into the system, by installing a bypass that runs from the output of the water pump back to the engine, circulating coolant in that loop even if the valve is closed. So all the valve controls now is shutting off the radiator. But coolant still circulates, avoiding hot spots.
They didn't build this bypass into the Vortec heads for some reason (weight? the location already in the block inconvenient?), so you add it externally.
Now, lots of SBCs run pretty cold, like mine. Low compression, torquer cam, runs cold as a witch's heart. So I took out the always-on mechanical fan, and added an electrical fan which hardly ever runs except at sitting (no air flow from movement) on a hot day.
So a guy like me, with that setup, doesn't know about the bypass line, say, so he doesn't put it on. No problem, because his heater circuit sort of acts like a bypass, per the later posts in the thread.
But then some guy comes along who puts in the Vortec heads, and doesn't know about the bypass line, and cuts the heater out because he lives in Cali, and all of a sudden he is trying to diagnosis overheating issues.
BTW, another thread I found, one poster said the reason for needing circulation when the thermostat is closed is avoiding local hot spots, especially where the exhaust ports are adjacent to each other, to avoid (ta-da!) cracking the heads.
So that's my take.
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