The ideal operating temperature for long engine life is 200-210 degrees. More complete combustion, less combustion byproducts to sludge the engine up. Chevy 4.3s cylinders wear more oblong than a V8 due to the split pin crank that even fire V6s use. Also the front two cylinders on 4.3s are known to wear faster because they are cooled more than the other 4. Too cold increases wear and promotes sludge.
If you are planning on swapping in a V8 later on, it makes sense to install a V8 radiator now, a 2 row aluminum rad should be adequate. If you are using a mechanical fan you will want a V6 shroud for now.
Here is a handy formula I posted in another thread. If you know the amount of fuel your engine will consume, then you can figure out how many BTUs that will produce and compute the expected waste heat through the radiator. Then choose a radiator by its BTU rating. One pound of gasoline produces 19,000 BTUs per minute. One gallon of gasoline weighs 5.92 pounds, so 5.92 x 19,000 = 112,480 BTUs per minute. So GPM X 112,480 ÷ 1/3 should give you a ballpark figure. 1/3 of the energy (BTUs) in gasoline goes out the radiator as waste heat, another 1/3 out as heat in the exhaust, only 1/3 is left to do work, and some of that bleeds off to operate the engine and accessories so less than 1/3 to propel the car
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