Not sure about how cold it gets in New Hampshire but -40 is the norm for January over here on the western Great Lakes. At that point, even the sweetest coolant mix can start to get. Only cure for that is a block heater. Factories offered an in-block version that looks like a miniature edition of the heater element from a house hot water tank. Better is a circulating heater that is plumbed into one of the heater hoses or into the delivery side or lower rad hose.
Issue with the block heater is that it generates a localized area of warm coolant that doesn't move all that far from the element; no large scale circulation. If your vehicle is computerized then the water temp sensor sends readings to the CPU. What happens when you start up cold is that the coolant is cold at the sensor and that is the reading sent to the Processor which adjusts the warm up cycle accordingly. Then , all of a sudden, here comes that pool of warm coolant, The sensor reads that and sends the amended reading to the processor, but then that pool moves on and the sensor reading changes again. These abrupt changes confuse the processor so it throws the equivalent of a "WTF?" code up on the dash. That ends up having to be scanned and deleted. to reset the system.
That is why the circulating unit can be the better option. Because it not only warms up the coolant but encourages it to move through the coolant passages, all the coolant more or less acquires the same ambient temperature, which, on initial start, the coolant sensor reads and submits. No violent shifts in sensor data due to hot and cold variants, so no confusion in the binary based peanut brain of the processor, so no codes thrown.
I personally went with a Champion 2 row with 1" tubes and cooled down a .030 455 and does great with my 6.0 ls swap. I bought a cheaper 4 core aluminum ebay radiator that was junk. I took the aluminum shroud off of the ebay one and added dual spal fans to mount on the Champion. 100 degree heat in Indy in traffic and car never went above 190-195*
Another thing to consider is you don't want to run the engine too cold either. 210 is a good operating temperature for street engines. So don't use colder thermostats as bandaids, a 195 degree one is good for most street usage. Water is a better heat conductor than antifreeze, so the higher ratio of antifreeze you use in the mix the worse the cooling system will perform. Glad it never gets -40 where I am, rare single digits were bad enough.
On the other end of the spectrum if flushed and only water is in the block , fill with straight antifreeze and the water in the block will delute it for a -40* mix Used to used air/ water surge to back flush the system .
man the memorys blast air n water with a flush gun.always good for a bath.lol.epa didnt care for it much.just part of why you dont see radiator shops anymore.moder flush machines suck.most place pull a flush job out back,drain an fill and thats it.
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