Some real world results since I replied on pg 1.
Even though the cooling system on my 49 3100 w/216 was functioning fine pressurized via a 4lb cap, I had also purchased a new stock "0" pressure cap for just in case at the same time and decided to try it and run non-pressurized this weekdend.
I took the truck out for an 80 mile run on Sat., high seed, low speed red light idleing etc., and I did notice a slight increase in engine temp as registered on the gauge, but it was minimal.
Normal temp would register w/the pointer just about centered on the "8" of 180, for this cruise the pointer moved over to the "0" and only rose slightly above that at long idle times (red lights, fast food drive through etc) but it never overheated.
I'm going to run non-pressurized for as long as the cooling system keeps the engine at proper temps but with warmer temps approaching I've added the 4lb cap to my travel kit.
re. overflow tank. Even non-pressurized the coolant level in the tank remained the same when up to operating temps and when I checked it a couple hrs after shutdown the radiator had sucked it back. So coolant overflow operation remained the same.
I highly recommend a coolant overflow tank no matter if the system is pressurized or not.
Dave
