actually Joe, it's the other way round - the higher pressure caps came about to cope w/ engines that naturally ran much hotter and the higher pressure actually keeps it from boiling over - the earlier trucks ran NO pressure caps, and do run normally quite cool, which is why they use thermostats, because an engine is more efficient at certain temps, like created by a 180* stat
the pre-46 used a non-pressurized cap, the AD and TF trucks used 3 & 7# caps - all use 160+ thermostats - all run the risk of popping rad seams w/ caps over 7#, and the 60 year old rads may go w/ even a 3# cap
my 46 GMC 228 would run quite well & with a 185* thermostat required having the window or cowl vent open a bit to prevent making the cab downright tropical - my 57 235 w/ a 185* gets the heater cookin too, but the gauge won't get above 1/4 unless the rad is half empty
Bill
