The only reason for pressurizing coolant is to allow it to go above 212 degrees without boiling. For every pound of pressure the boiling point rises 3 degrees, so a 15-lb. cap will raise the boiling point to 257 degrees. A 4-lb cap only raises the boil-over temp to 224 degrees, hardly worth the effort. The older radiators just don't have strong enough construction to handle over 4-7 lbs., max. Keep the core clean and flush the crud out of the engine block, and stick with a low-pressure cap. The manufacturers learned they could build cheap, inefficient radiators, if they just put enough pressure on 'em to keep the coolant from boiling. Most OEM electric cooling fan switches trigger the fan(s) at 220 degrees, so a bad radiator cap can cause a boilover before the fan ever comes on.
Jerry
