Operating at or above an actual coolant temperature of 200 degrees, even intermittently, is going to require a pressure cap to prevent periodic coolant losses due to boiling over at times. The sticking point is how much pressure the radiator and heater cores are able to tolerate without rupturing. The boiling point rises 3 degrees for every pound of pressure, so a 7 pound cap will raise the boil-over temp 21 degrees- - - -to 233. Modern aluminum or tube=type copper radiators can handle 14-17 PSI without a problem, but the "honeycomb" heater core might not live at that sort of pressure.
A 180 thermostat maintains a low enough minimum temperature for a 0,4,or 7 pound cap to handle momentary spikes in temperature. Going hotter than 180 with the thermostat temp requires a higher-rated pressure cap. If a system with a 180 thermostat is getting to 200 degrees or more routinely, there's not enough radiator to do the job, or it's dirty.
Jerry
