180°F (82°C) thermostat: The thermostat starts to open between the temperatures of 173°F and 180°F; the thermostat plugs the bypass orifice between the temperatures of 185°F and 192°F; and the thermostat is fully open between the temperatures of 198°F and 204°F.
192°F (89°C) thermostat: The thermostat starts to open between the temperatures of 185°F and 192°F; the thermostat plugs the bypass orifice between the temperatures of 197°F and 204°F; and the thermostat is fully open between the temperatures of 210°F and 216°F.
Thermostats are not ultra-precise devices, which is why it is necessary to write they open and close within a small range of temperatures, rather than at one precise temperature. Notice the Robertshaw 333 thermostat must open about 50% before it plugs the bypass orifice. If a cooling system is healthy proper thermostats (Robertshaw 333) of different ratings will regulate at different temperatures.
If a cooling system is stabilizing at the same temperature regardless of thermostat rating then one of two problems exist: (1) wrong thermostat or (2) the cooling systems heat removal capacity is marginal, the thermostat is operating wide open and not regulating anything.
I believe the Panteras cooling system has been marginal at low engine speed for 40 years. The SACC 4.9" pulley will help matters quite a bit. I think it will eventually be recognized as one of those "must have" type of parts.
-G