More important than temperature rating is making sure you have the right type as the temp rating is when it opens to allow water to run through the radiator. At operating temperature it will run hotter than any of those settings. More important is that it has the Triumph foot as seen on the thermostat on the right as that closes the by pass and allows all the water to run through the radiator. With out the foot only a portion of the water will run through the radiator drastically reducing the effectiveness of your cooling system.
One at left is from VB and the one at right is from Robsport (by special request.)
The 15 lb. cap runs the cooling system at 1 atmosphere above normal raising the boiling point of the water above 100C (212F) much like a pressure cooker. The coolant also raises the boiling point (and lowers the freezing point).
In Manitoba we used to change out the thermostat for one with a higher rating in winter time so the engine would warm up quicker and hence would heat the interior quicker. We would also block off the radiator with a piece of cardboard with a flap or circle cut out. Alternately you could buy a vinyl radiator covering that had flaps you could open.
That would be in 20 to 40 below zero. (40 below is the same in F and C). Nothing like that on the west coast.[8D]
TR7 Spider - 1978 Spitfire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT