What is a thermostat ?

The thermostat is a device sitting in a housing in the coolant flow between the engine and the radiator. Its function is to make the engine warm up as quickly as possible, thus reducing wear, deposit and emissions.
What is the process ?: When the engine is running cold (at start up), the water pump pushes coolant into the system but the Thermostat's job is to work as a tap, closing the coolant's way to the radiator thus limitating its flow to a "short" circuit around the engine. As a result, the coolant temperature increases quickly and also does the engine's temperature.
But to avoid overheating the short circuit around the engine has to be extended to a long cooling circuit including the radiator. The thermostat's function is to enable this, but only at a given temperature, known as the "engine's operating temperature". This temperature is usually specified in the car's manual.
Source: How Stuff work
How is the thermostat working in detail on the SSI ?

Refer to the sketch (source : B. Vogel)

1. Engine : COLD - Position : CLOSED
Water is pushed by the water pump and comes from both "A" and "B" banks of the engine (V8) and enters below the thermostat housing lid. As the thermostat's discharge valve is closed by a lid, water flows back to the water pump by a bypass port kept open and located at the bottom of the thermostat housing.

2. ENGINE HOT - Position : OPEN
When the thermostat's set up temperature is reached, the wax pellet inside the thermostat spool expands. This has two effects that are liked together : the spool moves the discharge valve's lid downwards, opening the outlet to the radiator an at the same time closing the bypass port.
As the temperature rises, a balance is made between the coolant flow to the radiator and back to the water pump.A fully opened thermostat (at the highest possible temperature) will route all the coolant to the radiator while an half-opened thermostat will route only half of the flow to the radiator and the other half back to the water pump.
Important notice
A thermostat has to provide a sufficient stroke to both open the outlet to the radiator ans close the bypass port. This stroke is 45mm for a Spur and a Silver Wraith.
Source: Thermostats issue and selection article by B. Vogel and Alii
Opening temperature
The workshop manual specifies that the thermostat opening temperature is stamped at its base. There is though a difference between the opening temperature the "fully open temperature".
For a Crewe 36600 thermostat, the engine design goal is 190 ° F. (88 C), the thermostat rating is 190 ° F. and the actual opening is 182 ° F. (83 C).
Source: Workshop manual
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