Cooling system - thermostat blanking disk for bypass flow path

[Tim Buja <buja@domain.elided>]

Guy Grunwald wrote:
> My understanding is that the disk on the back of the correct
> thermostat blocks the inlet manifold bypass port when the vehicle
> is up to operating temperature.  The theory is that as the car
> warms up the thermostat is closed and the bypass port is open.
> This allows the coolant to circulate around the engine via the
> water pump and inlet manifold.  This gets the inlet manifold up
> to temperature quicker and therefore leads to proper fuel
> atomisation and vapour flow, leading to smoother running sooner.

Entirely correct.  It also decreases emissions during warmup.

> Once the vehicle is up to temperature the thermostat regulates the
> coolant flow and allows some coolant to continue to flow through
> the manifold, with of course the majority through the radiator,
> particularly when the thermostat is fully open.  If the thermostat
> does not have the bypass blocking disk it means that some coolant
> can continue to circulate via the manifold, bypassing the radiator,
> when the thermostat is open.

This is incorrect.  The coolant flow path in the Stag engine is from
the water pump, to the block, to the heads, to the intake manifold.
At this point it reaches the thermostat, which can either direct it
to the upper radiator hose, through the radiator, and back to the water
pump, or directly back to the water pump through the bypass hose.  All
of the coolant flow exits the cylinder heads through the intake manifold
at all times, hot or cold.  When cold, the coolant returns to the water
pump through the bypass hose.  Once the thermostat reaches its operating
temperature, it opens the flow path to the upper radiator hose and uses
the blanking disk to close off the bypass flow path back to the water
pump.  This forces all of the coolant to flow through the radiator and
reject heat to the atmosphere, instead of bypassing the radiator and
flowing back to the water pump.

> This can lead to overheating which is not good for a Stag.

Very true!  I have had first hand experience...

Tim Buja - Rockford, Illinois, USA - 80 TR8, 73 Stag, 72 TR6