Below was cribbed from a website:



From MB tech literatu
"Sodium is a heat-conducting element. Sodium fuses and becomes liquid at
an approximate 97 degree in running engine and conducts the heat on
valve head to valve stem by convection. Heat on the valve stem first
conducted to valve guide second to cylinder head then finally
transferred to the cooling water. Cooled sodium sinks to the bottom and
fuses again. Cooling of the valves continuously repeated by this method.
When a sodium filled valve is compared with the same dimensional valve
without sodium, a heat difference of 80 to 150 degree is reported at
valve head circle as well as at the critical point where the valve stem
and head radius intersects. This is one of the major facts on resistance
of the material. Furthermore valve weight is reduced by 7% - 16% by the
drilled volume. This enables a considerable reduction of mass effect on
high performance valves."

--
Steve Walker
(remove wallet to reply)