This probably won't come out very smoothly, but...

Sodium filled (also known as sodium cooled) valves are used to transfer
heat away from the hottest part of the head (i.e. that nasty part right
in the combustion flame) to the cooler parts of the head.  This will
(theoretically) increase the life of the valves, make the heat
distribution in the head more uniform (i.e. no major hot spot at the
valves) which causes all sorts of problems, lower the temperature of the
air whoosing into the combustion chamber (more power and efficiency).

The sodium in the valves actually melts when the engine is running.
Neat, huh?  Some argue that sodium filled valves do more harm than good,
and that they are too expensive.  For more on that, search for sodium
filled valves in Lycoming aircraft engines.  They say the heat
transferred up the valve causes more valve guide wear and thus problems
because they heat transferred up the valve stem causes carbonization of
the oil.  Sounds like they have chronic valve guide problems, and
someone thought this would be a fix, but it wasn't, so they hate them
now.

Keith Adams
Calgary, Alberta, Canada
1986 Vanagon GL Westfalia "Roxy" (ne idea what valves are in it!)

> Dave Baker wrote:
>
> Sodium-filled valves.  There's a neat concept.  Can anyone tell me why
> they would want to do this?
>
> Dave in KC (who is learning tons of new stuff from the members on this
> list!)
> 85 Westy