Subaru Impreza 2002 Service Manual page 34

SODIUM-FILLED EXHAUST VALVES
Mechanical
12. Sodium-filled Exhaust Valves
The STi model's engine uses sodium-filled exhaust valves.
Each exhaust valve contains pure sodium in its hollow stem. Sodium has high thermal conductivity.
The entrapped sodium will liquefy at high temperatures and move inside the stem as the valve is
operated. Thus the sodium will effectively transfer heat from the valve head to the valve stem, con-
tributing to cooling down the valve head faster.
B2H4814A
(1) Pure sodium
ME-9