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Sodium filled valves

Purpose of sodium filled valves is to aid heat transfer from the head of
the exhaust valve up the stem, and thence away from the valve via the valve
guides. Hottest part of the valve is the head, hottest valve is the exhaust
valve. Inlet valve is cooled (partially) by the incoming unburned mixture.
Exhaust valve is opened while all that hot exhaust (some of which may still
be burning, especially at high revs) bathes it, top and bottom. Weakest
part of valve is the junction between head and stem - this is where the
valve separates (failure mode). Of course, it always happens at speed, and
usually wrecks the head and sometimes the block, too. The piston is always
trashed, and there's plenty of scrap metal in the oil pan, too.

Inlet valves don't need expensive tricks like sodium filled - generally
speaking, the exhaust valve is the single hardest-working part in an
engine. Notice what they look like at overhaul: burned deposits on stem
(from heat + oil), gas erosion of valve face and sometimes valve seat, crud
on back of valve head from exhaust deposits, etc.

High performance engines are even worse - due to more valve overlap, (inlet
and exhaust both open at tdc between end of exhaust stroke and beginning of
inlet stroke), valve spends more time open, and away from the comparative
coolness of the valve seat, also spends more time exposed (both sides of
valve head) to the extremely hot exhaust gasses.

side comment on radio on while ignition off - I wired mine to work like
that - sometimes I get to sit outside Wal-Mart while wife is in shopping -
I hate Wal Mart - my car gets scratched and dinged EVERY time I go there!

Best Regards,

Mike Arman