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Sodium-cooled valve

The sodium-cooled valve is filled with sodium. duh! When heated, the sodium melts and helps to cool things down.
I never heard of one, can someone explain this? From an engine quiz I missed one on, so y’all will probably score perfect.

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These have been around for decades in high-performance applications. Like you said, sodium is a better heat conductor, keeps the exhaust valves from failing, maybe decrease knocking from pre-ignition.

Sodium is a very light metal with a very low melting point, around 208 degrees F. So it sloshes around in the hollow valve stem transferring heat from the valve stem to the valve guide keeping the valve head a little cooler. I think sodium filled exhaust valves are mostly used on aircraft engines.
Sodium is also highly reactive and will react violently with water so don’t saw one of these valves open, unless you do it underneath oil.

My 1963 Ford Galaxie 406 cu in 405 hp Super High Performance had factory sodium filled exhaust valves. Replacement intake valves cost $1.69 ea. Exhaust valves $13.29 ea in 1972!

Here’s an description I found:

“SR20DET heads are nearly identical to their non-turbo counterparts except for one thing. The turbo heads have sodium filled exhaust valves. Sodium filled valves have a hollow head and stem filled about 60% with Sodium. Sodium has an extremely low melting point (about 100 degrees C) and boiling point (880 C). In a hard-running engine, the valves are hot enough that the sodium in the head of the valve vaporizes and travels up to the cooler stem where it condenses back to a liquid again. [edit - I doubt that, all the movement would slosh the liquid around, much faster means of heat transport, see discussion below]. The phase transition from liquid to gas absorbs a lot of heat, and the condensation dumps the heat in stem where the large metal-to-metal contact between the valve stem and guide transfers that heat to the cooling system. Without this heat pipe effect, vastly more heat would have to be transferred through the valve seat. Too much heat through the valve seat, and you get a burned valve.”

The heat pipe effect works only if the valves are vertical with the heads on the bottom and the stems on the top. On the radial engines used in WWII, some of the valves were upside down, so I think it is mostly the liquid sloshing from one end of the valve to the other that transferred the heat.

There were also salt filled valves, a salt mixture that melted at around 475F was put inside the valve instead of sodium. Note: “salt” does not necessarily mean sodium chloride, which is a salt, but there are also many other salts, some with relatively low melting points like a mixture of potassium nitrate and sodium nitrate.

Some high performance cars have sodium filled valves, it’s not all that uncommon today. GM’s LS6 and LS7 engines used them, as does the Road Runner variant of Ford’s Coyote V8’s and the Voodoo V8 used in the Shelby GT-350.

I think you’re right. And sodium boils at 1621 F, way too high to make the ‘heat pipe’ explanation work.

I recall that sodium cooled valves were developed by radial aircraft engine manufacturers in the 1930s. But how does the ‘heat pipe’ theory hold up when most of the cylinders are inverted or horizontal?