thermosiphoning?

i work at a metal shop and want to make an aluminum radiator catch tank that will thermosiphon cool fluid back into the system. does anyone know where to start? i saw the trick factory ones on race bikes in the mags so thats where the idea came from.

It's not really thermosiphoning, but to begin with, you need a cap that is designed for such an application. The difference is that there will be a gasket directly under the cap (not the one on the spring plate that seals the coolant in under pressure) to prevent air from leaking in when a vacuum forms in the radiator due to cooling. This vacuum forms in ordinary radiator systems, too, but is relieved by a small check valve on the bottom of the cap, and air enters through the overflow hose. In a coolant recovery system, you want the vacuum to draw coolant back into the radiator through the hose. Air leaking in around the top of the cap would spoil that scheme. Use a cap from a WR450.

The second thing you need is a tank that has a vented cap, so the fluid level can freely change up or down.

And the last element is the overflow hose from the radiator. That obviously has to go to the tank, but what isn't obvious to everyone is that the hose has to reach to a point below the normal coolant level in the tank in order to pull it back up.

Thermosiphoning is how some of the earliest water-cooled road racers circulated coolant without a pump. It depends on the heated water seeking to rise to a higher part of the system, while the cooler water would sink to the lower areas. It worked, sort of, at least at that kind of speed, but it's a lot like voodoo.

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