Thread: Aviation Gas
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Old 05-29-2019, 07:51 PM
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Originally Posted by Karl Bebout View Post
I don't understand how the molecular density of the fuel would make any difference in the float setting. Idle mixture and/or jetting, yes, but wouldn't affect the buoyancy of the "bobber", would it?

Molecular density usually is taken to mean the mass of one molecule, but it could mean the number of molecules per some unit of area. For simplicity purposes let just use the term fuel density, which is the mass of some volume of fuel at some temperature and some pressure. Generally it is expressed as mass per unit of volume like grams per cubic centimeter (g/cc).

The generally accepted theory is that when an object is put onto a liquid surface and it sinks into the liquid, the object will float, when the mass of the amount of liquid it has displaced is equal to the mass of the object. I would go so far as to say this is a physical fact proven a billion times over. Ship designers can tell you where the water line will be before the ship is built.

The more dense the liquid the higher the object will float. The less dense the liquid the deeper the object will sink. Go float in a fresh water pool (as in swim). Then go float in the Gulf of Mexico and see how much higher you float.

So if a fuel is more dense, the float will set higher, closing the needle valve sooner, which will make the fuel level lower. It is absolute fact, but what is the real world impact? Theoretically you can calculate how much a gnat landing on an 18" I beam will deflect the beam, but in the real world it does not matter.

What is the difference in density of the two fuels? Personally I expect it to be fairly small. Since we live in a world where temperature can vary from -20 F to 120 F in the lower 48 states, I would expect the density difference in pump gasoline to vary more from the affects of thermal expansion than the difference from pump gasoline to aviation gasoline.

So to summarize, a change in fuel density does change the fuel level in a carburetor with a float system, but it is not likely to be a significant change that you need to worry about.

But when the size of the molecules change, that has a big impact on how the fuel flows through a jet. So your instinct of what is more important was dead on. Just your understanding of buoyancy needed adjusted a tad, even though it likely doesn't matter.
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