Aviation Gas

[patrickt]

Originally Posted by Varmit

Av gas is designed to be run at altitude, thus it has a higher density. If you dont adjust the floats running a high percentage of
av gas, you will have this problem.

Most people would never even think of that. Here's more detail on that exact point: https://www.fuelexpert.co.za/canirunavgas.php

[rodneym]

Adjust the floats?
If it's designed for a different density then I could see an adjustment in ratio. But floats?

[austin.minter15]

I have a 390FE with a single Quick Fuel carb in my Cobra and I ran AvGas in it for a while
exclusively because there is a local airport that I can drive up to the pump and fill up. The car ran great, smelled great, and I was extremely happy! I still fill up there from time to time with no issues at all.

[RET_COP]

Light gray or the color of a cigarette ash in the headers is considered a good a color. So is light Tan

[novasscnc]

the compression will dictate the fuel I have 565 hp 11. to one engine I run 94 octane at Chevron in Canada no problems, and no running a high octane fuel that is not required does not make it run better

[Phx Mike]

Originally Posted by RET_COP

Light gray or the color of a cigarette ash in the headers is considered a good a color. So is light Tan

It’s been a long time since I saw that color on the inside of the tail pipe of any car I’ve owned. All my cars used to have a light gray color on the inside of the tail pipe back in the late 70’s and into the 80’s. Now they are all soot black on the inside of the pipe, including my new cars. Never understood why.

[Gaz64]

That's because of no lead in the fuel.

[66AC]

My local airport fuel service sells 101 octane low-lead fuel. This fuel has no ethanol. I have purchased it in my own 5 gal. jug for use in my hit and miss old time engines. I have also tried it in my 428 FE engine. It seemed to work perfectly. One thing I did notice is that the exhaust pipe outlet became a light gray color vs the typical black color it exhibits with unleaded. I do not use the av gas in the Cobra as a normal fuel. I am fortunate to be able to get 93 octane Sunoco here so the added cost and risk to tote fuel is not worth the effort. The airport will NOT pump the av gas into a car. And 5 gal. is the max they will sell at any time. As I recall the av gas was a very pale blue color and had a great odor.

[Varmit]

Originally Posted by rodneym

Adjust the floats?
If it's designed for a different density then I could see an adjustment in ratio. But floats?

Just don't run pure av gas, and you should be fine. I usually put about 7 or 8 gallons with the rest 91 and it runs just fine

[Karl Bebout]

Hauss, way back in the day of only leaded gasoline (Amaco high test excepted), any car, in tune, would have light gray exhaust pipes, after several miles at highway speeds. No plug issues, always thought the color came from the lead in the fuel. I'm not a chemical engineer, just a geezer with lotsa miles on my clock.
Varmit, 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?

[Blas]

Aviation fuel has de-icers as part of the mix. Also extremely clean by comparison to automotive gasoline.
Blas

[t walgamuth]

Originally Posted by Karl Bebout

Hauss, way back in the day of only leaded gasoline (Amaco high test excepted), any car, in tune, would have light gray exhaust pipes, after several miles at highway speeds. No plug issues, always thought the color came from the lead in the fuel. I'm not a chemical engineer, just a geezer with lotsa miles on my clock.
Varmit, 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?

Yes. The float is sealed so the air inside is probably at sea level when built more or less. So as you go higher up it should float lower. I have no facts, just talking off the cuff.

[Chilibit]

Volume is unchanged. Mass is unchanged in the float. Gravity changes the weight minutely as we move away from the center of the earth. The fuel becomes slightly less dense also as it is unrestrained. Technically the float might be lower in a less dense fluid. Where is was sealed matters not.
IMHO of course. Practically negligible.

[olddog]

Originally Posted by Karl Bebout

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?

Karl,

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.

[Chilibit]

Fuel density can change. Molecules obey strong intramolecular forces and do not change size. To change size would require a different atomic construction and then it would be a different molecule. So it is the distance between molecules that affects density.

The issue remains largely academic. Just as olddog said.

[Karl Bebout]

I'm telling ya...dem Buckeyes! (Old Dog) Confusing the issue with facts

I think I understand, but not really sure. 'way above my little grey cells. Oh well.

BTW Old Dog, I escaped Ohio in 1978 for the deserts of AZ. (it was a sunshine thing)

[olddog]

Originally Posted by Karl Bebout

BTW Old Dog, I escaped Ohio in 1978 for the deserts of AZ. (it was a sunshine thing)

That's why we have Buckeye fans everywhere!

I'm not sure what I will do, when I retire. I hate winter, now that I do not hunt. Summer is too hot, now that I'm fat. I only like spring when it is not pouring rain for days on end. I still love the fall. I just don't think there is one state that could satisfy me.