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AV gas

In the northern part of the state AV gas can be a pain to get. Has anyone found a 93 octane and additive that works as well?
If so what additive and ratio. Will typically run av gas, just looking for a solution in a pinch.


Slidin Gator

Well-known member
You can burn 93 pump gas, just got to know what you are doing. Assuming 8.7 or lower compression ratio. Drop timing to 20 deg BTDC and run the 93.

I run a Pacemaker ignition and I have it setup with 2 timing maps with a toggle switch to select. Leave it up for 100LL at 25 deg BTDC, throw it down for 93 if I need to burn some fuel I bum off my car motor buddies (bum's the wrong term, them pirates charge me more than AV gas for that stuff in the woods, something about delivery fees). I have it set for 22 BTDC timing which let's me run 93 if I don't heat it up too much, 20 BTDC is safer.

FWC got 3 new angle valve 540s about 4 years ago, they smoked about 3 cylinder heads first 20 hours "they run 93 octane pump gas" the technician said oh these engines suck! I said,,, what fuel are you running in them,,,, car gas,,, they are detonating and overheating cause the timing and compression is set up for 100LL,,, back the timing off to 20° and the problems ceased, the operators of the boats are ruthless.

What is available in the panhandle is E85. E85 has gotten a bad name over the years. It is deserved, but only because of the way they rolled it out vs. the fuel itself. That and of course the "General Public".

You can make more power running E85 than you can running 100 LL. Here is some info.
What is E85?

I just dragged this tank of Ethanol home from a failed distillery gin project (damn stuff is bitter as hell!). It looks like I'm gonna be doing some Ethanol experimentation soon, cause I should not drink all of this (I opened the cap on top in the middle of the afternoon sun and took a big wiff, about fell out of the bed).

Tank of Ethanol.jpg

I used to own a 1986 boat when the US fuel supply switched to E15. All the sudden I started seeing all this brown sand in my fuel filters, I was literally changing them every tank. When I looked into it I realized all that sand was the varnish in my 2X 100 gallon tanks getting cleaned out of the tanks. After a while the sand quit clogging filters and the next time I looked inside they were all shiny. Basically, the ethanol just cleaned out the fuel tank.

The 2nd issue with Ethanol, back in the day was fuel system compatibility. All the old fuel lines and gaskets were not compatible with Ethanol. A given setup would run on it fine for a while, but all that sitting in contact with ethanol and the old stuff deterieorates. These days, anything you buy from the auto parts or marine store is Ethanol compatible. Aviation fuel system seals not so much, O-rings are easy (get the same size in viton), gaskets depend on the material. Old automotive carbs are the same (need new seals), but updated gasket kits available these days work. The point being that you need to update your fuel system to support ethanol, even for 93 octane pump gas.

Both of the above are manageable issues, the 3rd issue is water. Gasoline and water are not miscible, but water and ethanol definitely are. Pour water into 100 LL and you will have water at the bottom of your tank. Pour water into E15-85 and you will have water mixed with your fuel and not burnable. Ethanol does not store well due to humidity and therefore the pump fuels these days do not store well (they really are not meant to be stored). The point here is that ethanol fuels need to be burned, don't try to store forever.

Octane is basically the resistance of a fuel to ignition, ignition temperature is a good guide. The higher the octane, the higher the temp/pressure required for it to self-ignite. We don't want self-ignition, that's knock, but higher ignition temps allow motors to run at higher internal temperatures/pressures that occur when making more power.

E85 is around 105 Octane, so it will allow you to make more power by adding timing, air and fuel. But..., energy content in ethanol is lower than gas, so you need to burn about 1.8X more (by weight, not volume) for the same power level. On aviation engines, all this added fuel is good, it cools the motor, but E85 requires a recalibration of the fuel curve (bigger jets for more fuel in a carb). Throw on the fuel and you can make more power with E85 vs 100 LL. Just be ready to burn a bunch of it.