Can I use E-85 in my airplane?

Patrick Puckett asks: “”Since E-85 is rated at 105 octane, can we use it in engines that require 100LL?””

Patrick Puckett asks: “”Since E-85 is rated at 105 octane, can we use it in engines that require 100LL?””

This is one of the best questions I’ve received in many years. The reason I like it is that I can give a definite answer, which is no. (I guess I could put the qualifier “”not without a significant amount of modification,”” but that would ruin the effect of my concise answer.)

First there is the octane number game. The rating for E-85 (85% ethanol) is 105, which is the average of the Research and Motor octane number. The aviation lean rating would probably be at or around 100, which is the same as the lean rating for 100/130LL. The problem is that we do not know what the rich rating is or, more important, how it would work in an aircraft engine. I am confident that E-85 will meet the octane requirements for all naturally aspirated aircraft engines whether they were 80/87 or 100/130 certified. The concern is whether it would meet the octane requirements for all of the turbo/supercharged engines today.

One major limitation on the use of E-85 in aircraft engines is the change in carburetor calibration. The stoichiometric air/fuel ratio is the ratio of the amount of air needed to mix with a fuel to support complete combustion. This ratio is based on mass, not volume. For avgas, it takes a little more than 14 pounds of air for every pound of fuel consumed. For E-85, it takes a lot more fuel for the same 14 pounds of air. (Based on EPA data, it takes about 25% to 35% more fuel to operate on E-85 vs. auto gas, which is very similar to avgas.) This means that if you were to use E-85, it would be necessary to recalibrate your carburetor.

One evening, I got into a discussion with several gentlemen as to whether a modern fuel-injected automobile that was not designed for E-85 could be operated on it. To settle the discussion, we took one guy’s car, which was almost empty, and filled it with E-85. After driving for about 50 miles, we subjected the vehicle to an array of different driving conditions. We found that the E-85 worked well under normal conditions, but when we went to full throttle acceleration, it was way down on power. I feel that the power loss was due to an overly lean condition. This would relate to a takeoff power setting where, even at full rich, the amount of fuel would be well below that needed for maximum rated power.

The other major problem with E-85 is its affect on composite fuel system components. Many years ago, the state of Illinois operated an aircraft on ethanol fuel. The fuel ruined the composite carburetor float in just a few miles. A metal float fixed that problem. However, there are a lot of composite fuel tank liners, lines and even fuel pumps that can be negatively affected by ethanol.

There are other problems associated with E-85, such as reduced range, water absorption and fuel system cleanup.

The bottom line is that an airplane can be built to operate on E-85, but the present fleet cannot just be changed over.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at

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