In search of the illusive octane

The GAfuels Blog is written by two private pilots concerned about the future availability of fuels for piston-engine aircraft: Dean Billing, Sisters, Ore., an expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.

By Dean Billing, director, Aviation Fuel Club

First, let me say, “OCTANE IS NOT AN INDICATION OF POWER”! If you learn nothing from this post, always remember octane is not a specification, or the measurement, of gasoline power or quality. The octane rating is only a measure of a fuel’s ability to burn in a controlled manner, rather than exploding or detonating in an uncontrolled manner. The higher the octane specification the more resistant the fuel is to detonation in a high compression engine.

Now comes the really confusing aspect of octane rating for gasoline. There are a myriad of them — RON, MON, ISO, AKI and some I don’t remember. The only ones involving avgas and auto gas are RON, MON and AKI. Here are the definitions from Wikipedia: “The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.”

“There is another type of octane rating, called Motor Octane Number (MON), or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load, as it is determined at 900 rpm engine speed, instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel’s knock resistance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON, however there is no direct link between RON and MON.”

Avgas uses the MON rating system exclusively. It is made up of two numbers, the lean MON and the rich MON. Back in the stone age of aviation, when I started flying more than 40 years ago, all avgas was specified with both numbers. I can remember when we had 80/87, 91/96, 100/130 and 115/145. Today the powers that be have decided to describe avgas with only the lean MON octane rating, which is the lower number of the pair. We have 82UL, 91UL, 94UL and 100LL, although 82UL and 94UL are not made by any refinery that I know of, and 91 UL, if it is being made, is being made for military drones with Rotax engines. 100LL has the same octane rating as 100/130 of the old days, but it meets that octane rating with the lower lead specification of 91/96 and has the same dye color, blue.

So what about unleaded auto gas, what some aviators call “mogas.” Throughout auto gasoline’s history, marketing has been key. Back before unleaded auto gasoline was required, gasoline companies generally marketed “regular” and “supreme.” The implication was that “supreme” had more “power” and would make your car perform better. This was back before World War II when auto engines had maybe a 5 or 6 to 1 compression ratio. The difference between regular and supreme was nothing more than the amount of Tetra-Ethyl Lead (TEL) blended into the gasoline.

Meanwhile aviation was making fuel advances and measuring it in “octane,” especially with the coming of World War II. I understand there was a 70 octane avgas at one time at about the same time auto gasoline was about 40 octane. During the war military performance demands got the industry to develop 100/130 and then 115/145. Drivers heard about these “octane” numbers and thought auto gasoline regular must have been about 80/87 and “premium” was 100 octane plus, although the auto gasoline companies didn’t have to advertise any octane numbers — and, if they did, they probably used the RON number because it was higher.

Beginning in the 1970s, the government committed to getting TEL out of gasoline for environmental reasons as it interfered with catalytic converters on cars. By 1996, specifications for unleaded gasoline were finalized as ASTM D4814. TEL, which had been used to raise octane in avgas and auto gas was then removed, as refinery processes had improved so that a high octane, lead free gasoline equivalent to the old leaded auto gas could be produced economically. Some gasoline producers thought that the new unleaded products should be marketed with their MON octane rating, similar to aviation fuel, and others wanted to market auto gasoline with the RON specification that was higher, because they believed it was a marketing advantage if the consumer thought the gasoline was more “powerful.”

To compromise, the industry agreed to an “Anti Knock Index” or AKI, which was determined by taking the average of the MON and RON octane ratings. (Mathematically AKI is computed by adding the MON and RON and dividing by 2). This is why you see a label on all gasoline pumps that gives the “octane” with a note next to it or below it that says (R + M)/2 Method. The industry settled on three grades of gasoline, 87 AKI, 89 AKI and 91 AKI (or higher). Those grades are not specified in ASTM D4814, but by the states or equipment manufacturers. That is why regular gasoline in the Rocky Mountain states is often 85 AKI, instead of 87 AKI that we find closer to sea level. However premium is always 91 AKI or above, with 93 AKI found in many states east of the Rockies. I know of no American cars whose owners manuals require higher than 91 AKI premium unleaded gasoline and only the Petersen “high compression” STCs and the Rotax 912ULS require 91 AKI auto gas. Lycoming has talked about requiring 93 AKI auto gas for its O-360 sport engines and at one time publicly stated that they were going to seek a unique ASTM specification for it, but they have never pursued it.

So remember, “octane” and “AKI” are both measures of detonation resistance and they have nothing to do with power or gasoline quality.


  1. Mike Arman says

    I’m an AGI and taught ground school for over 11 years. One of THE most difficult concepts to get across was that (as you say) higher octane does not give you more power. Everyone thinks it does, and it does NOT.

    Higher *compression* gives you more power – but higher compression results in detonation. Higher octane gasoline suppresses this detonation. This allows you to run higher compression (which is what gives you more power) without destroying the engine.

    Most of the students would nod uh-huh, and then go refuel their Hyundais and Geo Metros with high test anyway . . .

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