Detonation, AKI and Octane Number
Field Information Number 303
The number which is posted on the automobile service station pump is not a true octane number. It is what is called an "antiknock index" number (AKI). This number is the average of two octane numbers arrived at by two different kinds of tests. One is called ASTM Research Method and is often abbreviated R or RON. The other is the ASTM Motor Method, M or MON. The antiknock index number on the pump is then this average, or R + M divided by 2 = AKI. A rule of thumb is that the Motor Method octane number (MON) is approximately five points less than the AKI. The significance of the MON is that this is identical to the octane number for aviation gasoline.
Specification D-4814 (previously D-439) for automobile gasoline requires a minimum of 82 MON when the posted number is 87 AKI or more. When the EAA requested approval from the FAA, the request was for an AKI number of 87 to insure a safety margin of 2 octane numbers over the approved rating for aviation gasoline for these 80 octane engines.
Detonation will not be a problem when using any grade of automobile gasoline with an aircraft engine approved for use of 80 octane fuel.
The Department of Energy's (DOE) semi-annual report giving data on gasoline's selected at random throughout the United States shows that for more than 20 years, the lowest octane number measured for automobile gasoline in the US has been more than 80 octane by the Motor Method, which is the same as the aviation method. So, for 80 octane aircraft engines, the octane rating of even the lowest octane automobile gasoline is more than adequate.
The AKI is only 85 in some states. Is that a legal fuel?
No. The octane number requirement for any engine is reduced with reduced ambient temperatures and increased altitude. Therefore, mountain states are permitted to market fuel at lower octane numbers than others. In terms of the STC, the approved fuel must have an 87 antiknock index rating. This may mean that sometimes in mountainous states premium fuel should be used. In this case, the minimum octane number, according to DOE surveys will be at least 84 MON (equivalent to aviation gasoline octane number). Using regular automotive gasoline with an 85 AKI could possibly result in a minimum octane number of 79.1 MON. This difference is probably not significant, but in order to maintain a larger conservative margin, EAA requested approval for automobile gasoline with a minimum 87 AKI to insure 82 MON.