Octane Number Confusion

The Various Terms Used When Discussing Octane Quality



The octane quality of a gasoline is it's ability to resist detonation, a form of abnormal combustion. Detonation occurs when the air / fuel mixture reaches a temperature and / or pressure at which it can no longer keep from self igniting. Two types of abnormal combustion are common: the first is detonation as previously mentioned and the other is pre-igninition.

Detonation occurs after the spark plug has ignited the air / fuel mixture and the flame front is moving through the combustions chamber. If, during this burning process, the unburned air / fuel mixture reaches a temperature and / or pressure at which it is no longer stable, it burns very rapidly causing a new flame front to collide with the one that originated at the spark plug. Maximum pressure in the cylinder occurs before the piston reaches Top Dead Center (T.D.C.) and that pressure tries to push the piston down before it is ready to go down. Piston burning and rod bearing damage are the result as well as loss of power.

Pre-Ignition is the other bad actor and is usually started by a hot spot in the combustion chamber which causes the mixture to ignite before the spark plug fires. Under wide open throttle conditions, pre-ignition will destroy pistons in seconds.


RON is determined in a single cylinder variable compression ratio engine that operates to 600 RPM with a 125 degree F inlet air temperature at standard barometric pressure. Spark advance is fixed at 13 degrees BTDC. In a real world engine, RON is necessary to satisfy part throttle knock problems. A good quality racing gasoline has RON in the range of 110 to 115. A high quality racing gasoline can have a RON in excess of 120 which is the top of the octane scale. The difference in the spread of RON is not very important to racing engines.

The procedures and hardware for the RON test were originally developed in 1931. The hardware was revised in 1948 with procedural changes made until the late 1960's.


MON is determined in a single cylinder engine similar to the RON engine with a few changes that make operating conditions more severe and therefore the octane numbers are lower. The MON engine runs at 900 RPM with a 300 degree F mixture temperature. Spark advance varies with compression ratio. In a real world engine, MON is necessary to satisfy octane demands at wide open throttle. This is a very important number for racing engines since they spend a high percentage of their lives under high speed and high load conditions. Racing engines cannot afford to be short on octane quality, since detonation or pre-ignition will quickly reduce a racing engine to junk.

The Motor Octane Number (MON) appetite of an engine with 13:1 compression ratio and a four inch bore varies with operating conditions but is normally around 101. Good quality racing gasolines have MON in the range 100 to 110. High quality racing gasolines have a MON in excess of 110. If your engine requires a 101 MON, it is of no value to use a gasoline that has a 115 MON. To cover yourself for extreme conditions, it is wise to have an octane cushion, but there is no performance advantage to using a very high octane quality product if you do not need it.

The MON test was originally developed in 1932. Major hardware changes were made in 1948 with procedural changes made until the late 1960's.

(R+M) 2:

This is the average of RON and MON. It is sometimes referred to as the AKI or Anti-Knock Index. By law this number must be posted on the dispensing pump at retail outlets in most states. It is the most commonly used octane reference today. It came into use in the early 1970's as a compromise between RON and MON for advertising purposes, and to keep from confusing the consumer with too many different terms. It has erroneously been referred to as Road Octane Number (RdON).


This is derived from testing gasolines in real world multi-cylinder engines, normally at wide open throttle. It was developed in the 1920's and is still reliable today. The original testing was done in cars on the road but as technology developed the testing was moved to chassis dynamometers with environmental controls to improve consistency.


222 Industrial Drive
PO Box 155
Chickamauga, GA 30707
Toll Free (888) 394-3835 | Toll Free Fax: (888) 752-6457
Local (706) 375-2851 | Fax (706) 375-5798