Octane Number Confusion
written by, and thanks to
Tim Wusz
 

• Octane numbers can be very confusing due to several different terminology's used in discussions. I will try to clear up some common misunderstandings and define some of the various terms used when discussing octane quality.
  
  Octane
  The octane quality of a gasoline is its 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 preignition.
  
  Detonation occurs after the spark plug has ignited the air-fuel mixture and the flame front is moving smoothly across the combustion 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 (TDC) and that pressure try's to push the piston down before it is ready to go down. Piston burning and rod bearing damage are the result.
  
  Preignition 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 plug fires. Under wide open throttle conditions, preignition will destroy pistons in seconds.
  
  

  Research Octane Number (RON)
  RON is determined in a single cylinder variable compression ratio engine that operates at 600 rpm with a 125 degree 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 a RON in the range of 110 to 115. The difference in the spread of RON is not very important to racing engines.
  
  The test and hardware were originally developed in 1931. The hardware was revised in 1946 with procedural changes made until the late 1960's.
  
  

  Motor Octane Number (MON)
  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 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 preignition will quickly reduce a racing engine to junk.

  The motor octane 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 of 100 to 115. 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 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 was developed about 20 years ago as a compromise between RON and MON for advertising purposes and also to keep from confusing the consumer with too many different terms. It has erroneously been referred to as road octane number.
  

  Observed Road Octane Number (RdON)
  

  This is derived from testing gasolines in real world multi-cylinder engines, normally at wide open throttle. It was developed about 70 years ago but is still reliable today because engines are the test tools so we are able to take immediate advantage of current technology. The original testing was done in cars on the road but as technology developed the testing was moved to chassis dynamometers. this eliminated a lot of variables and some people have since built very elaborate chassis dynamometers with environmental controls to improve consistency.
  
  "76" has modified this test additionally to use it with racing engines on engine dynamometers. This has given us the opportunity to evaluate gasoline blends during our racing gasoline development that had good RON and MON but that did not respond well in the racing engine under a full throttle excursion through the entire rpm range. We felt these conditions were the true indication of how the fuel could be best developed. In our program we found that the blending components and their ratios are far more important to the racing engine response than high RON and MON numbers found from testing the gasolines in 40 year old single cylinder laboratory engines with antiquated combustion chamber designs. RON and MON can only be used as a guide, the final word must come from the road octane number.

  As an example, 76's current 108 octane leaded racing gasoline blend is the result of testing over 100 experimental blends. The final blend has a road octane number (RdON) of 110, the same RdON as one of our competitors gasoline that has advertised 116 RON and 116 MON. Using only RON and MON can lure a person into a false sense of security. If you want to be certain that your racing gasoline has been thoroughly tested in real world racing engines with the horsepower and road octane number maximized, choose "76" for your engine. If you want high RON and MON without knowing how it will work in your racing engine, buy from the other guys.
  

This article was written on 11/16/95

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