Understanding Octane Numbers

We all know regular, mid-grade and premium gasolines when we fill our automobiles at commercial gas stations. Regular is labeled as 87 octane, mid-grade is 89 and premium is 93 octane. You also know that premium gasoline with octane 93 costs more than mid-grade and regular gasolines. In simple words, higher the octane number, more expensive is the gasoline. In some states they also sell regular gasoline with 86 octane. Consumer may also see the sign posted on gas stations (R+M)/2 and that the gasoline also contains about 10% ethanol.

Now let us first look at these terms:

Isooctane is a C8 hydrocarbon and is considered to have very good combustion. This liquid chemical is used as primary reference standard and has been assigned an octane number of 100.

Normal heptane is a C7 hydrocarbon and is considered to have very poor combustion. This liquid chemical is also used in making standards and has been assigned an octane number of 0.

80/20 blend is a mixture of 80% volume of isooctane and 20% volume of normal heptane. This mixture is used in making standards for calibrating the octane engines. This mixture has an octane number of 80.

Now if we want to make a standard higher than 80 octane, we will add calculated amount of isooctane to 80/20 blend. Similarly if we need a standard less than 80 octane, we will add calculated amount of normal heptane.

Toluene is another chemical used to prepare a check standard for verifying the engine’s calibration.

In summary, the above chemicals are used to calibrate the octane engines and insure that the engines have been verified before running the gasoline samples to test their octane ratings.

Research Octane Number (R.O.N.) ASTM D 2699

R.O.N. relates to antiknock performance of your vehicle engine under mild conditions. Such as driving on a highway.

Motor Octane Number (M.O.N.) ASTM D 2700

M.O.N. relates to antiknock performance of your vehicle engine under severe conditions. Such as driving through downtown.

Principle of Testing Octane Numbers of Gasoline

Octanes are tested by using a single cylinder, CFR test engine. There are separate engines for R.O.N. and M.O.N. the main common parts of both engines are:

  • Air Humidifier Tube
  • Intake Air Heater Coolant Condenser
  • 3-4 Bowl Carburetor
  • C.R. Change Motor
  • CFR 48 Crankcase
  • Oil Filter
  • Detonation Meter
  • Knockmeter
  • C.R. Digital Counter

Appropriate ventilating hoods, special maintenance tools and special burettes for measurements of primary reference fuels are also there, where the engines are located.

Engines are first calibrated by using appropriate blends of reference standards. The actual procedure requires all operating engine controls to be right before running the standards. Several adjustments including but not limited to, checking air/fuel ratio, spread, barometric pressure, digital counter readings and others to optimize the maximum knock. After successful calibrations, the engines are verified by running a toluene based check standard/s. Once the operator is satisfied with engine verification, he then introduces the sample under test and tries to optimize the knock reading. This is followed by bringing the knock reading to the middle of the knock meter. This is done by going up or down on the digital counter. The reading on the counter is then used to record the octane number from the tables provided in both methods.

Good and trained operators confirm their number by going back and rechecking their check standards in most situations.

Separate results are recorded from each engine and the reported octane number is average of the two values.

Octane number = (R.O.N. + M.O.N.) / 2


This test requires full knowledge of engine parts and all controls. Proper maintenance including sand blasting of the carbon build up and periodic cylinder overhauls of the engines are fun to learn and an added expertise to do it. Almost all labs are using octane engines built by Waukesha as the sole manufacturer. Each new engine roughly costs us $ 500,000.00.

The above information is provided for non-operators and hope will enhance your understanding of octane numbers.