The Symptoms of Low Octane in Motorcycles

Octane ratings, and the method for determining them, are posted on all gas pumps.

Octane is the measure of gasoline's resistance to detonation. The higher the octane rating, the more resistant a particular variety of gasoline is to detonation. In the United States, gasoline is commonly sold at one of three different octane ratings: Regular, with a rating of 87; Mid-grade, with a rating of 89; and Premium, with a rating of 92 or 93.

How Octane Ratings Are Determined

Gasoline is composed of two primary parts, n-heptane and iso-octane. Heptane, especially in vapor form and in the correct concentration, is highly explosive. Octane is more stable. Gasoline manufacturers mix octane with heptane to retard heptane's highly volatile characteristics. The octane rating of gasoline represents the percentage of octane in a particular blend of gasoline. Gasoline contains more elements than just n-heptaine and iso-octane, so the octane rating isn't an exact reflection of the percentage of octane in the blend. Instead it represents that a blend of gasoline will behave as if it was composed of that percentage of octane. So that gasoline with an octane rating of 87 will behave as if it were 87 percent iso-octane and 13 percent n-heptane.

Different engine conditions affect the behavior of fuel octane ratings differently. Gasoline in an engine operating at low rpm in neutral will behave differently than in an accelerating engine at high temperatures. To account for this difference in behavior, two standards of measuring octane ratings were developed. The Research Octane Number (RON) is used to represent a situation with the engine at idle. The Motor Octane Number (MON) represents a hot engine under load and is generally about 10 points higher than the RON for the same fuel.

In Europe gasoline is normally sold displaying the RON rating. In the United States an average of the RON and MON ratings are used and are shown as the PON or Pump Octane Number. American gasoline pumps may sometimes display a different name for PON, but the sticker will show how the octane is figured, usually with a formula such as PON = (R + N) / 2.

Preignition

Preignition, or premature ignition, is the explosion of the fuel-air mixture in the combustion chamber prior to the firing of the spark plug. This symptom is also referred to as dieseling, since diesel engines don't use spark plugs and rely on high compression to ignite the lower-octane fuel-air mixture. Preignition causes a "pinging" sound in the engine.

Detonation

Detonation occurs when the fuel-air mixture burns unevenly, and after the spark plug fires some of the fuel explodes spontaneously due to the heat and pressure of the previous explosion. Detonation causes a "knocking" sound in the engine.

How To Determine Proper Octane For Your Motorcycle

Every motorcycle owner's manual lists a recommended octane rating for that particular motorcycle. There are factors that can cause a motorcycle to operate more efficiently by using a higher or lower octane fuel than what is recommended. Older engines can contain deposits in the combustion chamber, causing the compression ratio to be increased, which can lead to the need for higher-octane gasoline. Weather can also affect the octane requirements of your engine. High ambient air and engine coolant temperatures and low humidity levels can also cause an engine to perform better by using a higher octane fuel. Altitude has the opposite effect on octane requirements. Generally the higher the altitude, the lower the octane requirement.

References

• Motorcycle Performance; Tech Tips: Bad Gasoline; Bill Whisenant; July 2008
• Covington Creations: Steed Motorcycles; It's A Gas, Gas, Gas -- Don't Be Fuelish
• Motorcycle Anchor; How To Understand Fuel Octane Ratings And How It Relates To Your Motorcycle
• "Modern Motorcycle Technology"; Edward Abdo; 2009
• Total Motorcycle; Gasoline -- A Consumer's Guide