Morgans Blog

What is an octane rating?


This is my second blog on misused terms in the power sports industry.

The term Octane Rating refers to the amount of heat it takes to ignite fuel without an open flame or spark. High octane fuel will withstand more heat before reacting with the oxygen (spontaneously detonating). The benefit of this is that an engine can be built to operate at higher combustion pressures like turbo or supercharged motors or just high compression.  It is recommended to have a fuel with high potential energy to also have a high octane rating, but the octane rating itself is not related to power.
The definition by Wikipedia is as follows (for full article click here):

Octane is a hydrocarbon and an alkane with the chemical formula C8H18, and the condensed structural formula CH3(CH2)6CH3. Octane has many structural isomers that differ by the amount and location of branching in the carbon chain. One of these isomers, 2,2,4-trimethylpentane (isooctane) is used as one of the standard values in the octane rating scale.

As with all low-molecular weight hydrocarbons, octane and its isomers are very flammable. It and its isomers are components of gasoline (petrol).

Use of the term in gasoline

"Octane" is colloquially used as a short form of "octane rating" (named for the ability of octane's branched-chain isomers, especially isooctane, to reduce engine knock), particularly in the expression "high octane." However, components of gasoline other than isomers of octane can also contribute to a high octane rating, while some isomers of octane can lower it, and n-octane itself has a negative octane rating.]

My point is that (High Octane) can have high or low potential energy it has nothing to do with the power stored in the fuel, it relates to a temperature and pressure coefficient at which the fuel will ignite without a spark. However, as a motor builder, it is an important factor. When you plan to build a motor one of the factors that you need to consider, is what the octane rating of the fuel is that you plan to use, bearing in mind that not all fuels are readily available.

We can blame various advertising campaigns for misleading us, suggesting that by selecting a higher octane that you would increase your vehicles performance. The biggest factor in building power from fuel is the amount of oxygen locked up in the fuel.   This is a little dangerous, as the fuel becomes more oxygenated it requires more fuel to maintain the ideal air (oxygen) to fuel ratio.  An example of this is an alcohol carburetor is made with extremely large fuel passages to flow higher volumes of fuel.

At the end of the race it makes little difference, but by better understanding the terms used we can better use the high performance equipment that drives our passion for power sports.