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Salient: Victoria University Students' Paper. Vol. 28, No. 11. 1965.

Octane Rating

Octane Rating

Next to volatility, the most important single property of a petrol is its octane rating, a subject filled with mystery for the uninitiated, but nevertheless fairly easily explained in terms of gasoline and engines.

The archetypal combustion engine had a remarkable ability to produce some pretty ghoulish noises. Among the disharmonies of piston slap, tappet rattle, big-end thuds and other percussive variations, the discerning ear could frequently pick out a muted and high-pitched knocking.

Occurring most in cars with fairly high compression ratios, research proved that it was caused by the premature burning of the fuel-air mixture. It was also discovered that a relationship existed between compression ratios, knocking, the octane number of the gasoline and the energy output of the engine.

In brief, the octane number of the gasoline can be taken as a measure of its anti-knock qualities. Using a standard single-cylinder engine, it is possible to determine the octane number of a specific gasoline by comparing its performance in that engine with carefully measured quantities of heptane (its octane number is 0) and iso-octane (its octane number is 100).

The higher the octane number, the less will a gasoline knock. It is particularly important that high octane gasolines be used in cars with high compression ratios.

High compression means that cylinder temperatures are hotter, and premature burning of the fuel/air mixture is more likely. High octane fuel reduces the tendency of the fuel to burn before it is ignited by the spark, and eliminates the consequent loss of power. So if the little old bomb ain't what she used to be, change to high octane gasoline!