Octane Rating


A gasoline's ability to resist detonation is called its "octane" or anti-knock rating.
Gasoline from asphaltic base crude oil produces less knock than one from paraffinic base crude. 
Cracked gas has less tendency to knock than straight run gas.
All marketed gasolines are a blend of straight run and cracked gasolines,
so unless their blending is controlled, the anti-knock qualities will vary. 

A mixture of iso-octane, which has a very high anti-knock rating, and heptane, 
which makes a pronounced knock, is used as a reference fuel to establish an anti-knock standard.
The anti-knock value or octane number is represented by the percentage of volume of iso-octane
that must be mixed with normal heptane in order to duplicate the knocking of the gasoline which is being tested.
These ratings range from 50 in third grade gasolines to 110 in aviational fuels.
The rating of 100 means a fuel having an anti-knock value equal to that of iso-octane.
If the octane rating of a gasoline is naturally low, the fuel will detonate as it burns
and power will be applied to the pistons in hammer-like blows.
The ideal power is that which pushes steadily on the pistons, rather than hammer against them.
The octane rating of a gasoline can be raised by treating it with a chemical which is not a fuel.
The best chemical known is tetra-ethyl lead compound, which is added to the gasoline. 

Tetra-ethyl lead is a liquid which mixes thoroughly with gasoline and vaporizes completely. 
Ethylene dibromide prevents the tetra-ethyl lead from forming lead oxide deposits on spark plugs
and on valve seats and stems. Red dye is added to identify an ethyl treated gasoline
and to warn against its being used as anything but an engine fuel. 
In 1975, it became illegal to use a leaded gasoline except in cars built prior to this time.
With the addition of the catalytic converter, it is undesirable to burn leaded fuel,
because leaded fuel will clog the converter and increase the back-pressure of the exhaust.