Originally Posted by Organic Chemistry, 5th ed, Wade Jr

In addition to being volatile, gasoline must resist the potentially damaging explosive combustion known as knocking. The antiknock properties of gasoline are rated by octane number that is assigned by comparing the gasoline to a miture of n-heptane (knocks badly) and isooctane (2,2,4,-trimethylpentane, which is not prone to knocking). The gasoline being tested is used in a test engine with variable compression raio. Higher compression ratios induce knocking, so the compression ratio is increased until knocking begins. Tables are available that show the percentage of isoocatne in an isooctane/heptane blend that begins to knock at any given compression ratio. The octane number is assigned to the gasoline is simply the percentage of isooctane in an isooctane/heptane mixture that begins to knock at the same compression ratio.