Edgar rated gasoline by matching reference fuel blends

As engine compression rose in the 1910s and 20s, gasoline increasingly detonated inside cylinders — a violent 'flicker' that cost power, overheated engines, and wore out pistons. Yet knock resistance was impossible to put a number on: you could only fill a car and drive it to learn if a fuel knocked. Refiners produced fuel by countless non-standard processes, and the only way to know if a gasoline was good was to test it in a running engine.

The result was an industry that argued about quality it could not measure. Petroleum refiners, carmakers, and buyers traded vague claims — 'runs smooth', 'premium' — with no common unit to compare fuels across suppliers. The classic solution, cataloguing gasoline's chemical composition, had failed to give a useful handle on knock, because knock resistance is not read off composition.

Graham Edgar, a chemist at Ethyl Gasoline Corporation (a General Motors / Standard Oil venture), anchored a scale to two pure compounds: iso-octane, which never knocked in any engine, was set at 100; n-heptane, which always knocked, at 0. By blending the two in different proportions he built a ladder of reference fuels, and assigned every gasoline the number of the iso-octane/n-heptane blend whose knock behavior it matched — the octane number.

Rated gasoline by behavior, not composition: matched each fuel to an iso-octane/n-heptane blend that knocked identically, giving a stable, reproducible number.

Behavior is more repeatable than composition, so the number survives across suppliers and engines.

Pure, easily sourced anchors let any lab rebuild the ladder, making the number reproducible and trustworthy.

A single enforceable number moved the quality argument from vague words to a transactable unit.

When a quality can't be measured directly, anchor a scale to two extreme reference points and rate everything by which rung it matches.

The octane number became the global standard for fuel quality and is still on every pump a century later. Edgar's two-reference-fuels ladder outlived its creator and its company. The octane number remains the universal currency of fuel quality — contested, but in the numbers rather than in incommunicable claims — and its method of anchoring a scale to reference extremes is the template used again for cetane (diesel), and countless other quality indices since.