linguistic:paraphrasing
U.S., 1930–1941

In the five years prior to 1929, a great amount of experimentation was conducted on different testing methods for determining fuel resistance to abnormal combustion. It appeared engine knocking was dependent on a wide variety of parameters including compression, ignition timing, cylinder temperature, air-cooled or water-cooled engines, chamber shapes, intake temperatures, lean or rich mixtures, and others. This led to a confusing variety of test engines that gave conflicting results, and no standard rating scale existed. By 1929, it was recognized by most aviation gasoline manufacturers and users that some kind of antiknock rating must be included in government specifications. In 1929, the octane rating scale was adopted, and in 1930, the first octane specification for aviation fuels was established. In the same year, the U.S. Army Air Force specified fuels rated at 87 octane for its aircraft as a result of studies it had conducted.

During this period, research showed that hydrocarbon structure was extremely important to the antiknocking properties of fuel. Straight-chain paraffins in the boiling range of gasoline had low antiknock qualities while ring-shaped molecules such as aromatic hydrocarbons (for example benzene) had higher resistance to knocking. This development led to the search for processes that would produce more of these compounds from crude oils than achieved under straight distillation or thermal cracking. Research by the major refiners led to the development of processes involving isomerization of cheap and abundant butane to isobutane, and alkylation to join isobutane and butylenes to form isomers of octane such as " isooctane ", which became an important component in aviation fuel blending. To further complicate the situation, as engine performance increased, the altitude that aircraft could reach also increased, which resulted in concerns about the fuel freezing. The average temperature decrease is 3.6 °F (2.0 °C) per 300-meter (1,000 ft) increase in altitude, and at 12,000 meters (40,000 ft), the temperature can approach −57 °C (−70 °F). Additives like benzene, with a freezing point of 6 °C (42 °F), would freeze in the gasoline and plug fuel lines. Substituted aromatics such as toluene, xylene, and cumene, combined with limited benzene, solved the problem.

By 1935, there were seven different aviation grades based on octane rating, two Army grades, four Navy grades, and three commercial grades including the introduction of 100-octane aviation gasoline. By 1937, the Army established 100-octane as the standard fuel for combat aircraft, and to add to the confusion, the government now recognized 14 different grades, in addition to 11 others in foreign countries. With some companies required to stock 14 grades of aviation fuel, none of which could be interchanged, the effect on the refiners was negative. The refining industry could not concentrate on large capacity conversion processes for so many different grades and a solution had to be found. By 1941, principally through the efforts of the Cooperative Fuel Research Committee, the number of grades for aviation fuels was reduced to three: 73, 91, and 100 octane.

The development of 100-octane aviation gasoline on an economic scale was due in part to Jimmy Doolittle, who had become Aviation Manager of Shell Oil Company. He convinced Shell to invest in refining capacity to produce 100-octane on a scale that nobody needed since no aircraft existed that required a fuel that nobody made. Some fellow employees would call his effort "Doolittle's million-dollar blunder" but time would prove Doolittle correct. Before this, the Army had considered 100-octane tests using pure octane but at $6.6 per liter ($25/U.S. gal), the price prevented this from happening. In 1929, Stanavo Specification Board Inc. was organized by the Standard Oil companies of California, Indiana, and New Jersey to improve aviation fuels and oils and by 1935 had placed their first 100 octane fuel on the market, Stanavo Ethyl Gasoline 100. It was used by the Army, engine manufacturers and airlines for testing and for air racing and record flights. By 1936, tests at Wright Field using the new, cheaper alternatives to pure octane proved the value of 100 octane fuel, and both Shell and Standard Oil would win the contract to supply test quantities for the Army. By 1938, the price was down to $0.046 per liter ($0.175/U.S. gal), only $0.0066 ($0.025) more than 87 octane fuel. By the end of WWII, the price would be down to $0.042 per liter ($0.16/U.S. gal).

In 1937, Eugene Houdry developed the Houdry process of catalytic cracking, which produced a high-octane base stock of gasoline which was superior to the thermally cracked product since it did not contain the high concentration of olefins. In 1940, there were only 14 Houdry units in operation in the U.S.; by 1943, this had increased to 77, either of the Houdry process or of the Thermofor Catalytic or Fluid Catalyst type.

The search for fuels with octane ratings above 100 led to the extension of the scale by comparing power output. A fuel designated grade 130 would produce 130 percent as much power in an engine as it would running on pure iso-octane. During WWII, fuels above 100-octane were given two ratings, a rich and a lean mixture, and these would be called 'performance numbers' (PN). 100-octane aviation gasoline would be referred to as 130/100 grade.