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The mass of data accumulated by API Research Project 45 on the knock ratings of hydrocarbons has yielded a number of correlations between the molecular structure of a hydrocarbon and its knocking behavior in an internal combustion engine. The knock rating of a hydrocarbon may be expressed as an octane number, critical compression ratio, knock-limited indicated mean effective pressure (imep), or performance number. It is a single-cylinder engine measurement of the kinetics of the chemical reaction of the hydrocarbon with oxygen under the temperatures and pressures existing within the engine during the combustion cycle. Thus, the knocking behavior of a pure hydrocarbon is dependent on its molecular structure. The knock rating of a hydrocarbon may be sharply affected by the addition of antiknock agents (such as tetraethyllead), by the presence of other hydrocarbons, by different engines, and by engine operating conditions. Knock ratings for hydrocarbons so obtained provide correlation of molecular structures. These correlations serve as a guide to the knock ratings of hydrocarbons and may be used as a means of estimating the knock rating of a hydrocarbon which has not been studied. Because of the nature of knock ratings, the relationships thus established are empirical. Correlations between the various test methods and the variations of ratings with test conditions are discussed first. The hydrocarbons have been divided into ordinary chemical classes and are studied separately as to their octane level and response to tetraethyllead.