Significance and Use
Motor O.N. correlates with commercial automotive spark-ignition engine antiknock performance under severe conditions of operation.
Motor O.N. is used by engine manufacturers, petroleum refiners and marketers, and in commerce as a primary specification measurement related to the matching of fuels and engines.
5.2.1 Empirical correlations that permit calculation of automotive antiknock performance are based on the general equation: 
Values of k1, k2, and k3 vary with vehicles and vehicle populations and are based on road-octane number determinations.
5.2.2 Motor O.N., in conjunction with Research O.N., defines the antiknock index of automotive spark-ignition engine fuels, in accordance with Specification D 4814
This is more commonly presented as: 
Motor O.N. is used for measuring the antiknock performance of spark-ignition engine fuels that contain oxygenates.
Motor O.N. is important in relation to the specifications for spark-ignition engine fuels used in stationary and other nonautomotive engine applications.
Motor O.N. is utilized to determine, by correlation equation, the Aviation method O.N. or performance number (lean-mixture aviation rating) of aviation spark-ignition engine fuel.7
Scope
1.1 This laboratory test method covers the quantitative determination of the knock rating of liquid spark-ignition engine fuel in terms of Motor octane number except that this test method may not be applicable to fuel and fuel components that are primarily oxygenates. The sample fuel is tested in a standardized single cylinder, four-stroke cycle, variable compression ratio, carbureted, CFR engine run in accordance with a defined set of operating conditions. The octane number scale is defined by the volumetric composition of primary reference fuel blends. The sample fuel knock intensity is compared to that of one or more primary reference fuel blends. The octane number of the primary reference fuel blend that matches the knock intensity of the sample fuel establishes the Motor octane number.
1.2 The octane number scale covers the range from 0 to 120 octane number, but this test method has a working range from 40 to 120 octane number. Typical commercial fuels produced for automotive spark-ignition engines rate in the 80 to 90 Motor octane number range. Typical commercial fuels produced for aviation spark-ignition engines rate in the 98 to 102 Motor octane number range. Testing of gasoline blend stocks or other process stream materials can produce ratings at various levels throughout the Motor octane number range.
1.3 The values of operating conditions are stated in SI units and are considered standard. The values in parentheses are the historical inch-pounds units. The standardized CFR engine measurements continue to be in inch-pound units only because of the extensive and expensive tooling that has been created for this equipment.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
