Family of alkane molecules, formula C₈H₁₈. Several different isomers exist with different structures but the same chemical formula. One of the isomers, 2,3,4-trimethylpentane, is the standard for knock properties in automobile engines. The octane rating measures the percentage of 2,3,4-trimethylpentane in a mixture with heptane that would give the same knock characteristics as the fuel under test.