Editing Heptane (section)

===Octane rating scale===
''n''-Heptane is defined as the zero point of the [[octane rating]] scale. It is a lighter component in [[gasoline]] and burns more [[explosive]]ly, causing [[engine]] pre-ignition ([[engine knocking|knocking]]) in its pure form, as opposed to [[octane]] isomers, which burn more slowly and give less knocking. It was originally chosen as the zero point of the scale because of the availability of very high purity ''n''-heptane, unmixed with other isomers of heptane or other alkanes, distilled from the [[resin]] of [[Jeffrey pine]] and from the fruit of ''[[Pittosporum resiniferum]]''. Other sources of heptane and octane, produced from [[crude oil]], contain a mixture of different isomers with greatly differing ratings, and do not give as precise a zero point.