The Heptane reference article from the English Wikipedia on 24-Jul-2004
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Heptane

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Heptane is an organic chemical, specifically an alkane, with formula C7H16.  Heptane has nine isomers.  The n-heptane has by definition the octane rating 0, so it is undesirable in petrol (US:gasoline), because it burns in a way that often causes engine knocking, as opposed to octane, which delivers better performance.