I do not know for certain but the fuel we use is not just gasoline and some percentage of ethanol anymore. Gasoline was a mix of heptane (7 carbon chain) and octane (8 carbon chain). 8 carbon chain octane is highly resistant to detonation. 7 carbon chain heptane is cheaper to produce. We have octane rating to represent how a fuel (no longer gasoline) rating as to how it responds to compression iignition (knock).
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