Quote:
Originally Posted by Yakaru
Higher Octane means it's more resistant to detonation from pressure; so if you go higher you may get a less complete ignition of the gas and it can actually lower your mileage, since the engine is designed for gas that ignites relatively easily.
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No. Flame-front propagation speed is identical across all octane levels. The higher branch-chain and aromatic content of higher octane fuels just resist formation of radical alkyl groups better. Those radicals are sensitive and can spontaneously ignite ahead of flame front. Note at this point, ignition and combustion is well underway. It just hasn't completed yet when detonation event occurs.
Mileage is purely based upon number of hydrocarbons per gallon, regardless of octane level. Ethanol is an easy octane-booster @ 114-octane. It is used to cut lower-octane gas. However, due to its onboard oxygen, which is HUGE, this displaces hydrocarbons per gallon, leading to lower mileage.