Gasoline Octane and Energy

["Osterbrink, Mark (AFS-Warren)" <Osterbrink@xxxxxxxxxxxxxx>]

I have heard several people express the opinion that lower octane fuel
"contains more energy" than high octane fuel.  I posed this question to
the Mobil Oil Corporation, and this is the reply they sent me (I thought
that the list might be interested):

>There is no direct relationship between octane quality and energy
>content.  The relationship is between relative density and energy
>content, where the higher the relative density, the higher the energy
>content.  The higher octane fuels usually contain more of the denser
>aromatic materials (to provide higher octane) and therefore would
>tend to provide greater energy content.  However, because the
>differences in relative density are small, there are overlaps in the
>range of relative densities for the different octane fuels.
>
>The difference in energy content between "typical" RUL and "typical"
>SUL is estimated to give fuel economy benefits of about 2% for the
>SUL (a level that most consumers cannot detect).

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