RE: Octane vs. MPG

[John Bolhuis <bolhuijo@xxxxxxxxxxxx>]

On Wed, 18 Sep 2002, Jason Foster-Bey wrote:

> > Stan,
> >
> > Actually, I think it's the opposite. Lower octane gas will give you
> > better mileage. High octane gas is needed in high compression engines so
> > it won't ignite under pressure before the spark plug fires (pre-ignition
> > or ping). So it really renders less energy than lower octane gas.
>
> Uh, to be clear, high octane gas render more energy. Ignition of a highly
> compressed air-gas mixture results in more energy than the ignition of a
> less compressed mixture.

 The best you can do is use the cheapest fuel that makes your engine
run right, given its particular design.  As for fuel, pound for pound,
high octane fuel has less energy.  quoting from a faq:

===
 There is less energy (BTU's per pound) in higher octane fuels than in
lower octane fuels. Higher octane fuels (like octane itself) have a
benzene type of structure (ring structure), while lower octane fuels
have a parafine type of structure (linear chain structure). Now as too
why this difference in structure has a bearing on energy content, I
believe this is due to the differences in the bonds that hold the
individual atoms together. There is just not as much energy stored in
the ring structures. So when the molecule is oxidized, its energy
state drop is less than in a lower octane molecule.
===

 If you want even more energy, switch to diesel.  Gasoline has about
115,000 Btu/gallon, while diesel is 130,500 Btu/gallon.  Though nobody
recommends putting diesel in your gas engine.

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