Okay, if you will accept the view of a chemist:
Higher octane doesn't really mean higher power. The actual amount of
energy in the fuel may be more or less depending on the exact molecules
in there and this may not be correlated with octane. Octane as we know
it is really a measure of how evenly the fuel is combusted. If the
reaction proceeds too quickly it is more of an explosion and that is
perceived as knock. A more controlled reaction is what's wanted. This
is done mostly by supressing the action of free radicals (anything with
an unpaired electron). (If you don't have a taste for chemistry, stop
reading now) Although the ideal products of combustion are H20 and CO2
there are lots of byproducts and things produced along the way, free
radicals being very important. Chain free radical reactions are what
can really accelerate the reaction rate. Higher octane fuels such as
iso-octane form more stable radicals that will stay around for longer
periods. Less stable radicals react more quickly in order to make
themselves thermodynamically happier. In other cases, such as propane,
the hydrocarbon chain is short enough that it can go to CO2 and H20 and
some other stuff with fewer intermediates, so it has a high octane
rating. For those of you concerned about tetraethyl lead (PbEt4), it
boosts octane by acting as a radical trap, under the conditions in the
cylinder the PbEt4 becomes Pb(s) and 4Et*. The Pb(s) can go to lovely
places like your valves, or river trout. The Et* is a radical itself
and combines with other radicals generated in the combustion process
creating a 'neutral' species and slowing the overall reaction. Okay,
that's today's chemistry lesson, I'll go back to my cave now.
To be honest, I just put 87 octane in my 76 NYB, it seems to drink
premium and regular with no observable difference other than in my
wallet. Those of you with high compression, pre-emissions engines that
put out more than 215 goatpower may have a different situation.
James
Mike Pittinaro wrote:
>
> As Hugh mentioned, this is a thread that could go on
> forever, but as a student of engineering (and I'm sure
> Dimitrios will back me up here) I wish to correct a
> possible misconsception.
>
> > current engine set up. It seems to me there is no
> > such thing as premium
> > fuel. It is all the same stuff, only the plus and
> > the supreme have
> > additional additives. The latent power is exactly
> > the same. the additives
> > serve to provide a higher resistance to knocking or
> > pinging and that's it.
> > I have never heard of detergent in gasoline, but
>
> This is not true. Iso-octane is one of two (basic)
> hydrocarbons that comprise gasoline. In its purest
> form, gasoline is x parts iso-octane (which is by
> definition 100 octane) and 1-x parts n-heptane (which
> by definition is 0 octane). The mixture gives the
> overall octane rating. Octane is actually calculated
> empiracly using a standard engine with a variable
> compression ratio, so that the engine can be adjusted
> to run without pinging for any octane fuel.
>
> Octanes above 100 are possible with different
> substances such as methyl tert-butyl ether (MTBE),
> among others.
>
> The higher the octane, the greater the power that is
> available, but, if the engine is not tuned for it, it
> will be unable to utilize this extra power. Advancing
> the timing can take advantage of some of this extra
> power by "fooling" the gasoline into thinking you have
> increased the compression ratio. But, it only will
> work up to a point.
>
> As for detergents and additives, I have heard that all
> grades of gasoline from a given manufacturer have the
> same basic additives and detergents, and really there
> are only 2 or three producers of gasoline anyway..just
> lots of different station names!
>
> I run 93 octane in my Imperial, with 14 degrees of
> initial advance, very little vacuum advance, and an
> average amount of mechanical advance. I also put in a
> bit of lead substitute from time to time, prolly more
> for my peace of mind than anything else. But hey, its
> cheap. ;)
>
> =====
> --Mike Pittinaro
>
> Christmas is over now
> So I'm changing my haiku
> Thank you, David Brown
>
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