[A4] Need for Premium Gas in 1.8T quattro?

[David Ullrich]

Higher octane ratings correlate to higher activation energies. Activation
energy is the amount of energy necessary to start a chemical reaction. Since
higher octane fuels have higher activation energies, it is less likely that
a given compression will cause knocking. (Note that it is the absolute
pressure (compression) in the combustion chamber which is important - not
the compression ratio. The compression ratio only governs the maximum
compression that can be achieved).

It might seem odd that fuels with higher octane ratings burn less easily,
yet are popularly thought of as more powerful. The misunderstanding is
caused by confusing the ability of the fuel to resist compression detonation
(pre-ignition = engine knock) as opposed to the ability of the fuel to burn
(combustion). However, premium grades of petrol often contain more energy
per litre due to the composition of the fuel as well as increased octane.
A simple explanation is the carbon bonds contain more energy than hydrogen
bonds. Hence a fuel with a greater number of carbon bonds will carry more
energy regardless of the octane rating. A premium motor fuel will often be
formulated to have both higher octane as well as more energy.

Dave
1987 CGT 2.3
SE VA


On 9/21/06, Grant Lenahan <glenahan at vfemail.net> wrote:
>
> Nonsense. The main difference between low and high octane is:
>
> 1.) more resistance to knock
> 2.) sometimes, a better detergent package (since its more expensive)
>
> A turbo engine, by definition, operates at a high effective compression
> ratio.  So high octane is almost a requirement, mitigated only by an
> knock sensor which will retard timing and performance to protect the
> motor.
>
> More detergent will result in cleaner running.
>
> I have no idea what "more combustible" means in this context, so I wont
> comment.
>
> Grant
> On Sep 21, 2006, at 1:46 PM, thejimrose wrote:
>
> > i wonder about this too.. from experience with ducati's, the
> > reccomendation
> > is always run high octane, but lota of guys ran low octane as it's more
> > combustible and tended to build up much less carbon in the head +
> > valves.
> >
> > not sure how that might translate.. but i seem to get much better
> > mileage if
> > i run premium, which i think offsets the price of the gas..
> >
> > jim
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