Dave:
David Geesaman wrote:
[...]
> I've done two years of serious research tracking my fuel economy. I
> used only 93 octane for several years before switching to 89. My fuel
> economy did not change at all.
>
> I imagine under very heavy load there is a difference, but for routine
> driving you will not see anything.
>
> Dave
The higher octane rated, or equivalent to chemically pure octane, has no
benefit to power at all. Nor is there a difference in the quality of the
product. The octane rating numbers have nothing to do with quality or
power produced.
Higher octane rated gasoline has one primary purpose in the internal
combustion piston engine. The higher rated fuel burns slower.
If you have a higher compression engine, or that achieves higher mass
charge and cylinder pressures due to super/turbo charging, the lower
rated gasolines burn far too fast. In fact the sudden burning can
seriously damage the pistons by melting and predetonation. The piston
tops can be hammered into dents by pre-ignition. The higher octane rated
fuels burn more slowly and evenly, and the ignition timing may be set to
ignite in advance of TDC producing a longer burn in faster revving
engines.
If you burned septane or heptane in a modern engine with more than a 9:1
compression ratio there would be a lot of pre-ignition detonation, and
that would be quite audible as pinging or knocking. The method of
slowing combustion with those fuels is to retard the ignition. Later
ignition sparks reduces pre-ignition, however, the fuel is not burned so
efficiently.
The computer makes the ignition delays, and other changes to induction
timing and fuel metering, etc., and you may not be aware of the changes
made to the engine functions to avoid the knocking.
Using low octane fuel in a high compression engine without a computer
will result in knocking. The engine will not be using the fuel as
efficiently and producing as much power with the low octane rated fuel.
The manual states the minimum octane for that engine. Its not a matter
of quality or fuel economy; its a matter of the durability of the soft
aluminum engine pistons over time. The advantage is with the high
compression ratio. That creates higher cylinder pressures, greater
charge and more power. If the engine runs with the ignition retarded
pressure are lower and the power produced is less.
Its a matter of safety. A melted, holed or dented piston top can wreck
the engine. You may not be able to hear the sounds of pre-ignition that
the computer knock sensor detects. Stay with the recommended anti-knock
fuels. Formerly, the chemical called tetraethyl lead was used to raise
the octane rating, however, that shouldn't be used in engines that have
catalytic converters. Fortunately, the engine designers built in the
computers, and that makes it possible to use lower rated fuels. If so,
stay off the throttle and minimize the charge in the cylinders to avoid
knocking. Mixing rated fuels of the same type is Ok, until you run the
proper rated fuel.
A higher rated fuel than what the manual recommends has no effect on
power or fuel economy. Nor does it have any additional quality. Higher
rated fuels than necessary are simply money wasted.
Ralph Hertle
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