6.13 Can higher octane fuels give me more power?

On modern engines with sophisticated engine management systems, the
engine
can operate efficiently on fuels of a wider range of octane rating, but
there
remains an optimum octane for the engine under specific driving
conditions.
Older cars without such systems are more restricted in their choice of
fuel,
as the engine can not automatically adjust to accommodate lower octane
fuel.
Because knock is so destructive, owners of older cars must use fuel that
will
not knock under the most demanding conditions they encounter, and must
continue to use that fuel, even if they only occasionally require the
octane.

If you are already using the proper octane fuel, you will not obtain
more
power from higher octane fuels. The engine will be already operating at
optimum settings, and a higher octane should have no effect on the
management
system. Your drivability and fuel economy will remain the same. The
higher
octane fuel costs more, so you are just throwing money away. If you are
already using a fuel with an octane rating slightly below the optimum,
then
using a higher octane fuel will cause the engine management system to
move to
the optimum settings, possibly resulting in both increased power and
improved
fuel economy. You may be able to change octanes between seasons ( reduce
octane in winter ) to obtain the most cost-effective fuel without loss
of
drivability.

Once you have identified the fuel that keeps the engine at optimum
settings,
there is no advantage in moving to an even higher octane fuel. The
manufacturer's recommendation is conservative, so you may be able to
carefully reduce the fuel octane. The penalty for getting it badly
wrong,
and not realizing that you have, could be expensive engine damage.

6.14 Does low octane fuel increase engine wear?

Not if you are meeting the octane requirement of the engine. If you are
not
meeting the octane requirement, the engine will rapidly suffer major
damage
due to knock. You must not use fuels that produce sustained audible
knock,
as engine damage will occur. If the octane is just sufficient, the
engine
management system will move settings to a less optimal position, and the
only major penalty will be increased costs due to poor fuel economy.
Whenever possible, engines should be operated at the optimum position
for
long-term reliability. Engine wear is mainly related to design,
manufacturing, maintenance and lubrication factors. Once the octane and
run-on requirements of the engine are satisfied, increased octane will
have
no beneficial effect on the engine. Run-on is the tendency of an engine
to
continue running after the ignition has been switched off, and is
discussed
in more detail in Section 8.2. The quality of gasoline, and the additive
package used, would be more likely to affect the rate of engine wear,
rather
than the octane rating.

6.16 What happens if I use the wrong octane fuel?

If you use a fuel with an octane rating below the requirement of the
engine,
the management system may move the engine settings into an area of less
efficient combustion, resulting in reduced power and reduced fuel
economy.
You will be losing both money and drivability. If you use a fuel with an
octane rating higher than what the engine can use, you are just wasting
money by paying for octane that you can not utilize. The additive
packages
are matched to the engines using the fuel, for example intake valve
deposit
control additive concentrations may be increased in the premium octane
grade.
If your vehicle does not have a knock sensor, then using a fuel with an
octane rating significantly below the octane requirement of the engine
means
that the little men with hammers will gleefully pummel your engine to
pieces.

You should initially be guided by the vehicle manufacturer's
recommendations,
however you can experiment, as the variations in vehicle tolerances can
mean that Octane Number Requirement for a given vehicle model can range
over 6 Octane Numbers. Caution should be used, and remember to
compensate
if the conditions change, such as carrying more people or driving in
different ambient conditions. You can often reduce the octane of the
fuel
you use in winter because the temperature decrease and possible humidity
changes may significantly reduce the octane requirement of the engine.

Use the octane that provides cost-effective drivability and performance,
using anything more is waste of money, and anything less could result in
an unscheduled, expensive visit to your mechanic.

http://www.faqs.org/faqs/autos/gasoline-faq/part3/preamble.html

 The truck belongs to my sister, who will not run cheaper gas in it, despite

    I used a Hypertech programmer on mine.   I set it to the mid grade program, and
power and mileage actually went up.  Then I set it to the premium fuel programm.
Power went up just a little, but not much.  Fuel mileage went down.
    If she insists on using premium fuel anyway, might as well use the hypertech and
set it mid grade.  I gained about 2mpg on steady highway driving, and about 1mpg
around town.  The hypertech cost about $275-300.  I'll let you do the math on this one.

I've only owned two. A 90 and a 92. Both pinged until I put colder plugs in.
The 92 tows everything, and has most of its life. I run 87 in it and it
never pings. With the factory plugs it would ping under medium power output.
Give it more or less throttle and it would quit.

Al

92 W250, 360, 5 speed.