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Understanding The Properties
& Octane Ratings of Race Gas
Below we will attempt to
help you better understand
how race fuel works by
explaining to 4 basic
qualities of fuels. As in
everything, there are
trade-offs. You can't make a
racing fuel that has the
best of everything, but you
can produce one that will
give your engine the most
power. This is why there are
different fuels for
different applications. The
key to getting the best
racing gasoline is not
necessarily buying the fuel
with the highest octane, but
getting one that is best
suited for your engine.
1. OCTANE:
This does nothing more than
rate a fuel's ability to
resist detonation and/or
preignition. Be aware that
the ability of fuel to
resist detonation is a
function of more than just
octane. Octane is rated in
several different ways:
Research Octane Numbers,
(RON); Motor Octane Numbers,
(MON); and Pump Octane
Numbers (R+M/2). Pump Octane
Numbers are what you see on
the yellow decal at gas
stations, representing the
average of the fuel's MON
and RON. Many fuel
companies rate their fuels
using the RON in an effort
to make them appear more
resistant to detonation, so
it is important that when
comparing fuels to make sure
that you are comparing the
same method of rating.
Don't be fooled by high RON
numbers or an average --
MONs are the most relevant
ratings for a racing
application. How ever too
much octane can slow the
burn rate of the fuel
causing a loss in power so
it is important that you
chose the appropriate fuel
for your combination.
2. BURNING SPEED:
This is the speed at which
fuel releases its energy. At
high RPMs, there is very
little time (real time - not
crank rotation) for fuel to
release its energy. Peak
cylinder pressure should
occur around 20°
ATDC. If the fuel is still
burning after this, it is
not contributing to peak
cylinder pressure (which is
what the rear wheels see).
3. ENERGY VALUE:
An expression of the
potential energy in the
fuel. The energy value is
measured in BTUs per pound,
not per gallon, the
difference is important. The
air/fuel ratio is expressed
in weight, not volume.
Fuels with higher BTU’s will
have higher energy value,
this has a positive impact
on horsepower at any
compression ratio or engine
speed.
4. COOLING EFFECT:
The cooling effect on fuel
is related to the heat of
vaporization. The higher a
fuel's heat of vaporization,
the better its ability to
cool the intake mixture. A
better cooling effect can
generate some horsepower
gains in 4-stroke engines,
and even bigger gains in
2-stroke engines.
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OCTANE NUMBERS AND
WHAT THEY REPRESENT
A commonly asked
technical question
is: What is the
difference between
Motor, Research and
R+M/2 Octane
Numbers. Another
frequently asked
question is why some
fuel companies
represent their
fuels with Motor
Octane Numbers,
while other
companies use
Research or R+M/2
Octane Numbers.
To answer these
questions, we need
to first explain the
machines that do the
testing. These
machines were made
in the 1930s and
were designed to
test for octane
ratings between 0
to100, any number
above 100 is an
extrapolation. These
machines are
dinosaurs & are not
adequate for today's
high tech fuels or
engines, but they
the only means
available for
testing fuels. These
machines are
one-cylinder engines
that have an
adjustable head that
can move up or down
to increase or lower
the compression
ratio while the
engine is running.
The Motor and
Research machines
are the same in this
respect, but they
differ in several
other
characteristics. The
following is a
comparison of the
two machines used
for testing octane
numbers:
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Motor
Machine |
Research
Machine |
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RPM |
900 |
600 |
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Intake Air
Temp.
|
300
DEGREES F |
120
DEGREES F |
|
Timing |
VARIABLE
BASIC
SETTING 26
DEGREES |
FIXED AT
13 DEGREES
(DOES NOT
CHANGE) |
As the comparison
above shows, the
Motor Octane machine
runs at a higher
RPM, hotter
temperature and more
timing. This machine
will put more stress
on any fuel and more
accurately
represents a racing
engine. Since most
people reading this
are primarily
interested in fuels
for racing
applications then it
is best to look at
the Motor Octane.
The Research Octane
machine will always
produce a higher
number for the
obvious reason that
it does not put the
same amount of
stress on the fuel.
The R+M/2 Octane
Number is the
average of the
Research and Motor
Octane numbers and
is the number
displayed with
yellow labels on
retail level gas
pumps.
When comparing fuels
for racing purposes
make sure to compare
Motor Octane Numbers
because these are
the ones that count
when racing.
NOTE: Definition of “Specific Gravity”:
The specific gravity
of a substance is a
comparison of its
density to that of
water.
Specific gravity
MUST always be
referenced to a
particular
temperature
reading. The
specific gravity of
a liquid decreases
with a rise in
temperature or
increases when its
temperature is
lowered. Fuels with
a higher specific
gravity number have
a higher resistance
to detonation
because they fall
down the cylinder
faster. The faster
the fuel falls the
quicker the fuel
gets away from
largest
concentration of
heat (top of the
cylinder or
combustion chamber). |
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