It all comes down to fuel. You can build the hottest, most
throw-down thumpin big block that ever existed, but its
gotta have good gas.
What is good gas anyway? What separates the killer stuff for
your NMCA Pro-Streeter from the slag your lawnmower barely runs
on? After consulting experts in the field, we decided to check
into the various street fuels available, various types and grades
of octane booster, aviation gasoline (AvGas), and racing fuels.
By comparing the different options available to you, it may be
easier to choose the best grade for your ride.
What is octane anyway? Octane is a measurement of a
fuels resistance to ignition. Ideally, the air/fuel mixture
will ignite at the proper time and burn smoothly through the
power stroke. The idea is, one powerful combustion of better
than several. randomly-ignited small flame fronts. When you can
precisely control the point at which the fuel will ignite, maximum
performance of the engine can be achieved, and power-robbing
knock and ping will be eliminated. Knock and ping are a result
of abnormal ignition, or multiple flame fronts colliding within
the combustion chamber during the compression stroke.
All reputable fuel manufacturers determine the octane rating
of their gasoline in the research lab using a special, dedicated
single cylinder engine. Comparing the gasoline to a series of
standard reference fuels in the test engine results in either
a research octane number (RON) or a motor octane number (MON)
depending on a set of operating conditions. The RON is determined
with the test engine operating at 600rpm, at standard barometric
pressure, and the intake air temperature set at 125 degrees Fahrenheit.
RON is primarily used to address part-throttle knock and ping
problems. The MON addresses wide open throttle operation and
is determined with the test engine spinning at 900rpm, also at
standard barometric pressure, and the intake air temperature
pumped up to 300 degrees.
The best predictor of a fuels performance in a street/strip
machine is the Anti-Knock Index (AKI). This is simply the average
of the RON and MON numbers, or (RON+MON)/2. Most all octane ratings
posted at the pumps are determined by this AKI formula, and are
the minimum values you could expect to see. The minimum octane
requirement of your engine is determined by several variables
besides the compression ratio. The engine and cylinder head configuration,
air/fuel mixture, timing, coolant temperature, atmospheric pressure,
relative humidity, and ambient air temperature will also affect
the octane required to make your mill produce maximum power.
The burn rate of a fuel is a measurement of the time required
for complete combustion of the air/fuel mixture. The notion that
octane ratings affect the burn rate of fuel is about 180-degrees
from reality. Burn rate is a function of several variables, and
the two are completely independent, although there is generally
a correlation between octane ratings and burn rates.
To give you a good example of this, we contacted Jim Wurth
from Sunoco Race Fuels. He explains, "A perfect example
is Sunoco Maximal, which is our fastest burning fuel, and coincidentally
one of Sunocos highest octane fuels at 116 (R+M)/2. A lot
of Pro Stock teams rely on Maximal for those sub-seven second
runs. When they are turning 9,000rpm or more, the fuel has to
burn pretty quickly to achieve complete combustion."
Octane boosters offer little help in the quest for higher
octane. Most popular street-legal octane boosters claim increases
in octane ratings up to five points, and those boosters intended
for off-road use only claim up to seven points. Thats a
lot of octane to hope for simply by pouring an additive in a
tank. Sunoco told us that before they launched their GT-100 Unleaded
retail pilot program, they wanted to be sure that a 100 (R+M)/2
octane street-legal fuel would be of value, and that enthusiasts
would not be able to get the same (or better) results using an
octane booster. Nine of the most popular retail octane boosters
were put through a series of tests to determine where the consumer
could get the most bang for the buck. The test results were verified
by an independent testing facility, using several brands of regular
unleaded and premium gasolines, just to make sure everything
According to Mark Borosky, Vehicle Test Engineer for Sunoco,
"Of the nine octane boosters tested, none showed a significant
increase, and one actually lowered the octane number of the test
gasolines." Testing repeatedly showed a maximum increase
in octane of 3.5 points by only two of the six street-legal octane
boosters when the recommended treatment rate was blended with
lower base 87-octane gasoline. The best the remaining four products
could muster was less than a one point increase. "While
clearly no one would actually use an octane booster in a low
base octane fuel, we wanted to give the manufacturers the benefit
of the doubt relative to their claims of five-to-seven point
increases" explained Borosky.
When tests were performed using 98 and 94-octane fuel, even
the two best products from the previous tests produced a disappointing
1.5 to 2 point maximum increase. The remaining four street-legal
octane boosters showed less than a .5 point increase. Those products
designated for off-road use only didnt fare any better
than the street-legal products. Subsequent tests where the dosage
of octane booster was doubled, tripled, and even quadrupled produced
only minimal improvements in octane, regardless of the base octane
hum-ber of the test gas. In fact, quadrupling the treatment rate
of the most powerful additive produced only a 3.5 point increase
in octane when added to 98 premium, resulting in a cost of $3.25
An alternative path to octane euphoria is to blend gasolines
of different octane levels yourself. Its easier than you
may think, safe, and the results are predictable. The formula
for mixing gasolines of the same type is pretty straightforward.
When you mix a 50/50 blend of two unleaded fuels, simply average
the two octane ratings to determine whats in the tank.
If you mix 94 and 100, you get 97. The same generally holds true
for leaded gasolines, assuming the lead content is nearly equal.
Blending a leaded fuel with unleaded, however, pushes the
octane up a bit more than the math would suggest, due to the
effect of the lead. Just a gram or two of lead blended into the
unleaded fuel will raise the octane number significantly. Commercial
leaded racing fuels contain anywhere from a trace to six grams
of lead per gallon. If you were to mix 50 percent 110 octane
leaded fuel with 100 octane unleaded, you would actually end
up with an octane number around 106 to 107. Keep in mind that
even the smallest amount of lead or leaded gasoline with unleaded,
could spell the end of your catalytic converter or oxygen sensor.
The same holds true for using octane boosters intended for off-road
use only. A word to the wise, check for any lead content in all
the additives you might mix with your unleaded gasoline. And
check with your state emissions regulations for street use.
We asked Sunocos Wurth about using aviation fuel in
an automobile engine. He was emphatic when he said, "Dont
do it. Even though Sunoco is a major producer of aviation fuel,
this fuel is specifically blended for aircraft engines. Aircraft
operate under very different conditions than automobiles, and
the fuel requirements are quite different as well. Aircraft engines
generally use very small pistons and run within
a very narrow rpm range. Theres no need for transient throttle
response in an airplane because after the pilot does the initial
engine run-up, the throttle is set in one position and the rpm
doesnt normally change until landing. Also, airplanes fly
where the air is cold and thin, and the atmospheric pressure
is low. These are not even close to the conditions your street
machine will see on the ground. Also, since most piston-driven
aircraft cruise at 3,000rpm or so, the burn rate of aviation
gas is much too slow for any high-performance automotive applications."
What is it that makes race gas so different? Whats it
made of? Sunoco tells us their GT PLUS 104 octane unleaded race
gas is only 15-20 percent traditional gasoline, and about 85
percent additives! Actually there are about 120 different chemicals
in GT PLUS. One reason it isnt street legal is the high
oxygen content. The EPA requires that the oxygen content of a
street legal fuel cannot exceed 2.9 percent. GT PLUS is about
3.5 percent oxygen. This fuel is light in weight at only 6.14
1bs-per-gallon. The high oxygen content improves the octane,
and when the induction system is properly calibrated, this fuel
will help make additional horsepower. The high oxygen content
has a supercharging effect, since 3.5 percent oxygen is the equivalent
to about 17 percent more air. Different fuels can actually alter
horsepower 5-to-10 percent or more.
We wanted to know more about the different types of race gas
Sunoco had, and didnt realize there were five different
types of racing fuel alone.
GT-100 Unleaded, is a clear fuel with a pump octane
of 100, and will handle compression ratios of up to 12:1, and
is street legal in all 50 states.
GT PLUS, is also unleaded, and is rated at 104 octane.
It is suitable for compression ratios up to 14:1 and is colored
light blue. It will not harm oxygen sensors or knock sensors
in computer controlled engines. It is not street legal.
STANDARD, is a leaded fuel rated at 110 octane, is
colored purple and is intended for drag racing, road racing,
and race boats.
SUPREME, also a leaded fuel, rated at 112 octane, is
dark blue. It was developed to help resist vapor lock and meet
the demands of sportsman, modifieds, offshore powerboats, and
endurance racing where engines regularly run in excess of 7,000rpm.
MAXIMAL, we mentioned earlier, is colored red, has
116 octane, and is leaded. It is intended for exceptionally high
performance applications, like Pro Stock, where extremely high
cylinder pressures are common. Its extremely fast burn rate is
satisfactory where rpm exceeds 10,000.
Now that youre an expert on gasolines, you probably
would like to know where to buy and store the stuff. If you are
fortunate enough to live in the mid-Atlantic states, you can
take advantage of Sunocos GT-100 Unleaded retail pilot
program and get 100-octane race fuel at pumps located at select
service stations. The rest of us have to purchase from local
speed shops, at race tracks, or directly from Sunoco distributors.
When you plan on buying fuel in quantity, say a 55-gallon
drum, youll be happy to know that racing fuel has a shelf
life of about a year, if you store it properly. The container
must conform to all safety standards, and should be made from
metal or polymer. Make sure the container is opaque and solid
in color. The white plastic jugs we see at the track should be
used for short-term storage only. They let in sunlight, which
will affect the fuel The lead in leaded fuel and other chemicals
in unleaded fuel are photosensitive, and will dissipate if they
are exposed to the sun. Keep any container tightly sealed to
- Good gas is a great thing, and we learned a lot from the
Sunoco guys. They have been at this a long time (remember Mark
Donahues Camaro?) and we hope you learned as much as we
did. We couldnt talk them out of a tankful of the killer
stuff for Project X, but we had fun. Cmon guys! Its