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How Much Octane?
Street-legal racers are always looking
for better performance; 100-octane
gasoline is one answer
By Tim Wusz
Reprinted with permission from National Dragster
February 7, 1997
When thinking of octane numbers, most
automotive enthusiasts subscribe to “More’s Law,” which is: “If some’s good,
more’s better.” This is frequently the case high-octane race fuel for street
legal drag racing cars. So, what is octane, why is it needed, and how can it
help a street machine make an occasional great pass?
The engines in many street-legal cars have
been modified to improve performance. These modifications, which include
aftermarket headers, carb/intake manifold, camshaft, rockers, fuel-injection
system, higher compression, and modified cylinder heads with big valves, can
lead to increased horsepower as a result of getting a greater amount of the
proper air/fuel mixture into the engine. When more of the proper air/fuel
mixture is in the combustion chamber, the cylinder pressure is greater than
it would be in an unmodified engine. This increased cylinder pressure needs
more octane. If the octane number of the gasoline is not high enough, it can
result in detonation and destruction of the engine.
Even some unmodified engines may benefit
from higher octane gasoline. As under-hood temperatures increase, the engine
needs a higher octane gasoline. In a dry climate like much of the Southwest,
the lower humidty also contriubtes to increased demand for octane. On a
low-humidity, 100-degree day with the air conditioner on, an engine's octane
appetite can increase by three to four numbers.
76 Unleaded Racing Gasoline is legal for
street use and will not harm catalytic converters or oxygen sensors. It
provides street engines that require high octane the opportunity to operate
at maximum output with no damage from detonation. This gasoline can be used
in 1960s muscle cars as well as current performance cars and has been used
successfully in four-inch bore engines with aluminum heads and compression
ratios up to 12.5:1. Bigger bore engines may need to use lower compression
ratios.
The benefit to using a 100-octane gasoline
in high-performance street engines goes beyond the octane number. The
distillation curve of the gasoline defines the temperature at which various
amounts of gasoline are evaporated. Gasoline must be in a vapor form to
burn, so the more readily it vaporizes (within limits), the better it will
work in your engine. 76 Unleaded Racing Gasoline has a low 90-percent point
compared with street gasoline. This can be a performance benefit in some
engines because of the ability of the gasoline to evaporate more readily,
contributing to more complete burning, which in turn leads to greater
efficiency and power.
DEFINING OCTANE NUMBER
What is octane? How can it help my engine? two laboratory octane numbers
determine the overall octane quality of a gasoline. Both numbers are
determined in single-cylinder, variable-compression-ratio engines. The first
is the research octane number (RON), where operating conditions are mild.
This is the number that gas stations in the 1960s put on their advertising
signs out in front, numbers that ranged from a little over 100 to as high as
104. If an engine is detonating (pinging) at part throttle, it usually needs
more RON.
The second laboratory octane number is also
determined in a single-cylinder, variable-compression-ratio engine and is
called motor octane number (MON). The MON test is conducted under more
severe operating conditions (higher rpm and higher inlet temperature) than
the RON test, and as a result, the number is lower. If an engine is
detonating at wide-open throttle, a higher MON will usually satisfy it.
The number shown on the black and yellow
sticker on the service station gas pump is the average of the RON and MON,
or antiknock index (AKI). If the sticker shows an AKI of 92, RON is
typically between 96 and 97, and MON is typically between 87 and 88. The RON
and MON can vary slightly but still must average a minimum of 92 to
have a pump posting of 92. Higher octane unleaded fuels will have a greater
difference between RON and MON. For instance, 76 Unleaded Racing Gasoline
has a RON of 106 to 107 and a MON of 94 to 95, which gives an AKI of 100 to
102.
Higher octane does not necessarily mean
more power. If a racer's engine is detonating on 92 octane gasoline, using a
higher-octane quality fuel is definitely a plus. If your engine is
detonation-free on 92 octane gasoline, the benefit of using a higher octane
product is limited to the distillation characteristics discussed earlier.
OCTANE REQUIREMENT
The octane requirement of an engine in a vehicle can be measured on a
chassis dynamometer with a standard series of reference fuels. Racers are
not expected to conduct these tests but must be able to recognize when they
need a higher-octane number gasoline. Keep in mind that higher octane does
not mean more power unless your engine is experiencing detonation. If racers
are not sure if detonation exists, try a higher octane gasoline and see if
their speeds and e.t.s improve. If they do improve, probably the engine was
experiencing detonation.
CONTRIBUTING FACTORS
Many factors significantly increase an engine's octane requirement. They
include a higher compression ratio, a bigger cylinder bore, leaner mixtures,
higher coolant and intake-air temperatures, dry air (low humidity), higher
barometric pressures, altitude closer to sea level, and more spark advance.
Also, cast-iron heads need more octane than aluminum heads.
WHAT IS DETONATION?
The octane number of a gasoline is a measure of its resistance to
detonation. Detonation occurs when the octane number is too low for the
engine and its operating conditions. When the spark plug fires, the flame
moves through the air/fuel mixture, burning it very rapidly. Detonation
occurs if a portion of the unburned air/fuel mixture gets raised to a
temperature and pressure it cannot tolerate and ignites before the flame
front gets to it. Detonation causes the maximum pressure in the combustion
chamber to be reached before the piston reaches top dead center and pushes
down the piston before it has reaches the top of its travel. Much of the
gasoline's energy is wasted in trying to move the piston up while the
high-pressure gasses are trying to push it down. The extreme temperature and
pressure developed can cause broken rings, rod-bearing damage, piston
overheating, and erosion of the aluminum. Pistons sometimes end up with
holes in their tops from the high temperatures and high pressures.
THE GASOLINE FOR RACING
76 Unleaded Racing Gasoline is a clean burning, street-legal racing fuel
with a minimum-octane number (AKI) of 100, designed for high-performance and
speciality vehicles. It is the highest octane street-legal racing fuel
available. It contains a complete additive package approved by Environmental
Protection Agency and California Air Resource Board that keeps carburetors,
fuel injectors, and intake valves clean. 76 Unleaded Racing Gasoline can be
blended with motor gasolines with no adverse effects. It blends linearly
which means that if five gallons of the product are mixed with five gallons
of 92-octane gasoline, the end result is 10 gallons of 96-octane gasoline.
Use the custom blending
chart to economically produce the octane number that your vehicle
requires. For example, if you want 98 octane and your street gasoline is 92
octane, look at the upper half of the chart (for 92 unleaded). The number 98
appears in the body of the table in several places, one being where the
vertical column indicating six gallons of 100-octane unleaded intersects
with the line that indicates two gallons of 92 octane. Therefore, if you mix
six gallons of 100-octan unleaded with two gallons of 92-octane unleaded,
the result will be 98-octane unleaded gasoline. If your street gasoline is
93 octane, use the same procedure with the lower half of the table.
KNOCK-SENSOR-EQUIPPED VEHICLES
In the late 1970s, the automakers began using knock sensors on engines.
When the knock sensor senses detonation, it sends a signal to the main
computer, which retards the spark timing until detonation ceases. By the
early- to mid-1990s, knock-sensor systems were becoming much more
sophisticated and could be found on about 50 percent of all new cars and
trucks. The number increases slightly each year. The best thing about a
knock-sensor system is its ability to reduce or eliminate engine damage due
to detonation. It can, however, cause spark-timing retard, which reduces
horsepower and fuel economy. Still, the trade-off can save lots of money and
is a good compromise. Detonation normally occurs in one or sometimes two
cylinders unless a serious fuel-starvation problem exists. Most
engine-management systems reduce the spark timing in all cylinders, although
some reduce the timing in only the cylinder that is detonating. Using a
gasoline high enough in octane to satisfy the engine under all types of
condtions will ensure maximum performance from all cylinders.
EXHAUST-VALVE-SEAT CONSIDERATIONS
In the 1960s, all gasoline contained tetraethyl lead, which was an easy
way to increase the octane number. The lead oxide from the burned tetraethyl
lead left a coating on the exhaust valve seat that provided protection from
wear. If racers are using an older engine that does not have hardened
exhaust valve seats, they should use an aftermarket lead substitute. If they
are getting ready to do a valve job on that 1960s car, they should have
hardened seats installed so they don’t have to mess with additives.
LEADED RACING GASOLINE
Using leaded racing gasoline on the street is illegal. The user and the
seller can both be fined up to $10,000 each.
For the 76 Racing Gasoline distributor
nearest you, call (800) 345-0076. Or click here
for our list
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