Gasoline FAQ - Part 1 of 4
Octane is a measure of the antiknock quality of gasoline. The octane number
posted on service station dispensers and listed in car owner's manuals is an
average of two measurements: Research octane number and Motor octane number.
These octane numbers are determined using a laboratory test engine operated
at different temperatures and speeds to simulate different driving
conditions. The average of these two numbers, (R+M)/2, provides an
indication of on-road performance. Most car owner's manuals recommend a
certain octane but advise owners to increase octane if they detect audible
knock or unsatisfactory performance.
Many new vehicles are equipped with knock sensors to prevent engine damage
resulting from knock or to enhance performance. When engine knock is
detected, the knock sensor system retards ignition timing until knock is
reduced or eliminated. When ignition timing is retarded, power and
efficiency are reduced so drivers can experience poorer acceleration
performance and gas mileage. These vehicles require a higher-octane fuel to
provide optimum performance.
Sunoco has the distinction of offering a range of gasoline octane levels to
satisfy the vehicle population. Sunoco produced leaded gasolines in the
1970s such as Sunoco 190 with 89 to 90 octane and Sunoco 260 with 97.5 to
98.5 octane. Sunoco's current unleaded gasolines include Economy with an 86
octane and Ultra with 94 octane. In order to provide this high-octane fuel,
Sunoco has tested different octane enhancers over the years. These tests
included oxygenated compounds (methanol, tertiary butyl alcohol (TBA),
ethanol, methyl tertiary butyl ether (MTBE), isopropyl alcohol (IPA), ethyl
tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME)), metal-based
additives (lead, methylcyclopentadienyl manganese tricarbonyl (MMT)) and
Sunoco has produced leaded racing gasoline since late the 1970s and unleaded
racing gasoline since the mid-1980s. Typical properties for these fuels are
listed in Table 1. One of the unleaded racing gasolines, Sunoco GT100 is
available at select Sunoco retail stations.
Table 1: Sunoco Leaded and Unleaded Race Fuels
Reid vapor pressure
Initial boiling point
Final boiling point
From the Staging Light
Sounds of an unhealthy engine
Disclaimer: While based on facts about drag racing, this page contains the
personal views and opinions of one racer - Michael Beard
Three weeks ago at Beaver Springs Dragway, Beaver Springs, PA, we used a
higher octane fuel than the usual kind. Typically we use 93 Octane from
Coastal in our Duster drag car, but we decided to try the much-heralded
Sunoco Ultra, which is 94 Octane. The car ran exceptionally well that
weekend, and it seemed that our fuel consumption had decreased. It seemed
obvious that the higher octane provided the engine with more power and
better gas mileage. Hence, you had to wonder what octane was really all
First of all, what do the octane ratings mean? If you have ever been bored
enough while filling up your car at the local gas station, you may have
noticed the stickers on the fuel pumps. One of them indicates that the
method used for calculating the octane number is (RON+MON)/2. The RON refers
to the Research Octane Number, and MON to the Motor Octane Number. The two
numbers are derived from different test conditions. The RON method
represents normal mild driving conditions, while MON tests are done under
severe conditions and high engine speeds.
Sensitivity is the difference between the two octane ratings (RON - MON Sensitivity), and indicates how the fuel will respond under different
driving conditions. Gasoline in the United States is required to have a high
MON, thus keeping the Sensitivity number low. This is important because it
means the fuel will operate consistently despite changes in driving
conditions. Now, with some of the basics out of the way, we can ask some
more interesting questions about octane.
The formula (RON+MON)/2 is referred to as the anti-knock index. This leads
us to ask exactly what knock is. A good description comes from E.F. Obert,
in "Internal Combustion Engines and Air Pollution" (1973, Harper & Row).
"During the compression stroke of a spark ignited engine, the pressure,
temperature, and density of the mixture are increased and, depending on the
fuel, chemical reactions ... may begin. The spark ignites the mixture, then
the flame travels across the combustion chamber at a more or less orderly
pace with the pressure rising uniformly throughout the chamber. Ahead of the
flame front, the unburned mixture ... is compressed by the rising pressure,
with an accompanying rise in temperature and density. ... If the ignition
delay (chemical) of the end gas is consumed before the flame arrives,
autoignition takes place. With autoignition, the orderly process becomes
uncontrolled and a violent rise in pressure may occur. Energy may be
liberated at such a rate such that the walls of the chamber ... vibrate, and
knock is said to be present."
The octane rating is the measure of the fuel's resistance to autoignition.
When autoignition occurs the gas pressure wave it causes superimposes on the
normal pressure wave of the combustion chamber. These two waves interact to
create a third sawtooth-shaped wave pattern of pressure oscillations. The
pressure oscillations create the knocking sound. Pressure waves caused by
knock can build up quickly and shorten the life span of an engine.
Knock can be reduced by adding chemicals to the fuel. Common examples are
tetra ethyl lead, aromatics, and oxygenates. While these substances increase
the octane rating, their composition does not contribute to the energy of
the fuel when it is burned. The result may be less energy per unit volume of
fuel used, and thus less efficiency. In short, higher octane fuels may
require more fuel to be burnt in order to produce the same amount of energy.
But wait! Didn't I say at the beginning that we experienced a gain in power
and fuel efficiency?
There is one piece of information we have been missing. The compression
ratio of an engine has a lot to do with knock and the apparent effects of
octane. Compression ratio can be thought of as the pressure in the
combustion chamber. High compression ratio engines have more of a tendency
to knock than lower compression ones. Given what we know, since a low
compression engine is not as likely to knock, it does not require a fuel
with as high of an octane rating. A high compression motor, however, needs
more octane to reduce knock. In such a case, switching to a higher octane
fuel would reduce the effects of knock, leading to a more efficient
combustion process. This creates more power and better fuel economy. Using a
higher octane fuel in an engine that does not experience knock will not help
performance. In fact, if the higher octane was achieved through the use of
oxygenates, higher octane than necessary may actually hinder the performance
of an engine!
The goal, then, is to find a fuel with an octane rating that is high enough
to prevent knock, but not much higher. It is possible that the tiny
performance boost we experienced with the drag car is related to the use of
the higher octane fuel. The compression ratio of our motor may be a
borderline case where it is able to use the slightly higher octane. Sunoco
116 would not help our relatively low compression motor any more than the
Sunoco 94, but it is a necessity in most of today's extremely high
compression race engines. In fact, at over $4.50 per gallon, Sunoco 116
would do nothing more than drain your wallet faster! Technology is not
WARNING: Use extreme caution when attempting to use a fuel with a lower
octane! If the octane you use is too low, knock will occur, and your engine
can be damaged. Follow your manufacturer's guidelines as to what octane is
suitable for your vehicle.