numbers can be very confusing due to different
terminologies used in discussion. I will
try to clear up some common misunderstandings
and define some of the various terms used
when discussing octane quality.
octane quality of a gasoline is its ability
to resist detonation, a form of abnormal
combustion. Detonation occurs when the air-fuel
mixture reaches a temperature and/or pressure
at which it can no longer keep from self
igniting. Two types of abnormal combustion
are common: the first is detonation as previously
mentioned and the other is preignition.
occurs after the sparkplug has ignited the
air-fuel mixture and the flame front is
moving through the combustion chamber. If,
during this burning process, the unburned
air-fuel mixture reaches a temperature and/or
pressure at which it is no longer stable,
it burns very rapidly causing a new flame
front to collide with the one that originated
at the sparkplug. Maximum pressure in the
cylinder occurs before the piston reaches
Top Dead Center (TDC) and that pressure
tries to push the piston down before it
is ready to go down. Piston burning and
rod bearing damage are the result as well
as loss of power.
is the other bad actor and is usually started
by a hot spot in the combustion chamber
which causes the mixture to ignite before
the plug fires. Under wide open throttle
conditions, preignition will destroy pistons
Octane Number (RON)
is determined in a single cylinder variable
compression ratio engine that operates at
600 RPM with a 125 �
air temperature at standard barometric pressure.
Spark advance is fixed at 13 �
In a real world engine, RON is necessary
to satisfy part throttle knock problems.
A good quality racing gasoline has a RON
in the range of 110 to 115. A high quality
racing gasoline can have a RON in excess
of 120 which is the top of the octane scale.
The difference in the spread of RON is not
very important to racing engines.
procedures and hardware for the RON test
were originally developed in 1931. The hardware
was revised in 1948 with procedural changes
made until the late 1960�s.
Octane Number (MON)
is determined in a single cylinder engine
similar to the RON engine with a few changes
that make operating conditions more severe
and therefore the octane numbers are lower.
The MON engine runs at 900 RPM with a 300
temperature. Spark advance varies with compression
ratio. In a real world engine, MON is necessary
to satisfy octane demands at wide open throttle.
This is a very important number for racing
engines since they spend a high percentage
of their lives under high speed and high
load conditions. Racing engines cannot afford
to be short on octane quality, since detonation
or preignition will quickly reduce a racing
engine to junk.
Motor Octane (MON) appetite
of an engine with 13:1 compression ratio
and a four inch bore varies with operating
conditions but is normally around 101. Good
quality racing gasolines have MON in the
range of 100 to 110. High quality racing
gasoline has MON in excess of 110. If your
engine requires a 101 MON, it is of no value
to use a gasoline that has a 115 MON. To
cover yourself for extreme conditions, it
is wise to have an octane cushion, but there
is no advantage to using a very high octane
quality product if you do not need it.
MON test was originally developed in 1932.
Major hardware changes were made in 1948
with procedural changes made until the late
is the average of RON and MON. It is sometimes
referred to as the AKI
or Anti-Knock Index . By
law this number must be posted on the dispensing
pump at retail outlets in most states. It
is the most commonly used octane reference
today. It came into use in the early 1970�s
as a compromise between RON and MON for
advertising purposes, and to keep from confusing
the consumer with too many different terms.
It has erroneously been referred to as Road
Octane Number (RdON).
Road Octane Number (RdON)
is derived from testing gasolines in real
world multi-cylinder engines, normally at
wide open throttle. It was developed in
the 1920�s and is still reliable today.
The original testing was done in cars on
the road but as technology developed the
testing was moved to chassis dynamometers.
This eliminated many variables. Some companies
have since built elaborate chassis dynamometers
with environmental controls to improve consistency.
Brand Racing Fuels
has modified this test additionally to use
it with racing engines on engine dynamometers.
This has given us the opportunity to evaluate
gasoline blends during our racing gasoline
development that had good RON and MON but
that did not respond well in the racing
engine under a full throttle excursion through
the entire RPM range. We felt these conditions
were the true indicators of how the fuel
could be best developed. In our program
we found that the gasoline blending components
and their ratios are far more important
to the racing engine response than high
RON and MON numbers. RON and MON can only
be used as a guide; the final work must
come from the Road Octane Number
an example, Rockett Brand 111
Racing Gasoline is the result
of testing over 100 experimental blends.
The final blend has a Road Octane
Number (RdON) of 111, the same
RdON as one of our competitor�s gasolines
that has advertised 116 RON and 116 MON.
Using only RON and MON can lure a person
into a false sense of security. If you want
to be certain that your racing gasoline
has been thoroughly tested in real world
racing engines, choose Rockett
Brand Racing Gasoline for
your engine. If you want high RON and MON
without knowing how it will work in your
racing engine, buy from the other guys.
Your Nearest Distributor, Call: 1-800-345-0076