Contents of the FUEL.TXT file
The fuel we use is one of the most important links in the
performance chain, yet it is surrounded by myth and
misinformation. It's obvious that octane is important, but
what is octane, and how much is too little or even too
A couple of fundamental concepts are necessary to explain
this mess. In simple terms the octane number you see at
the pump is the average of two octane numbers; the
Research Octane Number (RON) and the Motor Octane Number
Each of these octane numbers is determined by standard
laboratory tests, but the details of the tests are not as
important as what they mean in terms of performance. Low
to medium-speed knock characteristics are determined by
the Research (RON) method, while high-speed and partial
throttle heavy load knock characteristics are determined
by the Motor (MON) method. MON testing is conducted under
more stringent conditions than RON testing, so the MON
number tends to be lower but also more valid for
Keep in mind that octane number is simply a measure of the
anti- knock characteristics of a given fuel. Knock can
come from a couple of sources most often called detonation
and pre-ignition. Simply put detonation is the
uncontrolled burning of the fuel in the combustion
chamber. Pre-ignition can be defined as the starting of
the burning process by any source other than the spark
plug, either before or after the plug has fired.
To truly understand what detonation is, its important to
understand that if you raise the temperature of any
combustible mixture high enough, it will ignite on its
own. This is sometimes called the "spontaneous combustion
point" or the "auto ignition temperature".
Following the ignition process through a cycle should help
complete the explanation. During the combustion cycle the
spark plug fires somewhere before the piston reaches Top
Dead Center (TDC) and starts burning the compressed
mixture in the cylinder and as a consequence raises the
combustion chamber temperature. While this burning is
taking place, the piston is still rising and still
compressing the air/fuel mixture to raise the cylinder
pressure as well as the cylinder temperature.
At this point, the pressure rise in the cylinder is very
rapid. The unburned mixture (end gases) at the edges of
the combustion is being raised to extremely high
temperatures. Ideally, the burning of the mixture will be
completed before any of these end gases have an
opportunity to reach the point of auto ignition.
As we all know, this isn't always the case. If the
combination of pressure and temperature is to great for
the fuel being used, auto ignition of these end gases
begins a second flame front which collides with the first
flame front started by the spark plug. That causes an
incredibly rapid increase in pressure which produces the
pinging sound that signals the death of your engine is
near. That is detonation.
We defined pre-ignition previously as the starting of the
burning process by a source other than the plug. Well,
technically, that's what auto-ignition is, a source (HIGH
temperature) other than the spark plug that starts the
burning process. Keep in mind that any time you raise the
pressure in the cylinder you get a corresponding rise in
temperature, or conversely raising the temperature also
raises the pressure. Usually , we think of pre-ignition as
a hot spot in the combustion chamber that causes the
mixture to start burning before the plug fires. Common
causes for pre-ignition are carbon build up and spark
plugs of too high a heat range. Any time the engine
produces more heat than it can get rid of, it is a prime
candidate for both detonation and pre-ignition.
Therefore, pre-ignition starts the mixture burning before
the plug fires. That can lead to detonation but doesn't
always. Detonation raises the combustion pressure which
raises the temperature of the surrounding parts, and
that's what leads to pre-ignition. Or simply remember that
combustion temperature increases are more likely to lead
to pre-ignition than increases in combustion pressure
which tend to lead to detonation.
As you may have guessed from the earlier discussion of
octane numbers, high octane fuels have a considerably
higher auto ignition temperature along with a slower
burning rate to aid in controlling the rate of combustion.
All of these things help keep abnormal combustion from
rearing it's ugly head.
How much octane do we need? Well, just enough, not much
more, and certainly not much less. If this seems a little
vague, read on and you will soon understand why this is
the only answer that can make much sense.
Every engine can have radically different requirements.
Even two similarly modified engines can have requirements
as different as 5 to 8 MON numbers in some cases. The
factors affecting octane requirements should be of great
interest to every racer. By changing these factors around
you can raise or lower the octane requirement of your
engine. Some of the more obvious factors are :
D E S I G N F A C T O R S
Intake air temperature
Combustion chamber shape
Cooling system efficiency
Spark plug location
Spark plug heat range
O P E R A T I N G C O N D I T I O N S
Outside air temperature
In looking at this list, it should be apparent to you that
a number of these factors/conditions are pretty much out
of our control. We will concentrate on those that are more
Is it better to raise fuel octane or lower the octane
requirement of the engine? The answer to that question is
yes. You want to lower the octane requirement of the
engine as much as possible without lowering engine
performance. You also want to use a fuel with an octane
rating just high enough to keep your engine from ever
Engine timing is one of the factors on the list, but on
most modern engines the timing is fixed or electronically
controlled. You have more to lose than gain if you play
with the timing of these ignitions, it's best to leave
Premix ratio has real potential as a way to vary the
octane requirement of an engine. A simple rule of thumb is
that more in the mix requires more octane. By switching
from a 20:1 mixture to a 50:1 mixture, you will lower the
octane requirement of the engine dramatically.
Another way to scare off detonation is to make sure the
engine's cooling system works as efficiently as possible.
That means clean radiators at all times. Find a way to
measure the water temperature of your engine. It's
important to keep the water temperature between 50 and 70
degrees Centigrade. This range produces the most power
while keeping the octane requirement low.
Most two-cycle engines have a squish band machined at the
outer edge of the cylinder head. Unfortunately, on most
stock engines that squish band doesn't serve much purpose.
In theory squish bands work very well, but production line
tolerances leave squish clearances so great that the only
thing being squished is horsepower. Cutting the cylinder
head to bring the squish clearance into spec while still
retaining the original cylinder head shape and volume is
not the easiest thing to do, though it will definitely pay
dividends in horsepower gained and octane requirements
For the truly adventurous, it is possible to use two spark
plugs per cylinder to lower your engine's octane needs.
Adding a second spark plug shortens the fuel burn time and
decreases the distance the flame front needs to travel.
Without a doubt, the single most important thing you can
do to lower the octane requirement and save your engine
some self destruction via the detonation express is to
LEARN TO JET! Read the jetting article from a few months
back, ask friends you can trust or look over someone's
shoulder - whatever you have to do, do it. There is more
hidden horsepower in jetting than in any bolt-on item you
This all brings us back to the octane question : How much
do you need. Well, we still don't have an answer for that,
but here are a few suggestions.
Start simple and work your way up. Try a good grade of
premium gas. Most well modified normally aspirated engines
can run on 93-95 octane gasoline. Good porting with flow
matched transfer ports can significantly lower octane
requirement on two cycle engines. If your engine detonates
on premium, try one (or all) of the measures to lower the
octane requirement of the engine. If all these measures
fail, try mixing a little bit of good quality race gas
(Power Mist, UNION 76, etc..) with your gasoline.
Four-parts premium to one part racing gas should be a good
cost effective starting point.
Keep in mind that octane requirement is lower at high
altitude and high humidity. An engine that ran fine at
10,000 feet could very easily detonate at sea-level.
Remember you only need enough octane to stop detonation.
Too much more octane than that will cause sluggish
performance at high rpm. The higher your engine revs, the
more of a problem this can become. The same is true of
octane boosters, practice moderation. Most octane boosters
tend to melt paint and plastic. So avoid any octane
booster that comes in a plastic container, if it does the
chances are it won't be effective.
Just a few last points. Stay away from voodoo stuff like
toluene, benzene or anything with alcohol including
gasahol. Most of that stuff is toxic at worst and stupid
at best. There is nothing magical about lead either. In
most cases you can ignore the lead content of your gas.
Oil companies only used it because it was cheap and
effective. Aviation gas is too involved a subject for this
article, but for the time being, it is best to steer clear
of it as well.
So if you pay attention and experiment, you will be able
to beat the guy who races on bad advice and a prayer.