Why use race fuel?

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By Martin Saint, Stealth-Engineering.
Compiled with assistance from
and thanks to,
Dan Wade, Patriot Racing
Kelsey Manning, Torque-Line Cylinder Heads


Whether in the pit's, along a trail or in-fact anywhere that two-stroke riders gather you can bet that
sooner or later the talk will get to fuel, especially high octane race fuel.
( for the purposes of this article read fuel as gasoline )

Given the importance so many of us place on obtaining more power from our engine's, it is important
to understand what race fuel can and cannot do for us

One of the first way's to improve performance on a two-stoke engine is to increase the compression
ratio, but this is not without it's pitfall's, mainly detonation. Make no mistake, detonation is an engine
killer, destroying piston's, ring's and bearing's. To understand detonation we have to look at the
combustion process.

When a spark plug ignites the mixture in the combustion chamber it starts to burn, it does not
explode. It burns at a constant rate around 40-50 cm/second. Expanding outwards from the
plug, the burning mixture increases the pressure in the cylinder, pushing the piston down the
bore. The pressure increase is fairly constant at around 20-30 psi per degree of crank rotation.
In front of the burning mixture is a pressure wave that travels much faster ( around 300 meter's/
second ) This pressure wave rebounds off the chamber wall's and the piston crown. As it rebounds
it can become amplified due to the combining of the initial wave and the rebound wave. Due to
higher temperature's and pressure's within a "high compression" engine, this pressure wave
can become strong enough to ignite the mixture ahead of the normal burning flame front, usually
around the edges of the combustion chamber( generally called the end gases ) This leads to
even higher pressure's and temperature's and we get even more detonation that will wreck an
engine quickly.
Basically detonation is uncontrolled burning of the mixture, as opposed to the controlled burn
that we require.

So how do we avoid detonation? We choose a fuel that has a higher resistance to detonation,
called it's octane rating.

The first myth to dispel is that high octane fuel has more power. All gasoline fuel has roughly
the same amount of "chemical power"
So high octane fuel is not some magical elixir that will give an engine more power on it's own.

The octane rating of a fuel is only it's measured ability to resist detonation, as compared to a
reference fuel.

Octane is not the only consideration in choosing a fuel, but it's a good place to start

Octane ratings

An what to

Do with them

As stated before the octane rating of a fuel is a measure of it's resistance to detonation.

When you start to investigate octane rating's, the first thing you find is that two or three
numbers are quoted.

First you get the research octane rating. (RON)

Next is the motor octane number. ( MON )

Finally you might get the antiknock index.

When fuel's are assessed as to there octane rating, they are tested using a variable
compression ratio engine called the Co-operative Fuel's Research Engine. Two tests
are carried out to determine the detonation or "knock" resistance of the fuel.

The first test to determine the RON rating of the fuel is done at low rpm's and cool conditions,
this is not realistic for use in a high revving two-stroke

The second test to determine the MON is more realistic as it is carried out under higher rpm's,
higher temperature's and grater load's.

The final number, often quoted is the "antiknock index." This is the average of both RON and
MON ( RON + MON /2 ) Make sure when assessing octane number's that you know which
number is being quoted.

Now that you have found the numbers, what relation do they have with the engine?

The main factors that determine the octane requirement for a two-stroke engine are:-

1, Compression ratio.

2, Mean squish velocity.

3, Ignition timing.  

4, Combustion chamber shape.

5, Scavenge and trapping efficiencies.  

The main factors that can influence octane rating's

1, Ambient temperature and the intake temperature.

2, Air/fuel ratio, lean jetting can increase the chances of detonation.

3, Cooling efficiency.

4, Spark plug heat range.


The easiest way for an engine builder to determine what engine set-up you need is
for you to decide on what fuel you want to run first, rather than trying to find a fuel that
matches your set-up.

One big determining factor is related to whether you are going to race or not. If the
answer is yes you are going to race, then what constraints do the race organiser's
have on the fuel you can use. Its no use building an engine to run on very high octane
fuel, if the legal fuel will not cope in your engine.

Before you decide on a fuel there are a few more things to consider, not all high
octane fuel is the same. Just because two fuel's have similar octane rating's does
not mean that they will act the same way. Fuel's are blended for specific purposes,
so just looking at the octane rating will not give you the full picture.

Other reasons to choose a fuel

So you have decided that you want to run a high octane fuel, but which one. A lot of
companies make race fuel in various octane's , so how do you choose wisely.

The first question is what fuel can you legally run if you are racing. Then there is the
question of how deep are you pockets? Make no mistake race fuel is expensive and
generally the higher the octane the more it will cost. When you have decided on what
octane rating you can use, you then need to look at other aspects of the fuel to determine
which manufacturer and which of there products.

Gasoline is a blend of many different chemical's, all of which boil at different temperature's  
This is graphed out into what is known as the fuel's distillation curve. The boiling temperatures
are usually given for the initial boiling point and then the 10%, 50%, 90% and end point's.
The important points for a two stoke are the initial boiling point and the 90% and end point's.

The initial boiling and 10% points let you know how quick the fuel will vaporise to start with, to
low and vapour lock can be a problem, to high a starting can be a problem.

The 90% and end points are very important for a two-stroke as they tend to rev higher than
most four-stokes, this obviously means there is less time for the fuel to burn. Link this with the
fact that two-strokes have a shorter power stroke ( due to the exhaust port ) then the fuel has
to boil quickly at its 90% and end points or power is lost down the exhaust port. Some race
fuel has a distinct tail on it, meaning the gap between the 90% and end points is far to big for
our uses. If the fuel does not vaporise it will not burn properly and we loose power.
Different fuel's have different specific gravity's, that is how much they weigh in comparison to
distilled water. I know you are thinking why do I need to know that? The density contributes to
you air/fuel ratio. The air/fuel ratio is measured in weight rather than volume, so if you change
the density you change the ratio.

This has been written in a very simple way, so you do not have to have a grasp of chemical
engineering that would be necessary to explain all the why's and wherefore's of fuel selection.

The best thing you can do is talk to the fuel companies and tell them what your needs are,
tell them what average altitude you ride at, the ambient temperature and what you expect
from there fuel ( realistically ) they should be able to guide you to the best fuel for the job.

To Finnish here are a few commonly heard misconceptions

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Race Fuel Myth's

Race fuel burns slower than pump gas:- NOT TRUE, the burning speed of all gasoline's are
fairly constant, the burn rate has more to do with the distillation curve and the turbulence in the
combustion chamber. So its not a generalization you can make.

Race fuel is expensive so I'll just add octane booster to pump gas:- BIG MISTAKE, first pump
fuel is notoriously inconsistent in its blend, often changing weekly if not daily. Different manufacturer's
mix fuel in different ways. If you travel across the country to ride then fuel can and does change
between area's. therefore you have no baseline to work from. How much octane booster will you need?
You will never know. Add to this the fact that octane booster's will only increase the octane by
small amounts, no matter how much you add, you will get problems.

OK I'll use avgas:-   On your head be it, Avgas is designed for high altitude constant running
engines as found in aircraft. The distillation curve is all wrong for a two-stroke and can give
rise to poor throttle response and bad starting. This does not mean you can not use it, but its
not the best bet for two-strokes.

Race fuel is more dangerous:- All gasoline based fuels are dangerous, highly volatile and poisonous.
Treat all fuel's with the respect they deserve. Handle carefully, avoid spills and minimize you exposure
to the vapors.
Race fuel will make me quicker:- NOT TRUE, just adding high octane fuel to your stock engine
will not make more power, an engine need's to be set up to gain the benefits and it will just hurt
your pockets

Octane ratings An what to Do with them
As stated before the octane rating of a fuel is a measure of it's resistance to detonation.
When you start to investigate octane rating's, the first thing you find is that two or three numbers
are quoted.
First you get the research octane rating. (RON)
Next is the motor octane number. ( MON )
Finally you might get the antiknock index.

When fuel's are assessed as to there octane rating, they are tested using a variable compression
ratio engine called the Co-operative Fuel's Research Engine. Two tests are carried out to determine
the detonation or "knock" resistance of the fuel.
The first test to determine the RON rating of the fuel is done at low rpm's and cool conditions, this is
not realistic for use in a high revving two-stroke
The second test to determine the MON is more realistic as it is carried out under higher rpm's, higher
temperature's and grater load's.
The final number, often quoted is the "antiknock index." This is the average of both RON and MON
( RON + MON /2 ) Make sure when assessing octane number's that you know which number is being
quoted.
Now that you have found the numbers, what relation do they have with the engine?
The main factors that determine the octane requirement for a two-stroke engine are:-
1, Compression ratio.

2, Mean squish velocity.

3, Ignition timing.  

4, Combustion chamber shape.

5, Scavenge and trapping efficiencies.  

The main factors that can influence octane rating's
1, Ambient temperature and the intake temperature.

2, Air/fuel ratio, lean jetting can increase the chances of detonation.

3, Cooling efficiency.

4, Spark plug heat range.
The easiest way for an engine builder to determine what engine set-up you need is for you to
decide on what fuel you want to run first, rather than trying to find a fuel that matches your set-up.
One big determining factor is related to whether you are going to race or not. If the answer is yes
you are going to race, then what constraints do the race organizer's have on the fuel you can use.
Its no use building an engine to run on very high octane fuel, if the legal fuel will not cope in your engine.
Before you decide on a fuel there are a few more things to consider, not all high octane fuel is the same.
Just because two fuel's have similar octane rating's does not mean that they will act the same way.
Fuel's are blended for specific purposes, so just looking at the octane rating will not give you the full
picture.