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

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 generalisation 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 minimise you exposure 
to the vapours.
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 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.
 Martin Saint, Stealth-Engineering.