............................................................................................................................2

Detonation...............................................................................................................................................................2

Pre-Ignition..............................................................................................................................................................2

Motor Octane Rating ...............................................................................................................................................2

Research Octane Number ......................................................................................................................................3

(R+M)/2 Method ......................................................................................................................................................3

Specific Gravity .......................................................................................................................................................3

Reid Vapor Pressure...............................................................................................................................................3

Aviation Gasoline ....................................................................................................................................................3

Alcohol........................................................................................................... 4
Types of Alcohol......................................................................................................................................................4

Air-to-Alcohol Ratio .................................................................................................................................................4

Cost.........................................................................................................................................................................4

Race Fuel Specifications................................................................................... 5
Racing Gasoline......................................................................................................................................................5

Fuel System Plumbing Recommendations............................................................ 6
Carbureted Gasoline Engines.................................................................................................................................6

Fuel Injector Selection...................................................................................... 7
Brake Specific Fuel Consumption (BSFC)..............................................................................................................7

Injector Sizing..........................................................................................................................................................7

Fuel Pressure..........................................................................................................................................................7

Injector Flow Rates At Various Pressures ..............................................................................................................7

Horsepower Support ...............................................................................................................................................7

Estimated Injector Size by Airflow...........................................................................................................................8

Impedance ..............................................................................................................................................................8
2
About Gasoline

Source:
www.grapeaperacing.com
Octane Rating

Octane is a rating of a fuels resistance to
ignite. The higher the octane, the harder it is to
ignite. A higher octane may be necessary to prevent
pre-ignition and detonation in a high performance
engine.
A higher octane fuel will generally burn
slightly slower than a lower octane fuel which could
require a change in ignition timing. Using a higher
octane fuel than you need will not help power, the
slower burn rate can actually cause you to lose
some power.
To get technical, octane is actually a
hydrocarbon with 8 carbon atoms chained together.
If your gasoline were made up of 100% octane, it
would have an octane rating of 100. We use octane
as the rating because it is the hydrocarbon in
gasoline that resists detonation and pre-ignition the
best.
On the other end of the scale, we have
heptane, a hydrocarbon that has 7 carbon atoms
chained together, which resists detonation and pre-
ignition very poorly. It does not take much
compression at all to get it to spontaneously ignite.
A fuel made up of 100% heptane has an octane
rating of 0. Gasoline with an octane rating of 87,
means that it is made up of 87% octane and 13%
heptane or a combination of fuels that give the same
anti-knock characteristics.
So if pure octane yields a 100 octane rating,
how can we get a higher octane than 100%? Well,
we cant, but certain additives can make a fuel resist
detonation and pre-ignition better than pure octane.
A chemical called tetraethyl lead was very popular in
the past to boost octane ratings. It has been banned
for use in automotive gasoline due to pollution.
Leaded fuels are not compatible with catalytic
converters or oxygen sensors found on all cars
today.
A common additive these days is MTBE
(Methyl Tertiary Butyl Ether). This chemical boosts
octane ratings and it is also an oxygenate. Being an
oxygenate, it reduces the amount of unburned
hydrocarbons and carbon monoxide in the exhaust.
It is not important to know exactly what the
chemicals are that the boost octane rating of
gasoline. What is important is that you understand
that they increase the octane rating, and not actual
octane.

Detonation

Normal combustion will take place at a
pretty steady rate (for a given rpm and load). When
a large amount of the charge burns extremely fast
and uncontrollably, it is known as detonation.

Detonation can destroy an engine in a matter of
seconds.
It is common knowledge that heat causes
detonation, but heat is not the only cause. Pressure
plays a big role in it also. You can get detonation by
running with too much ignition advance; if heat was
the only cause, signs of detonation would around the
exhaust valve, but this is not the case. Usually
when detonation takes place from too much
advance, the detonation occurs on the intake side of
the chamber, which is the coolest side of the
chamber. This happens because detonation did not
occur until the pressure got excessive, which was
after the spark. By the time that happened, the
charge near the exhaust valve has already been
burned. Most of the time, detonation will occur after
normal combustion has started.
Apart for destroying pistons and spark plugs,
light detonation can cause all sorts of other
problems, like fatiguing cranks and rods quickly and
pounding bearings to death, so avoid detonation at
all costs.

Pre-Ignition

When the charge ignites before the spark,
it's called pre-ignition. This can happen with or
without detonation, but usually will cause detonation
in a high performance engine. Hot spots in the
combustion chamber are the usual cause of pre-
ignition. A spark plug with a high heat range can
cause a hot spot.
When pre-ignition occurs, the charge begins
to burn, When the ignition system fires, the result is
usually 2 separate flame fronts. What happens after
that is very unpredictable, but it usually leads to
detonation. It will act much like too much ignition
timing, but it is not controllable or consistent.

Motor Octane Rating

The motor octane rating, referred to as MON
(motor octane number), is the best rating to use
when selecting fuel for your race or high
compression engine. When testing MON, the fuel is
heated to 300° F and the intake air is heated to 100°
F. The test engine is a single cylinder 4-cycle
engine that is run at 900 rpm. Ignition timing is
varied with compression ratio. Engine load is varied
during test.
3
Research Octane Number

Known as RON (research octane number).
Tested at 600 rpm with a fixed timing of 13° BTDC.
The fuel temp is not controlled at all and the intake
air temp is varied with barometric pressure. This is
done to covert everything to a SAE standard day,
which is 60° F, 0% Humidity, and 29.92 inches
barometric pressure. The RON should not be used
when selecting fuel for a race or high performance
engine. The RON will almost always be higher than
the MON.

(R+M)/2 Method

This is what you get at the gas pumps. It is
average of the RON and MON. It is ok to use this
for lower compression street motors, but when you
get much over 10:1, you should really pay attention
to the MON. The closer the RON is to the MON, the
more stable the fuel is. This can be very critical
when running 7000+ rpm.

Specific Gravity

This is the weight of the fuel compared to
water. If a race fuel has a .75 specific gravity, it is
3/4 of the weight of the same amount of water at the
same temperature.
Checking your float bowl level in your
carburetor is important when changing your fuels
specific gravity. A float will sit furthe