Fuelish Facts
By Ken Mosher
Midlands Chapter Member
Back to Basics
Before we get down and start stirring the witch's brew of petroleum distillates needed to make a
good racing fuel, let's take a few moments to define and explore a few basic terms and
definitions. When talking about racing fuel, there seems to be many myths and half-truths that
are delivered to the novice as absolutes. The following article tries to clarify some of the jargon
associated with racing fuels and the manufacturer's literature.
How big is an Octane?
First, let's take the term "Octane". Octane is a measure of a fuel's anti-detonation resistance.
Octane is a unit for detonation resistance, just as grams are a unit of mass and inches are a unit of
measure. Having more octane does not necessarily indicate the fuel has "more power" or is a
magic elixir, it just means that the engine can run more compression, boost, timing, etc. (and that
makes power).
Octane is measured using three methods: motor octane (MON), research octane (RON), and
"pump" octane. With the motor octane method, a specially calibrated single cylinder test motor
runs on test sample at 900 RPM with intake charge heated and a spark advance engaged until
knock is detected. The test sample is rated on an octane scale that has two reference fuels. NHeptane is at the bottom end of the scale with almost no antidetonation qualities and Isooctane
(Octane rating 100) is on the high end of the scale with the maximum resistance to detonation.
The research octane method involves running a single cylinder motor running at 600 RPM and
results in a higher number, since it a less stressful test.
Generally, the motor octane is the number to pay the most attention to in the Turbo Regals. This
number is much more indicative of detonation resistance under the high boost situations favored
by the little blown V6s. The research octane number applies more to low load, low RPM
situations.
The common octane rating found posted at your local filling station is the "pump" method, which
averages the two ratings (R + M/2, where M = Motor Octane and R = Research Octane). This is
a standard way to compare different gasolines, but isn't quite as informative as the individual
ratings.
When Things Go Boom!
What is "Detonation"? Detonation is basically the uncontrolled ignition of the fuel/air mixture
that results in a sharp explosion in the combustion chamber. A typical combustion cycle consists
of four parts in the conventional four stroke automotive engine. The first cycle is the intake
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stroke, where the piston goes from Top Dead Center (TDC) to Bottom Dead Center (BDC) with
the intake valve open to allow the mixture of fuel and air to fill the cylinder. The next cycle is
the compression stroke, in which the piston goes from BDC to TDC with the valves closed,
compressing the fuel/air mixture. The third cycle is the power stroke resulting from the ignition
of the fuel/air mixture in the compression stroke. The piston again goes from TDC to BDC and
power is transmitted to the crankshaft. The final cycle is the exhaust stroke where the piston
goes from BDC to TDC and forces the spent combustion gases out throught the open exhaust
valve.
Detonation is a phenomenon that occurs during the compression stroke. As the piston
approaches TDC, a spark is fired prior to TDC (BTDC), such that the peak of the cylinder
pressure will occur at or slightly after TDC. Normal combustion produces a nice controlled
propagated wave front that propogates smoothly from the spark plug to the edges of the
combustion chamber. As it does so, it compresses the unburned gases ahead of it, which
increases their temperature and pressure. If the spark plug fires late, burning continues to occur
after TDC and the exhaust gas temperature rises dramatically. This waste heat represents lost
power and fuel economy. If the spark fires early during the compression stroke, the pressure
peak happens before the piston has finished its upward stroke and this,. in combination with a
wide open throttle condition, can lead to extremely high cylinder pressures and temperatures. The
extreme conditions cause small pockets of the fuel/air mixture not yet reached by the flame front
to detonate and begin burning outward from the ignition point. It is different than normal
combustion, because it happens at a point in front of the advancing flame front as it propagates
away from the spark plug. At these high temperatures, chemical reactions in the unburned gas
can lead to extremely rapid uncontrolled combustion before the flame front arrives. This fast
combustion can produce very high (even damaging) pressures and temperatures and results in
"knock" or "ping". This sound is the audible pressure oscillation caused by this abnormal
combustion.
This condition results in extreme turbulence in the burned mixture, which greatly increases the
rate of heat transfer into the cylinder walls and piston crown. This hot, pressurized, turbulent
mixture is what causes engine damage, because it exceeds the combustion chamber's capacity to
remove waste heat. Burned and pitted pistons, burned valves, and ring damage will result from
severe pinging or knocking. The mechanical shock of the detonation can also damage
connecting rod bearings. The power output of the engine drops dramatically when detonation
occurs because the pressure peak occurs while the piston is still on an upward stroke. In the
Turbo Regals, the ECM detects the detonation via the knock sensor and immediately cranks the
timing back several degrees to try to stop the condition. Black smoke is often seen when
preignition occurs because the turbulent conditions in the chamber result in incomplete
combustion of the mixture, and the unburned gases create black smoke.
In the Turbo Regal, it is important to use enough octane to prevent and/or limit detonation when
operating the car in a given condition. Detonation is caused by too much cylinder pressure for
the octane (boost), too much ignition advance, an air/fuel ratio that is too lean, or hot spots in the
combustion chamber (sharp edges, carbon build up, insufficient cooling). Fuels with higher
octane are more resistant to detonation. It's always nice to have a little "cushion" of detonation
resistance to adjust for changing conditions (i.e., the air gets very cold and dense and more octane
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is required). However, using a fuel with more octane than is required doesn't have any benefits
other than the ability to "turn the wick up" another notch without having to refill the tank with
more expensive gas. This can be done with more boost or a more aggressive chip.
A word of caution here, ALWAYS USE ENOUGH OCTANE to account for the conditions. If
you are driving around on the street with the boost turned down to 16 pounds, a mild chip and a
160 degree thermostat you may only need 92-93 octane gas. However, you turn the boost up to
18-20 pounds and I'll guarantee that you'll need more octane to prevent harmful detonation. In
fact, if you push these limits hard enough, all the pretty shiny metal parts on the INSIDE of the
motor may end up on the OUTSIDE of the motor! Detonation is extremely hard on rings, piston
tops, connecting rod bearings, and even valves.
Why do I need "Curves" in Drag Racing?
Racing gasoline contains many different petroleum ingredients, all of which add to a distinctive
signature that is called the distillation curve. The distillation curve is compiled using a test that
heats a sample of gasoline and then measures the temperature at which the fuel vaporizes in 10
percent increments until it is completely vaporized. These curves can tell you a lot about the
performance of a racing gasoline. Flat spots in the curves can indicate large amounts of
oxygenates or additives. A very steep curve may indicate poor starting characteristics and poor
throttle response. A very shallow curve may indicate a gasoline with to many "light" components
(also referred to as "front-ends"), which may effect its ability to be stored and may induce vapor
lock. What you are looking for is a gasoline that has a high enough initial temperature to prevent
vapor lock on a hot day or under high heat conditions, yet have a low enough upper temperature
to ensure that all the fuel is converted to energy in the combustion chamber.
Some example distillation curves are shown in Figure 1. The data for these curves was extracted
from data obtained by an independent test conducted by Texaco for Circle Track magazine.
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310
300
290
280
270
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
90
80
Turbo Blue
VP C-12
Unocal
CAM 2
Phillips Trick 66
Initial
T10% T20% T30% T40% T50% T60% T70% T80% T90% T95% T100%
Figure 1 Distillation Curves of Several Popular Racing Gasolines
The Pressure's On
Reid Vapor Pressure (RVP) is another helpful indicator as to the contents and performance
characteristics of a racing gasoline. This measure is affected by chemicals that are blended with
the gasoline, such as butane and isopentane. These components are the "front-ends" or volitiles.
This measurement refers to how easily a gasoline will vaporize, with a higher RVP indicating
more front ends and a motor that is easier to start when cold. A lower RVP indicates less frontends, which sometimes indicates that it is a "heavier" fuel and may contain more energy
(measured in BTUs) per gallon than a lighter gasoline. This difference is normally so small in
this respect that it doesn't have any practical effect in real world situations. The exception to this
would be if you are injector limited, which means you need to put more BTU value into the
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combustion chambers per volume of fuel delivered. This compensates a small degree for the
lack of ability to deliver the total volume actually needed to produce a given power level.
A winter blended street gas will typically show an RVP of between 11 and 13, a summer
blended street gas will typically show an RVP of 7-9, and racing gas usually shows an RVP of
between 5-8.
Are You Dense?
One last number you often see in the manufacturer's literature is Specific Gravity. Specific
Gravity is basically a measure of how dense the fuel is at a given temperature and pressure and is
related to the RVP. It can give you a basic feel how much BTU content is in the fuel, but is again
really only critical in limited volume situations.
So Now What?
Below are listed some gasolines that you can practice some of what you learned. The data
displayed came from the manufacturers' literature:
SUNOCO "Standard"
Color.....................Purple
Research Octane...........115
Motor Octane..............107
R+M/2 ....................111
Specific Gravity........0.725
Reid Vapor Pressure........ 8#
-- Distallation temp, degrees F -Initial............90
10%...............160
50%...............220
Final.............360
5
360
350
340
330
320
310
300
290
280
270
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
90
Initial
T10%
T50%
T100%
VP C-10
Research Octane...........105
Motor Octane...............96
R+M/2 ....................100
Specific Gravity........0.760
Reid Vapor Pressure....... 1.9#
-- Distallation temp, degrees F -Initial...........198
10%...............212
50%...............224
Final.............281
6
290
280
270
260
250
240
230
220
210
200
190
Initial
T10%
T50%
T100%
VP Performance Unleaded
Research Octane...........104
Motor Octane...............93
R+M/2 .....................99
Specific Gravity........0.760
Reid Vapor Pressure....... 6#
-- Distallation temp, degrees F -Initial...........105
10%...............160
50%...............225
Final.............390
7
390
380
370
360
350
340
330
320
310
300
290
280
270
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
Initial
T10%
T50%
T100%
SUNOCO "GT Unleaded"
Color....................Natural
Research Octane...........105
Motor Octane...............95
R+M/2 ....................100
Specific Gravity........0.760
Reid Vapor Pressure....... 8#
-- Distallation temp, degrees F -Initial............90
10%...............150
50%...............210
Final.............230
8
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
90
Initial
T10%
T50%
T100%
SUNOCO "Supreme"
Color......................Blue
Research Octane............116
Motor Octane...............109
R+M/2 .....................112
Specific Gravity.........0.715
Reid Vapor Pressure..... 8#
-- Distallation temp, degrees F -Initial............90
10%...............155
50%...............215
Final.............260
9
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
90
Initial
T10%
T50%
T100%
SUNOCO "Maximal"
Color......................Red
Research Octane...........118
Motor Octane..............115
R+M/2 ....................117
Specific Gravity........0.700
Reid Vapor Pressure....... 6#
-- Distallation temp, degrees F -Initial............100
10%................150
50%................220
Final..............240
10
260
250
240
230
220
210
200
190
180
170
160
150
140
130
120
110
100
90
Initial
T10%
T50%
T100%
Comparing these different offerings from the these two companies reveals that they are very
similar to one another, which is not unexpected. However, there are some important differences
here. Notice that the Initial Boiling Point (IBP) is about 10 degrees higher with the CAM 2
Maximal from Sunoco and it is a whopping 100 degrees higher with the VP C-10. This could be
important in trying to start the motor when it''s cold. Another thing to notice is the T100% or the
Final Boiling Point. There is a significant difference between the "Standard" fuel and the others.
The much higher FBP could indicate a tendancy to have heavier distallates and might be better in
in volume limited circumstances.
In summary, racing fuel is a complex issue that is fraught with old wive's tales and magic claims.
However, hopefully I have provided you with some data that will help you do some research to
locate the fuel that is right for your combination.
-- Midlands Chapter GSCA --
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