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EFI System Fuel Control
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Now that we have covered how information travels between the engine and PCM, it’s time to start
processing. Internal combustion gasoline engines only operate within a specific window. Things
have to happen in the right order with the right amounts of each input for the desired result. Much
like baking a cake, there is a recipe for turning gasoline into usable power. Whether you are
making yellow sheet cake or triple chocolate surprise depends upon the engine and parts with
which you start. Calibration cannot miraculously add airflow. That is up to the mechanicals in which
the limit resides. However, just like forgetting to stir the mix properly or setting the oven temperature
too high ruins the baked goods, the calibrator has the ability to prevent an engine from running
well. Likewise, a good cook can bake the moistest cake with the best frosting design, the calibrator
can polish the engine’s performance to perfection as well. All internal combustion, spark-ignited
engines have a few rules that must be obeyed.
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Combustion only occurs between 8.0:1 and 25.5:1 air/fuel ratios. The closer to the edge of this
envelope the engine gets, the more likely misfire becomes. The stoichiometric mixture is 14.68:1 (l
= 1), which would theoretically leave the least amount of leftover ingredients. Slightly leaner mixes
result in better fuel economy and slightly richer mixes result in better torque. Excess fuel can be
used as a cooling agent to limit combustion temperatures at high load.
Peak flame speed is found at l ª 0.9. At this point, less ignition lead is required to reach peak
cylinder pressure at the same time in the cycle. Adding excess fuel or air slows down the
combustion process. When operating under significant power enrichment (l << 0.9), leaning the mix
out increases flame speed as we approach l ª 0.9 again. Additional fuel enrichment (to ratios below l
ª 0.9) slows the flame front’s travel, requiring more ignition advance to achieve peak cylinder
pressure at the correct time.
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The spark initiates the combustion process in normal operation. For this to happen, the spark
energy must be high enough to create an arc across the plug gap. As the air/fuel charge mixture
density increases, more energy is required to create the electrical arc in the gap. Charge density is
directly proportional to engine load, so it is affected by throttle position, port velocity, compression,
and manifold “boost.” The wider the gap, the more energy is required to jump it. More surface area
of the arc from a wider gap yields better initial combustion due to an increased number of oxygen
and gasoline molecules in contact with the arc. Ignition energy is directly proportional to input
voltage, coil winding, and coil saturation time.
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The importance of efficient manifold
tuning can be seen in this BMW
design. Long individual runners are
sized to improve midrange torque of
this engine, and the plenum can
clearly be seen as well. (Nate Tovey)
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Engine speed is a result of the difference between loads (friction, accessories, pumping losses,
vehicle movement) and output (current engine power, often a result of throttle position). Any time
output exceeds the current loads, speed increases. When output is less than current loads, engine
speed drops. Holding a constant engine speed requires making just enough power to equal the
current loads on the engine.
Idle speed is a perfect example of this delicate balance since load changes from accessories and
friction as a function of speed must be offset by careful adjustment of engine power from throttling
and spark. Low speed idling is one of the most difficult balances for any engine to make. EFI
systems can experience significant trouble due to the relatively large amount of time between power
strokes when attempting to make relatively small corrections. If the loads on the engine exceed
output by enough, stalling occurs.
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Spark advance at most operating conditions should be set as close to MBT (maximum brake
torque) as possible to take advantage of maximum engine efficiency. It is difficult to damage an
engine from over-spark at light load. MBT may make best power at WOT, but it is often wise to
retard timing slightly to provide some safety margin and avoid knock. It is not ideal to operate at
MBT at idle. Timing should be intentionally slightly retarded from best torque to allow for spark
trimming of idle speed. If an engine is idling with a spark advance equal to MBT, it is not possible to
add timing to quickly compensate for any added load or drop in speed.
Most engines are perfectly happy to run at stoichiometric (l = 1, 14.68:1 A/F) ratio 95% of the time.
The primary exception is WOT performance, where a richer mixture not only makes more torque,
but also allows for more spark lead and cooler exhaust temperatures. Once the fuel mixture is right,
adjusting spark for best operation becomes much easier.
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Some vehicles with excessively large duration camshafts simply do not idle well at l = 1, and adding
up to 10% fuel can help idle quality. This increase in fuel delivery yields a slight torque increase
which helps stabilize combustion, even at light load. These same vehicles are typically fine at l = 1
for cruise, but require larger amounts of acceleration enrichment (AE) to prevent stumbling on tip in.
The bottom line is that once the calibrator has given the engine the right set of operating
parameters, vehicle performance should be a direct result of the parts of the system. Too large of a
camshaft and the calibrator has little hope of good idle quality. Too small of an intake runner and
total power suffers regardless of what air/fuel ratio or spark lead is run. Keep in mind as a calibrator
that it is possible to adjust things to a certain extent in the name of driving behavior, but there are
limits. Often it’s best to recognize the situation for what it is and either change parts or deal with the
less-than-optimal results.
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Modeling Airflow
To begin the actual calibration process for an engine, one of the most important steps is to
recognize the instantaneous airflow. This airflow can then be processed to determine the necessary
fuel delivery to maintain smooth engine operation. The most important core function of any PCM is
this airflow modeling, which determines subsequent fuel commands. To this end, the calibrator must
create adequate representations in the PCM code of what is happening in the engine’s physical
environment. Modeling of the engine airflow is done in one of two methods: Mass Air Flow
Measurement or Speed Density Calculation. Either system is capable of properly controlling an
engine, has its own pros and cons, and sometimes both are used together.
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Mass Air Flow
Mass Air Flow systems rely heavily upon input from the MAF sensor discussed earlier.
These systems take the output of the MAF sensor as a direct representation of current
engine airflow. This approach makes for very simple and straightforward calculation of
engine load and fuel requirements. In this case, engine load (Volumetric Efficiency) can be
instantly shown as:
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VE = MAF(Displacement x rSTP x Speed)
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Changes in throttle position simply restrict airflow. Part load (vacuum) is seen by the PCM
simply as a smaller mass flow value. Since forced induction engines end up moving more
total air mass per cycle, actual boost pressure is not required to calculate load and fuel
demands. Knowing exact air mass flow makes fuel demand calculations simple as well:
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Fuel flow rate = MAF x (desired A/F ratio)
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The crucial point to making mass air systems work properly is that the output of the MAF sensor
must reflect reality. This is where the calibrator must spend some time to ensure correlation
between actual mass flow and indicated mass flow. A large portion of the calibration process on a
mass air system is spent tweaking the MAF transfer function in the PCM to reduce the variation
across a wide range of steady state conditions. Making the MAF data more reliable to the PCM
forces many other operating conditions to simply fall into place easier.
Any errors between the MAF output and actual engine airflow directly result in inaccuracy in fuel
control and engine operation. The MAF must have adequate resolution, range, and repeatability for
the system to work properly. This is particularly important when high RPM, high load flow rates have
the potential to exceed the measurement range of a particular MAF sensor. In performance
applications, it is not unusual to see the “pegged” MAF reporting a constant maximum value to the
PCM. In turn the PCM commands a constant fuel delivery against what is actually an increasing
airflow. The result is a progressively leaner air/fuel ratio and often detonation. The solution is either
a change in MAF sensor hardware or some creative compensation in the PCM tables.
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The placement of the MAF sensor in the inlet tract can create some challenges. As discussed
earlier, the MAF should be positioned such that it sees laminar flow across its element. This often
means that some tight inlet routes may not be conducive to proper MAF placement. Many
aftermarket engine packages such as “spider EFI” intakes with a single throttle plate under a
central air filter do not even leave any room for a MAF sensor. The only way to run a mass air
system on these applications is to extend the inlet to a remote MAF and filter assembly, making for
a somewhat more complex inlet system.
The primary benefit to the mass air system is that any changes in actual airflow that fall within the
MAF sensor’s range and resolution can be instantly accommodated by the PCM. This means that a
change to a larger camshaft or higher flowing intake manifold simply show up to the PCM as slightly
higher airflow rates, resulting in slightly higher fuel delivery. Mass air systems tend to be very
forgiving of relatively drastic modifications in the name of horsepower. If it is possible to cleanly
install a MAF sensor, this is the most desirable method of engine control due to its flexibility and
accuracy.
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Previous | Next
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This has been a sample page from
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Engine Management: Advanced Tuning
by Greg Banish
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As tools for tuning modern engines have become more powerful
and sophisticated in recent years, the need for in-depth
knowledge of engine management systems and tuning techniques
has grown. Tuning engines can be a mysterious art, as all
engines need a precise balance of fuel, air, and timing in order to
reach their true performance potential.
Engine Management: Advanced Tuning explains how the EFI
system determines engine operation and how the calibrator can
change the controlling parameters to optimize actual engine
performance. This book takes engine-tuning techniques to the
next level. It is a must-have for tuners and calibrators and a
valuable resource for anyone who wants to make horsepower with
a fuel-injected, electronically controlled engine.
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Click below to view sample
pages from each chapter
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Author Greg Banish is a calibration engineer with extensive
aftermarket performance calibration experience. With over a
thousand unique calibrations performed over five years, he has
worked with enthusiasts and OEMs alike to improve the
performance and driving behavior of a wide range of vehicles.
The book contains detailed equations, graphs, and illustrations.
Also included are valuable and practical examples, including real-
world examples based upon the author’s experience that will help
more advanced readers apply this new information to situations
that are commonly seen during calibration.
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1 - Introduction to EFI
2 - Basics of Fuel Injection
3 - Carbureted Engines
4 - EFI System Inputs
5 - Fuel Injectors
6 - EFI System Fuel Control
7 - Ignition Systems with EFI
8 - Data Logging
9 - EFI System Calibration
10 - Idle Calibration
11 - Tuning for More Power
12 - Fine Tuning EFI
13 - Tuning EFI with Blowers
14 - Tuning Ford EFI Systems
15 - Aftermarket EFI Systems
16 - INCA OEM Calibration
17 - External EFI Controllers
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8-1/2 x 11"
Soft bound
128 pages
200 color photos
Item # SA135
Price: $22.95
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Click here to buy now!
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How To Build High-Performance Ignition Systems
The complete guide to understanding automotive ignition systems.
Covers components, systems & subsystems for street & race
applications. This book will help you understand how your car’s ignition
works, and it will help you choose the right components for your car’s
performance needs, whether it’s a 1965 289 or a 2003 Cobra with a
4.6-liter modular motor.
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Price:
$22.95
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How to Tune and Modify Engine Management Systems
Drawing on a wealth of knowledge and experience and a
background of more than 1,000 magazine articles on the
subject, engine control expert Jeff Hartman explains everything
from the basics of engine management to the building of
complicated project cars. This book is updated to address the
incredible developments in automotive fuel injection technology
from the past decade, including the multitude of import cars that
are the subject of so much hot rodding today. Hartmans text is
extremely detailed and logically arranged to help readers better
understand this complex topic.
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Price:
$27.95 |
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Turbo High Performance Turbocharger Systems
This book is the most detailed and up-to-date resource on
turbocharging. You'll learn how turbochargers work, how to
choose the right turbo or turbos for your engine by reading
flow maps, and how to tune your engine to run perfectly with
your turbo system. Uses more than 300 photos and technical
information to help you make more horsepower. It also
discusses the various components of a turbocharger and
explains how to decode turbocharger model numbers,
compressor maps, other specifications and includes a
complete step-by-step turbocharger tear-down and rebuild.
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Price:
$22.95
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Turbochargers
How to select and install the correct turbo for big or small
horsepower gains. Discusses turbocharger design, sizing,
matching, controls, carburetion, exhaust, ignition,
intercooling, marine and high altitude applications. The most
comprehensive book available. Turbo suppliers and kit
maker addresses are included. “Everything you could possibly
need to know about turbochargers for automotive applications
is in this book.
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Price:
$18.95
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Payment, Shipping & Sales
Tax: Iowa
residents must pay 7% sales tax. Items usually ship
within one
business day of receipt of payment! Standard shipping is a flat rate of
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POLICY
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