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Ignition Systems with EFI
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Ready for another routine
9-second run, this Mustang
warms the slicks in an
attempt to plant all of its
power. (Nate Tovey)
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Knowing that engines produce power by harnessing the dramatic pressure increases resulting from
the ignition of the air/fuel mix, let’s take a look at how that pressure plays into the calibration
requirements. Cylinder pressure is directly related to engine efficiency and output. Engineers often
refer to BMEP, or brake mean effective pressure, to describe engine operation. Throttling airflow
and changing ignition advance have the biggest impact on BMEP, but cam timing, manifold
pressure, and lambda also play a large part in determining cylinder pressure. The underlying
concept is that higher cylinder pressures generally indicate a more efficiently running engine under
normal operation. More efficient combustion requires less ignition lead. If the calibrator can
recognize where the engine is most efficient, it becomes easier to determine ignition needs.
If one were to look at a map of actual engine efficiency and compare this to the ignition map
showing MBT timing, the relationship between cylinder pressure and required advance becomes
evident. Conversely, we can employ this knowledge to use ignition advance to increase cylinder
pressure (and torque) in areas where the engine is less efficient. Advancing ignition timing at low
RPM under load can help improve throttle response before the camshaft and intake runner
efficiency come up in the midrange. Likewise, at higher RPM after the engine passes the peak
efficiency range of the cam and intake runners, cylinder pressure begins to fall off. More power can
be made by increasing ignition advance to make up for the decreasing engine efficiency as the
engine approaches redline.
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Using the AVL Indicom software and
pressure transducers in the cylinders,
we can see both the P-V diagram
(right) and pressure versus time (left)
for motoring the engine. The triangular
spike on the left is the spark event.
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Here we see the pressure traces for a
running engine with less than ideal
spark timing. Notice how the pressure
rise happens very late in the cycle and
each cylinder has its own unique trace
line.
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Calibrators must be keenly aware of two unique conditions of increased cylinder pressure: Knock
and preignition have very similar symptoms and results with slightly different causes. Knock is
caused by an uncontrolled fast burn in the chamber after the initial spark event. This is often the
result of excess ignition advance, lean mixtures, or elevated temperatures. Preignition is the
premature start of combustion before the normal spark event. This is often caused by hot spots in
the combustion chamber or residual glowing hydrocarbon deposits that ignite the mixture before the
spark plug has a chance to fire. Either way, both conditions mean that peak combustion pressure
has been reached before the piston arrives at TDC. This means that not only is normal combustion
pressing on the cylinder head, gasket, and piston, but dynamic compression happens as well. The
symptom for this condition is usually a sound similar to marbles in a coffee can. While not always
audible, the intensity of this noise grows with the intensity of engine stress resulting from the
condition. The forces associated with knock and preignition can be as much as two or three times
the normal combustion forces. This can be incredibly destructive to engine components, often
resulting in a blown head gasket, broken piston, bent connecting rod, or failed bearings. Knock
should be avoided, period.
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Burn Rate
Burn rate of the air/fuel mixture has a dominant impact upon just how much ignition lead an engine
wants. The speed at which the combustion process occurs directly influences the rate of cylinder
pressure rise.
Most spark ignition engines have an effective flame propagation speed of 706 to 985 in/s (18 to
25m/s). This means that a flame starting from a central spark plug in a 4.00-inch bore engine can
reach the cylinder wall in about two and a half milliseconds. At an idle speed of 800 rpm, this
translates into roughly 11.5 degrees of crankshaft revolution. That same flame speed and bore size
at 3,000 rpm yields more than 43 degrees of crankshaft rotation during our theoretical burn time to
reach the chamber walls. This is why ignition timing must advance at higher speeds to ensure
combustion before the piston has a chance to run away from the reaction.
Actual flame propagation speed changes with conditions. Faster flame speeds and better charge
mixing during combustion speeds up the reaction that is producing the expanding gases and rising
pressure. Burn rate is influenced by a number of engine operating conditions beyond just chamber
design.
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Fuel octane is a number representing the ratio of octane to heptane in gasoline. Higher octane
numbers indicate a stronger concentration of the slower, more stable- burning octane molecules
than the more volatile heptane. Since high-octane fuels have a slower burn rate, larger amounts of
ignition lead can and should be used. Increasing the ignition lead results in an increase in torque,
so higher fuel octane levels allow for more power resulting from the necessary ignition advance
increase. Keep in mind that fuel octane is a global change to the engine’s ignition advance needs.
Running higher octane fuel means more spark advance at idle and part load as well as WOT.
Changing from 93 octane pump fuel to 110 octane racing fuel may allow 4 degrees of additional
spark advance at WOT (with an accompanying increase in power), but may idle poorly and exhibit
reduced fuel economy and throttle response unless all other spark tables are increased by a like
amount.
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Pressure trace at part load with ignition
adjusted to MBT. Cylinder pressure is
maximized just after TDC and area
under the curve in the top portion of
the P-V diagram is also greatest.
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Pressure trace showing knock
resulting from a spark event triggered
about 6 degrees before MBT at part
load. Notice the ragged tips of the
pressure traces and increased
cylinder-to-cylinder variation.
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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
$4.95 to
anywhere in the United States with USPS Media Mail.
Priority Mail shipping is available for an
additional $2.95, or
$7.90 for shipping. Shipping is combined and discounted for multiple item
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International for $11.95, and to most
locations in Europe, Australia, Asia, Japan and South
America for
$14.95. Satisfaction is Guaranteed. Our store has a NO HASSLE RETURN
POLICY
within 7 days of purchase.
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