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EFI System Idle Calibration
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Now that the engine has been mapped under most stable operating conditions, it is possible to
move on to a less stable condition: idle. Because of the slow engine speed, there is a relatively long
time between possible corrections the PCM can make at each TDC event. The longer pause before
the next feedback signal makes it easy to over- or under-correct at idle speeds. Following trends
found under stable operating conditions at medium engine speeds and loads downward gives a
better estimate of what the engine wants at idle speed. Remembering that idle speed is the result of
the delicate balance between engine torque and engine drag from loads, small changes in engine
torque can yield relatively large changes in idle speed.
A couple layers of control are required for this delicate balance at the lowest acceptable speed
without stalling. The largest controlling factor to idle speed is engine throttling. Throttling can come
from either the throttle blade itself or the IAC motor. While very large changes can be made to
engine torque by changing throttling, actual changes take a relatively long time to fill the manifold
and allow the engine to react.
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A datalog of engine behavior at idle
shows relatively low calculated engine
loads and airflows. Notice how spark
advance moves rapidly to help
maintain a steady engine speed.
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To prevent stalling in the mean- time, a faster correction to engine torque can be made by
changing spark advance. This is why it is desirable to set base ignition timing lower than MBT at
idle, leaving room to adjust timing when necessary to reduce speed fluctuations and prevent
stalling. Idle spark advance values can be surprisingly small, especially in engines with more
efficient combustion chamber designs. Many modern DOHC engines with stock cams idle with single
digit spark advance.
If a larger than stock camshaft design simply does not allow stable idle at the stock speed, adding
100 rpm at a time can quickly bring it into stable operation. Once stable, reducing an additional 50
rpm may help quiet engine operation at idle. This is where carefully choosing load and speed set
points in a speed density application or MAF scaling come in very handy. There is no reason to
simply increase idle speed from 800 rpm to 1,000 rpm after a camshaft change when a little extra
calibration time allows the engine to be stable at 850 rpm. Careful control of fuel delivery, airflow
mapping, and spark compensation are the keys.
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The MAF on this engine is installed on the
airbox. The hook-shaped appendage on
the inlet tube is a Helmholtz resonator,
used to cancel out standing wave
vibrations that would otherwise corrupt the
MAF signal at low speeds as well as make
an audible noise.
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This engine uses two Helmholtz resonators
(arrows) to further dampen intake tract
oscillations. Each chamber is tuned to a
different frequency. The PCM is hiding
beneath the right arrow.
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Little by little, engine speed can be reduced to continue mapping airflow from the stable areas
mapped earlier to speeds approaching desired idle. As long as the fuel injector properties have
been modeled correctly, it is possible to continue with airflow mapping for progressively slower
speeds and lower loads. With each progressively lower engine speed, the airflow model can be
adjusted as before. The same process of checking actual lambda against the target value applies.
With proper airflow modeling, the engine is likely to exhibit loads at idle of 10 to 18%.
Changes in injector offset (dead time) show up most prominently at low speed where actual
pulsewidths are smallest and dead time represents a larger percentage of total injection time. If the
injector flow was not modeled accurately before starting, this is where the calibrator sees issues.
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When starting from scratch on a throttle stop set point without ETC, allow the engine to idle with the
blade held slightly open. Remove any IAC control (unplug the motor from the harness) and slowly
close the blade position until the engine barely runs near the desired speed. Reinstalling the IAC
harness lead should result in a more stable idle. This prevents the IAC from leaning to either a fully
open or closed static position. The object is to get the IAC motor in the middle of its adjustment
range so that it has maximum flexibility to adjust airflow either up or down. The TPS output should
be checked at this time to ensure that the output value is within normal operating range and
registers as “closed throttle” to the PCM. In some cases, it is not possible to open the throttle blade
enough and remain within the range the PCM considers “closed throttle.” In these rare instances, a
small bleed hole can be drilled in the throttle blade to allow slightly more air bypass when closed.
Start small and increase in very small increments to avoid scrapping the throttle body. A final check
should be made to verify that commanded idle speed in the PCM matches actual engine speed and
that the IAC is near the middle of its range.
During idle speed calibration, it is helpful to monitor data in real time for MAF, MAP, lambda, injector
pulsewidth, and temperatures. Verify that all signals are within normal operating range for the PCM.
Some PCMs have a value for minimum allowed MAF that may need to be reduced when using a
scaled meter. Actual MAF or MAP values give an indication of exactly which cells should be
modified to complete airflow modeling at idle speeds. Pulsewidth is also useful to monitor since it
can show any “hunting” activity in the PCM for target lambda that may be caused by background
functions such as evaporative or EGR compensations. Some PCMs also have a lower limit value for
pulsewidth that may need to be reduced when using larger fuel injectors.
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After finding the stable idle speed at normal operating temperature, a temperature compensation
curve can be built. To offset the poor combustion, higher fluid viscosities, and undesirable
emissions at cold start, idle speed should be increased at low engine temperatures. If idle is stable
at 800 rpm and 190 degrees F, this idle speed is usually fine down to about 160 degrees F. Expect
to add up to 400 rpm at freezing to allow for smooth engine operation, with a smooth transition as
temperature changes. A side benefit to increased engine speed at cold temperatures is faster
warm-up times for components, coolant, and oil. As a measure of precaution, temperatures above
220 degrees F may also be aided by increasing idle speed 100 rpm or so to boost water pump flow
during cool-down after extreme operation.
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Dashpot
After a stable idle has been found, the PCM needs a way to get to it smoothly. Just like a
carburetor, most PCMs have a dashpot function built in that allows for a softer landing to the actual
idle speed. Dashpot is usually shown as bypass airflow from the IAC at closed throttle versus
engine speed. It is up to the calibrator to contour this function so the engine speed drops quickly
once the throttle is closed at higher RPM, and slows its rate of change as idle is approached. Too
fast and the engine drops right past idle speed and stalls; too slow and the engine hangs at higher
RPM when lifting off the throttle. Changes in throttle body size, camshaft design, and intake
manifold design and volume have noticeable effects on required dashpot. Lower dashpot values at
medium to high engine speeds can be used to add engine braking when the throttle is lifted. Care
should be taken to make sure this does not interfere with normal cruise throttle position, which may
be relatively low for larger throttle bodies.
When adding a supercharger to a naturally aspirated engine, it is important to recognize the source
of air feeding the IAC mechanism. When the throttle is closed, the intake manifold is under vacuum
and higher pressure is present in front of the throttle blade. This pressure differential is
exaggerated in supercharged applications. It is desirable to have the IAC draw air from an
atmospheric source rather than the pressurized section between the compressor and manifold.
This reduces the force-feeding effect on the IAC, keeping its actual flow range low where it belongs,
and increases the adjustment precision of the PCM. Dashpot values need to be greatly reduced
when the IAC is fed from a pressurized source. The same area of opening in the IAC with higher-
pressure differential across it yields greater flow in the exact same manner as increased fuel rail
pressure increases injector flow. This also means that each “step” size of adjustment to the IAC
position is larger, reducing resolution and control.
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The dashpot function controls the amount of
airflow at closed throttle. At higher speeds
more air is allowed through the engine to
control pumping losses and drag.
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The actual amount of dashpot airflow must
be reduced over time for the engine to
return to idle. This table slows the rate of
reduction as speeds decrease to provide a
gentle return to idle speed without stalling.
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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 cars
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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Tune and Modify Engine Management Systems
Drawing on a wealth of knowledge and experience
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.
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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.
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International for $14.95, and to most
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America for
$18.95. Satisfaction is Guaranteed. Our store has a NO HASSLE RETURN
POLICY
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