7.2 Fuel Settings 7 ADDITIONAL ITEMS: BEYOND BASIC FUEL AND IGNITION CONTROL
Simple algorithm with narrowband sensor A narrowband sensor is only accurate at exactly stoichiometric
mixtures for the fuel being used (14.7:1 for gasoline). At around 0.5 volts, the mixture is stoichiometric. For leaner
mixtures (above 14.7:1 for gasoline, above 1.0 lambda) the voltage dips slightly below 0.5 volts. For richer mixtures,
the voltage goes above 0.5 volts. This behavior means that it is not possible to hold an exact mixture when running
closed-loop with a narrowband sensor.
Because of this, the best algorithm to use with a narrowband sensor is the "simple" algorithm.
The simple algorithm adjusts the mixture richer if the sensor reads lean, and leaner if the sensor reads rich.
It adjusts Controller Step Size percent every Ignition Events per Step. This can lead to a small oscillation in
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-based correction once the AFR reaches close to stoichiometric.
The following steps are recommended when tuning the simple algorithm with a narrowband sensor:
1. Ignition Events per Step - When first tuning the engine, this should be set to a fairly low number (4-8) so that
if the AFR is very far off, it is corrected quickly. Once the engine is better tuned, this number can be switched
to a higher number to gain more stable correction behavior (8-16 or more).
2. Controller Step Size - When first tuning the engine, this should be set to 2% so that when correcting, the
engine reaches stoichiometric quickly. Once the engine is well tuned, this should be reduced to 1% to gain
more stable correction.
3. Controller Auth - This is the maximum percentage of fuel change the MS3Pro can make as a result of O
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sensor feedback. When first tuning the engine, this should be set to 20% or higher. Care must be taken to
watch how the algorithm is correcting. In some situations, it is possible for the sensor to read very lean when
really the engine is running very rich. Once the engine is tuned, this should be set between 5% and 10%.
4. Engagement Settings - Most of the remaining settings control how and when the closed loop algorithm is
engaged. Engagement with a narrowband sensor should happen when the engine is nearly fully warm, 500-
1000 rpm above idle, below 80% throttle, below about 80% load, just above the lowest load seen when barely
pressing the throttle, and at least 30 seconds after the engine starts. These settings are because the sensor
must be hot to operate, must not be used at high load due to the fact that the engine should be operated rich
of stoichiometric, and must not be used at very low load because the oscillations will cause the engine speed
to oscillate.
Simple Algorithm with Wideband Sensor Tuning the simple algorithm with a wideband sensor is essentially
the same as tuning it with a narrowband sensor with the caveat that the AFR target table is used to set the AFR
target. It is still recommended that the EGO algorithm not be used at high throttle position/load due to the fact that
the accuracy of the wideband sensor decreases dramatically with pressure and temperature changes caused by
high load.
PID Algorithm with Narrowband Sensor When using a narrowband sensor with the PID algorithm, all the
same recommendations for settings given in the section describing the Simple algorithm should be followed.
Additionally, since it is nearly impossible to keep the narrowband sensor from oscillating, it is recommended
to start by tuning the "I" term until the target is reached with minimal oscillation. Once this point is reached. It is
recommended that very little (if any) "P" term is used since the "P" part of the PID algorithm causes instantaneous
reaction, and the response of the sensor is not proportional to the distance from stoichiometric.
PID Algorithm with Wideband Sensor When using a wideband sensor with the PID algorithm, the same
steps as when using a narrowband sensor can be followed for tuning the "I" term.
Additionally, since the response of most wideband controllers and sensors is linear with AFR, a larger "P" term
can be used to help correct for fast changes in AFR. Caution must still be used however since there is a significant
delay between the amount of fuel being injected changing and MS3 registering an AFR change as a result.
Finally, a small amount of "D" term can be used to help slow response during very fast changes. This helps
reduce overshoot of the target.
7.2.7 Narrowband EGO targets
This is a 3D table that behaves like an AFR table, but instead of specifying a target air/fuel ratio, it specifies what
voltage from a narrow band the MS3Pro should target.
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 160
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