Bill and anyone else interested:

The issues involved with mixture changes when the EC2 controller stages can be understood by considering two simple concepts:

1. The first is the concept of injector lag, also referred to as injector latency and injector dead time, as well as other names.  The fuel injectors are not perfect.  They do not instantaneously respond to the controller pulse.  That is, they take a measurable time to open after the application of the pulse and they take a measurable time to close after the pulse ends.  These two times are not equal and the result is that the injector flows fuel for a smaller period of time than the injector pulse width.  In addition, the injector flow rate is changing during the opening and closing time periods.  Measurements of the amount of fuel delivered as a function of pulse width show that the deficit in the amount of fuel delivered for any pulse is the same whether the pulse width is long or short.  For stock 40 lb Mazda injectors with a fuel pressure of 40 psi and a system voltage of 13 volts, this loss in the amount of fuel delivered corresponds to an injector lag or dead time of about 1.2 ms.

This 1.2 ms is not trivial.  Consider a rotary engine running at 6000 rpm.  The time it takes for one revolution is 10 ms.  Limiting the injector pulse to an 80% duty cycle limits the injector pulse width to 8 ms.  The effect of the injector lag is to decrease the amount of fuel delivered by 1.2/8 x 100 or 15% at 8 ms pulse width,  1.2/4 x 100 or 30% at 4 ms pulse width, and 1.2/1.2 x 100 or 100% at 1.2 ms pulse width.  It is obvious that the shorter the pulse width, the larger the fuel deficit.

2. The second concept to realize is that the EC2 controller does not take injector lag into account.  This has been demonstrated with my EC2's at default settings by measuring the injector pulse width on either side of the staging transition.  It has also been demonstrated by pulse width measurements taken when engaging the injector back up function where the change from using two to using four injectors is forced to occur without a change in manifold pressure or change in MAP table address.  These measurements show that, before tuning, the EC2 staging transition is a transition to using all the injectors at half the pre-transition pulse width.  The essential point here is that even if the primary and secondary injectors are the same size, staging, by using twice as many injectors at half the pulse width does not result in the same fuel flow, but rather a substantial fuel flow decrease.  Using the above data, this is easily seen by considering what would happen if staging took place at an initial pulse width of 1.2 ms.  After staging, the fuel flow would then be zero.


Before tuning and with primary and secondary injectors of the same size, the effect of injector lag is always for the controller to deliver less fuel than it calculates as being needed.  This problem becomes progressively worse as the injector pulse width decreases.  When the secondary injector flow rating is larger than that of the primary injectors, the mixture change due to staging, while predictable, is not intuitive.  This is because the tendency to go lean due to the injector lag effect accompanying the pulse width being cut in half is now combined with the tendency to go rich due to the two larger injectors now in use.  Whether the mixture goes leaner, richer, or is unchanged upon staging depends not only on the difference in the primary and secondary injector flow ratings but also upon the conditions (initial pulse width) under which staging takes place.

The discussion which follows is for the case of equal sized primary and secondary injectors and initial tuning starting with default parameters:

The percent decrease in fuel flow upon staging is most severe for low initial fuel flow rates.  This explains the difficulty in achieving a smooth staging process if the staging threshold is set too low.  Even for reasonable initial fuel flow rates in the range of 6 to 10 gal/hr, the fuel flow will decrease significantly.  If the mixture is rich enough when staging takes place, the decrease in fuel flow may not result in a large change in engine power output.  However, if the mixture is already somewhat lean when staging occurs, the mixture may become very lean and result in a marked decrease in power.

I've used three methods to compensate for the injector lag.

1.  The first method is to follow the instructions in the manual.  Tuning is started by adjusting modes 3 and 2.  After modes 3 and 2 have been adjusted, then mode 1 or 9 is used to adjust the mixture correction table.  Tuning the mixture correction table can compensate for the fuel flow decrease on staging caused by injector lag.  This results in a discontinuity in the correction table at staging.  Since defining the step in the mixture table can be difficult, programming the mixture somewhat rich across this region is helpful.  Cruise operation at MAP values close to the staging threshold then simply requires some adjustment of the manual mixture control.  Because of the step in the mixture table at the staging point, changing the staging threshold also requires retuning of the mixture table in this region. Tuning the rest of the table compensates for other factors such as changes in volumetric efficiency in addition to injector lag.  Injector lag affects tuning of the entire table although it is just not as obvious as its effect on staging.

2.  A second way to compensate for the effect of injector lag on staging is to use the mode 6 secondary injector differential adjustment even with primary and secondary injectors of matched flow rates.  Since the mode 6 setting only has effect above the staging threshold, and since the effect of injector lag is to make the injectors act as if they became smaller upon staging, mode 6 can be used to eliminate the step in the mixture correction table.  This is possible because mode 6 has the ability to compensate for smaller as well as larger secondary injectors compared to the primaries.  Compensation for the effect of injector lag and other factors is still necessary throughout the table, but the changes are gradual and much easier to implement than a discontinuity.  This method removes the injector lag effect upon staging and works quite well.

The tuning procedure then becomes the same whether the primary and secondary injector flow rates are different or identical:

a.  Set the staging threshold at a MAP corresponding to the high end of the primary injector flow limit using mode 7.  (For 40 lb injectors, about 20" MAP works well.)

b.  Adjust Mode 3 to get a mid scale O2 sensor reading at a MAP just below the staging threshold MAP.

c.  Adjust mode 6 to get a mid scale O2 sensor reading at a MAP just above the staging threshold MAP.

d.  Adjust mode 2 for best operation at minimum idle MAP.

e.  Adjust the mixture table throughout the useable MAP range using mode 1 or 9 to keep the O2 sensor reading mid scale.  (To be honest, I skip step "e" and simply use the manual mixture control to adjust the mixture in cruise.)


Steve Boese
RV6A 1986 13B NA RD1A EC2