Rules for Multi-Channel Ignition and Fueling

To set up your MegaSquirt® MicroSquirt®/Sequencer™ controller for ignition timing control and fuel timing use with a missing tooth crank wheel (or crank wheel and cam synch set-up) you have to set a number of user parameters under 'Advanced Ignition Options' in your tuning software, shown in red in the illustration below:

There is also the InjStart parameter (aka. 'Injection Timing Delay (%)' in the tuning software) under 'Fuel Set-Up/General Settings':

The following defines the primary inputs for MicroSquirt®/ Sequencer™ multi-channel ignition and fueling, and describes the operation of this logic as well as how to choose correct configuration(s) for your engine.

  1. Tooth Pulse. Every tooth of the crank wheel creates a changing voltage signal. The ECU 'sees' only the rising or falling 'edge' of this signal. Whether the rising or falling edge is used is set as per the user specification in the Ignition Input Capture (IC) selection. The tooth pattern is specified as the total number of 'Trigger Wheel Teeth' and the number of 'Missing Teeth' (this is sometimes referred to as "M-N", with M the total number of teeth and N the number of missing teeth). Typical wheels are 60-2, 36-1 and 12-1.

    Note that if you set Trigger Wheel Teeth equal to zero (0), then every tooth pulse will be a tach pulse. This is sometimes referred to as 'distributor mode' since that is how a distributor with one pulse per ignition event is generally handled.

  2. Tach Pulse. This occurs with the arrival of particular crank wheel tooth pulse edges via a variable reluctor (VR) sensor or Hall effect sensor. So tach teeth must occur at specified tooth pulses, but not all tooth pulses are tach teeth.

    The first tooth in the crank wheel sensed after the missing tooth gap (or after the arrival of a second separate synch signal from a crank or cam signal) is always numbered wheel tooth 1 (which we will call "T1" in this document).

    The first tach tooth (called "Tach 1") then occurs Delay_Teeth later, where the user specifies Delay_Teeth. So if Delay_Teeth is 0, then Tach1 = T1, and in general:

    Tach1 = T1 + Delay_Teeth

    In the example below, we have a 12-1 crank wheel with delay teeth=1 and skip tooth=6 (and the input capture set to capture the leading edge of the teeth as 'rising edges'):

    Tach1 is always where synch is declared by the ECU. It does this when:

    1. It has measured similar tooth times between at least 2 regular teeth and then
    2. Sensed a missing tooth time gap consistent with the number of missing teeth (or the ECU has detected a secondary sync signal like a cam synch signal), and then
    3. Counted Delay_Teeth more, all of similar elapsed times.
    That is, the ECU declares a 'synched state' only when it knows the engine's position within its 720 degree cycle in crank degrees accurately.

  3. Skip Teeth (No_Skip_Teeth in the code). The next tach tooth (Tach2) occurs at Tach1 + No_Skip_Teeth. Subsequent tach teeth occur every No_Skip_Teeth thereafter up to the number of cylinders in the engine and then repeat.

    For 4-stroke engines there are No_cylinder tach pulses over 720 degrees of crank rotation. Therefore:

    No_Skip_Teeth = 2 x No_Teeth / No_cylinder

    Note: the similarly named user input 'Skip Pulses' (no_skip_pulses in the code) has nothing to do with tach pulses do not change this variable when setting up an initial configuration.

  4. Top Dead Center (TDCn). Each tach tooth corresponds to a particular cylinder being near or at compression TDC. The Tach tooth (Tachn) for cylinder n must arrive either exactly at TDC, OR at some position AFTER TDCn. This amount is the user input Trigger_Offset (adv_offset in the code, triggerOffset in the INI, Trigger Offset in the tuning software).

    Trigger_Offset is in negative crank degrees.

    Note that Tach1 is an exact tooth signal edge in crank degrees from T1 (rising or falling edge, user defined by the input capture (IC) setting). TDC can occur at, or between, teeth (if the TDC occurs at the tooth edge, then the Trigger_Offset will be zero). While the tooth edge is sensed by the ECU, TDC is not sensed by the ECU, so the user must tell it where TDC is relative to the corresponding tach tooth by specifying Trigger_Offset.

    For the next few examples we will assume the Trigger_Offset is zero (i.e. that tach tooth occurs exactly at TDC for a particular cylinder), then we will show the effects if it is not zero in the last examples.

    For an even-fire engine, the same offset applies to all cylinders.

    Note also that TDC1 can be any cylinder, but as far as the ECU is concerned, this is cylinder number 1, and subsequent cylinders are in firing order, not the cylinder numbering convention for your model of engine. TDC2 refers to the TDC that comes after the first TDC, not necessarily the TDC for cylinder #2. So whatever cylinder is selected for Tach 1 must have its coil and injector wired to the IGN1 and INJ1 pins on the ECU and trigger offset is referenced to this cylinder.

    For example, if you engine's firing order is 1342, and cylinder #1 is #1 according to the ECU (i.e. it is the first cylinder to TDC after the missing tooth and delay teeth - see below), then:

  5. Delay Teeth (Delay_Teeth in the code). The only thing left is how to choose Delay_Teeth, that is, how do you align the th tach teeth signals (Tachn) and and the top-dead center events (TDCn) in the engine rotation.

    When Tach1 arrives, the ECU sets up the timing for dwell for coil 3 and spark in cylinder 3, which correspond to Tach3/ TDC 3. At the same time it opens Injector 3 (unless there is an InjStart delay, in which case it delays a specified number of degrees; or unless there is a SparkFuelOffset, in which case it picks another injector to open).

    The main consideration here is the ECU must fit the dwell plus spark advance plus trigger offset inside 2 tach cycles at high rpm. To determine if this is possible for your combination of user settings, you have to decide the minimum amount of time you can dwell to get acceptable spark, then convert this to crank degrees at the maximum expected rpm and add the maximum expected spark table advance plus trigger offset.

    The milliseconds per revolution is equal to 60000/rpm, because there are 60000 milliseconds in a minute. The revolutions per milliseconds is rpm/60000. Since one revolution is 360°, the degrees per milliseconds is rpm/166.7, (where 166.7 is 60000 millisec/rev 360°/rev). The degrees for dwell (which is in milliseconds) can then be calculated as:

    CrankDegrees = dwell * rpm/166.7

    This total amount of crank degrees must fit between Tach 1 and Tach 3, with a few degrees of slop for prediction error. If it does not fit within that time span, you will either get a short dwell or a spark at Tach 3 (0° advance). So move the Delay_Teeth and corresponding trigger offset until proper dwell is achieved. Since trigger offset can be a fraction of a tooth, adjust it exactly with a timing light. This is all shown in figure 2. below:

  6. Injection Timing Delay (InjStart in the code). Once a satisfactory dwell and spark is attained, you can adjust InjStart and SparkFuel Offset to put the pulse width where you want it.

    InjStart is a percentage (%) of one tach cycle for which the injection is delayed relative to the start of the tach cycle.

    You can not delay the injection so much that the injection starts with less than 2 milliseconds remaining before the end of the tach cycle. This is because the router must receive the starting edge of the pulse width before the next tach cycle, and the 2 milliseconds allows for prediction error, interrupt latency and pulse width calculation time.

  7. Spark/Fuel Offset (spk_fuel_offset in the code, spark_fuel_offset in the INI) allows the user to connect different injectors to a given injector pin without messing up the fuel trim inputs. As mentioned above, the router normally opens injector 3 when it reaches tach 1. If you want it to open injector 4 instead, set the SparkFuelOffset = 1. Note that this setting has absolutely no effect on spark timing or trims, it only affects fuel. Note also that while using this offset in a wasted configuration may move the injection timing, it is not meaningful since one can not be certain which injector is really being moved.

  8. InjStart and SpkFuelOffset can be combined to move injector timing across all cylinders. Examples are shown below. In the first figure, with SpkFuelOffset = 0, Injector 1 start can be moved from just after Tach1 to 2 milliseconds before Tach2.

    The 78% in the illustration below is just an example at a specific rpm. At low rpm you could reach 90% or more, at higher rpm less than 78%, but at high rpm fuel timing is less critical because fuel/ air mixing is much better. Likewise, Injector 2 covers most of the region from Tach2 to Tach3.

    Figure 5.b shows the same configuration, but with SpkFuelOffset = 1, which is the same as swapping the injector wires for channels 1 and 2. Injector 1 can now be made to cover the Tach 2 to Tach 3 region and Injector 2 covers Tach 1 to Tach 2:

  9. InjStart and SpkFuelOffset with Odd-Fire. When used with an Odd-Fire engine, InjStart and SpkFuelOffset can also be combined to move injector timing across all cylinders. However, there is not as much range and flexibility, because tach cycles are still defined as for an even fire engine, that is, they are all evenly spaced every Skip_Teeth apart, and the odd-fire injector can not be opened until after the tach cycle tooth has been sensed.

    So, if the Odd_Angle offset (aka. OddAng in the code, Offset (advance) for Output #2 in the tuning software) is positive, meaning the offset is to be added to Ign 2, Inj 2 outputs to advance(+) rather than retard(-) relative to Ign 1, Inj 1, then InjStart can not be 0%. It can be entered as 0 in the tuning software, but the delay relative to cylinder 2 TDC will actually be:

    (Odd_Angle degrees / tach cycle degrees) x 100 = 16%

    as shown in the examples shown below.

    So in order for the fuel to be injected at the same relative position in each cylinder, InjStart must be set to 16% or else Inj 1 will open immediately after TDC1, but Inj 2 is forced in the code to open 60 deg after TDC2 at the earliest.

    Note that setting SpkFuelOffset = 1 will reverse the injections in the same manner as in Figure 5.b, but because of the Odd_Angle characteristics, it then may only be possible to find a start time that is only approximately equally distant from the respective TDCs for each cylinders.

    With a negative Odd_Angle, there is no problem with both injectors opening right after tach, but the range through which you can move the injection is limited to 56% before the 2 milliseconds before tach limit is reached on the second injector channel.

  10. InjStart and Trigger_Offset (trigger offset is adv_offset in the code). In the preceding examples, Trigger_Offset (adv_offset in the code, triggerOffset in the INI, Trigger Offset in the tuning software) was 0, so the tach pulse and TDC were simultaneous.

    For an even-fire engine, trigger offset is defined as the difference in degrees between Tach n and TDCn.

    For an odd-fire engine the same is true for Tach 1 and TDC1; for the pair that arrives next, the trigger offset in the code remains unchanged, but the Odd_Angle input, which defines the lead or lag of TDC for the second cylinder or cylinder bank relative to the first, is subtracted, so that the actual Injstart for the Odd_Angle pair is:

    Input InjStart% - Odd_Angle (in percentage of tach cycle degrees)

    Because of the way the code is structured, InjStart is defined with respect to the tach pulse. This is the earliest that the fuel injection can start, and for the first Tach, TDC pair, InjStart = 0% means it starts immediately after the tach pulse is processed and the ignition and pulse width variables are calculated.

    For the second tach, TDC pair, the same is true except that the Odd_Angle input (60 degrees in the example), converted to percentage (16%), is subtracted from InjStart, but the result is forced to be 0 or greater.

    The example below (figure 7.a) shows how you can set up so that InjStart covers the same range relative to TDC1 and the actual TDC2. It is similar to the setup in Figure 6A, except now there is another 60 deg section due to trigger offset being -60 degrees.

    In the preceding, you could choose trigger offset = -30 deg and get similar results, with the 60 degrees|60 degrees segments becoming 30|60 for Inj1 and 60|30 for Inj2. Likewise, a trigger offset of -80 deg would give 80|60 for Inj1 and 60|80 for Inj2, but again the actual injector start times for both injectors would be the same for both relative to their TDCs.

    Unfortunately, the same result is not possible for a negative Odd Angle, as can be seen in the example below (figure 7.b). Choosing InjStart = 0% means the earliest Inj2 can start is when TDC2 arrives, because the actual start = input start (-80 degrees in percentage = 80°/(720°/2) = 22 %).

    However, this value of InjStart causes Inj1 to start 60 degrees ATDC1, and increasing InjStart does not improve anything. So in this situation, the most equitable result is to minimize trigger offset, by either moving the sensor or using Delay_Teeth, or reverse the injector wiring.

Note that on most engines fuel timing, unlike ignition, does not require great precision. Timing the fuel shows marked improvements primarily if the timing is such that fuel is being injected when both the intake and exhaust valves are open (the overlap period near TDC between the exhaust/intake strokes). As long as the timing is moved enough to avoid this, further tuning is not critical.

Click here to see the Procedure for Configuring Dual Spark Wheel Parameters


In this document, some parameter terminology may be used interchangeable with the code, INI, or tuning interface designations (which are sometimes identical). The equivalent nomenclature is:

Preferred in this document:Tuning Software:Code:INI:
No_Skip_TeethSkip TeethNo_Skip_TeethNo_Skip_Teeth
Delay_TeethDelay TeethDelay_TeethDelay_Teeth
SparkFuelOffsetSpark/Fuel Offsetspk_fuel_offsetspark_fuel_offset
InjStartInjection Timing DelayInjStartInjStart
Trigger OffsetTrigger Offsetadv_offsettriggerOffset
Odd_AngleOffset (advance) for Output #2OddAngOddAng


→→→→→→ →→→→→→ →→→→→→→ ↓↓ Under Construction ↓↓ ←←←←←← ←←←←←← ←←←←←←
Number of cylinders
2-stroke   4-stroke
Crank Sensor Position with Cylinder #1 at TDC← Number of teeth between the sensor and the first tooth after the missing tooth(s) when cylinder #1 is at TDC (can use decimal fractions of a tooth).

'Ignition Set-Up/Base Ignition Settings'↓↓ Suggested Values ↓↓
Trigger Offset (degrees)
'Advanced Ignition Options'
Trigger Wheel Teeth (M)
Missing Teeth (N)
Skip Teeth
Delay Teeth
'Dual/Multi Spark Ignition Options'
Offset (advance) for Output #2
'Fuel Set-Up/General'
Injection Timing Delay
Spark/Fuel Offset

Results With the actual settings above (not the suggested settings):
For Cylinder in the firing order
The tach pulse occurs on tooth:
The tach pulse occurs at crank position: (degrees)
TDC occurs at crank position: (degrees)
Fuel can start no earlier than: (degrees)
Fuel can start no later than: (degrees)
:1 AFR



©2010 Al Grippo and Bruce Bowling and Lance Gardiner - All rights reserved. MegaSquirt® and MicroSquirt® are registered trademarks.