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MegaSquirt This/Next Ignition Mode

Only for use with the MS1 Extra code (MS1 - 68H908 based microprocessors)

By Philip Ringwood (daxtojeiro), James Murrey (jsmcortina) and Ken Calver (muythaibxr)

Please Note:

All of these instructions / diagrams are to be used at your own risk, like most things there is more than one way to do the same thing, what we have tried to do is to offer a method that we have tested or that others have tested for us. No warranty expressed or implied.

Use at your own risk.

MegaSquirt requires a pulse from the ignition to syncronize the fuel injection. This signal provides the position of the crank and may be used different ways to provide spark during normal running and during cranking. There are two modes of operation; Next Cylinder Mode and This Cylinder mode. The mode is determined by MegaSquirt from the entry of Trigger Angle. . If the Trigger Angle is from 0 to 20 then MegaSquirt is in Next Cylinder mode. If the Trigger Angle is from 50 to 90 then MegaSquirt is in This Cylinder mode. We will look at how these two modes work, review the advantages and disadvantages, and then show some examples of how to implement each..

To fully understand how these two modes operate we have to look at a timing diagram when normally running, and when cranking. In the below diagram the engine is running, the spark advance is 25 BTDC. Waveform 1 is in Next Cylinder mode and waveforms 2 and 3 are in This Cylinder mode. The timing of the IRQ-1 at the output of the MS opto isolator is shown for waveform 1-3. In the Next Cylinder mode IRQ goes low from 20 - 0 BTDC. In This Cylinder mode that signal goes low 90 - 50 BTDC. In waveform 1 the signal goes low at 10 BTDC, the Trigger Angle entry is 10 and therefore is in Next Cylinder mode. In waveform 2 & 3 the IRQ goes low at 60 BTDC, the Trigger Angle entry is 60 and therefore these waveforms are in This Cylinder mode.

In both modes and all three waveforms, the time to begin charging the ignition coil, and the time to fire (spark) the ignition coil is calculated. In the Next Cylinder mode that calculation determines the time to charge and fire the Next cylinder. So, even though the trigger occurs at 10 BTDC (Trigger Angle) of cylinder #1, the trigger determines the calculation and timing for the spark at #3 (4 cylinder engine with 1-3-4-2 firing sequence)

In This Cylinder mode the IRQ goes low at 60 (Trigger Angle) which is well before the spark needs to happen for This cylinder. The trigger in this case determines the timing of the spark for #1 cylinder. The spark occurs quickly after the Trigger Angle. Waveform 3 is a special case of waveform 2. In waveform 3 the time between the Trigger Angle (falling edge at 60 BTDC) and the rising edge (Trigger Return) is greater than 50. This characteristic can be useful for cranking. Waveform 1 & 2 are representative of the Crane trigger wheel. The plastic Crane trigger wheel was modified to achieve waveform 3.

The width of the coil voltage pulse is the dwell time. This is a fixed time for charging the coil when dwell is on. When dwell is off it is a fixed angle.

You can view the trigger on the front panel of MegaSquirt. The second LED on the front panel (Normally showing warmup ) can be set to show the current status of the trigger (IRQ) to make it easier to adjust the distributor. This LED is only of real use for hall sensors and optical sensors and needs to be configured in TunerStudio. On Menu Codebase and Outputs function set LED18 (D15) to IRQ trigger.  

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