Originally Posted by Blaize
2. I have no idea why or what it is even for
The combination of Trigger Angle and Trigger Angle Addition describe to the Megasquirt the physical relationship between the CAS and the crankshaft. Specifically, you are telling it how many degrees BTDC the crank will be at when it receives the trigger pulse (CKP) from the CAS, so it knows how many degrees of engine rotation it has to wait (as computed in microseconds) before it actually fires the ignition to achieve whatever spark advance it wants.
Put simply, the CAS needs to be set up so that it gives a trigger pulse which is further advanced (further before TDC) than the maximum possible spark advance plus the maximum dwell time. So typically, we see trigger angles of 60-70° BTDC. In essence, we tell the MS "When you see a trigger pulse, that means that the crank is at 65° BTDC (or whatever),"
and from that, the MS will figure out how long to wait to fire the ignition.
Now, the reason that there are two different fields (angle and angle addition) has purely to do with some internal software constraints. The code can't deal with advance angles larger than 90°, so if you find that you actual advance angle is greater than 90°, you select one of the standard adders, subtract that from the actual angle, and type in the difference. So if your actual trigger advance is 100° (impossible on the Miata) you'd select an addition of +22.5, subtract 22.5 from 100 (giving you 77.5) and then enter 77.5 into Trigger Angle.
In the Miata, we typically don't need this. So we leave the addition at 0, and just enter the actual advance angle.