I can also confirm that rotor phasing is not an issue when using an SC distributor in a road car.

If, however, you have a high revving early race car, the reduced cap diameter can mean that the phasing could start become an issue once you get above 8000 RPM in a true multispark system (like our CDI+). The phasing can be dialled in on the cap by filing the location tab, allowing the cap to be turned and set. On our system, the time before the second spark can also be reduced. This is all best done on a dyno.

Because the MSD doesn't deliver multiple sparks above 3000 rpm, it doesn't have so much of an issue.

I have the Electromotive XDi twin plug system on my 3.3SS engine. Compression is about 10.8:1, large port CSI system, Carrera large port heads and 964 cams. The Electromotive manual suggested a starting advance curve of 10 degrees initial, 22 degrees added at 3000 rpm and -2 on the top end. After lots of "twiddling", I now have the advance set at 5 degrees initial, 20 at 3K with 0 retard (total advance of 25 degrees). The engine idles well and pulls like a scalded cat all the way up to the 7500 rpm ignition cutout. I found that anything more than 5 degrees initial advance made it difficult to get the engine to idle at 950 rpms. This is the best setting that I have found to date, but will keep on experimenting!

I'm exposing my ignorance here...but..

I was trying to figure out how the MSD box got the rotor position data to determine the degrees to advance? Meaning, does it just time the triggers from the dizzy and interpolate degrees from that, or is there an actual measuring source that you install (like a trigger wheel)?

I guess that spider would give 60 degree triggers, and it might be able to interpolate from there, but it seems like calculating 1 or 2 degree rotation with a rapidly accelerating timing source would be hard. Maybe it's not?

Yes, it's as simple as that, i.e. just determine the time between trigger inputs and convert that
to a RPM, and then have a lookup table for the advance. The next spark is determined from the
next trigger input and the process is repeated.

The algorithm used can compensate for rapid RPM changes.

One such algorithm (MS) is described here:

Alpha-Beta-Gamma Filtering

As long as you have enough data (e.g. multitooth crank wheel sensor) you can make that one work. With only three trigger points per rotation, the prediction algorithm becomes a little trickier.

There're other algorithms that can be used that solve the basic issue without a ring gear type
of input signal.

Hello,SmileWavy
i come back with this post because i would like to lock the distributor like snbush67.
I have an distributor for a porsche 2.7 with points and i would like to install a hall effect sensor.
If your read my message, is it always ok with your plate in alumunium:)? No problems any more?
Thank you for your answer.
Regards

No problems with the lock-out plate that I installed or the position that I locked the distributor advance at.