timing error test with 99+ 4-tooth trigger wheel
 

I have an AEM which ignores 2 of the teeth on the 99 wheel (because they are unevenly spaced). So I did a quick check and blipped the throttle in neutral while looking at the timing with a timing light (AEM set to timing test mode, always 10*).

Result. As the engine accelerates up, it briefly goes retarded by about 4* (while RPMs are low), and as RPMs climb, it diminishes to 2 then 1*. On decel, it retards by about 2*. This makes sense because at low RPM the effective sampling rate is slower so the error is larger.

I would very much like a 12-1 trigger wheel...