C&H Ignitions "SS" or Synchro Spark ignitions electronically retard the timing from the fully-advanced position verses advancing from the fully retarded position. C&H uses auto-retard logic and senses within the first flip of the prop what is required. C&H Ignitions are timed at full advance and retard from this setting. Others are timed at say 4 degrees and advance either electronically or mechanically from this setting. Others are fixed, this is a compromise between starting and maximum power, with max power suffering.

Our engines can accelerate from idle to full power in maybe 250 milliseconds. At an average rpm of, say, 3000, the engine fires 50 times per second, so in 250 ms. you'll see something like 10 to 15 power strokes. So if the timing must increase linearly (it doesn't which makes it more complicated) you need to advance the timing something like 2 to 3 degrees every rotation. This is quite a bit of change.

If set up for electronic auto-advance, the electronics module gets a signal from the crank position sensor/pick-up coil long after the time when the spark should have been generated. Therefore on that particular rotation it is too late and the only thing it can do is fire the plug immediately, so you get at least one cycle when the timing is off. The ignition module has to remember that the engine is accelerating, remember the rpm, and try to anticipate at what point the spark should be fired for the next rotation. As you can see things can change pretty rapidly, a full rotation could see the engine no longer accelerating.

The whole Auto-advance operation is some guesswork and a lot of computation to get it right. In an environment with rapidly changing RPM values some error may occur. The calculations are much simpler and error is much less with an auto-retard system. Auto-advance works, but it's not quite as precise in all conditions.

With auto-retard the module can calculate the firing angle over just a few degrees of crank angle in the current firing cycle. Auto-advance systems inherently must calculate over the last full 360 degrees of crank rotation and then average that with several previous cycles. This is because when auto advance senses in the current firing cycle that it should have fired, it is already 20 odd degrees too late to do so.

Auto-retard provides extreme accuracy and anticipates rapid increases in RPM and adjusts the timing for better engine response. This is achieved because the calculation time required is so minuscule as compared to the ignition firing rate.

Auto-retard requires an extremely accurate calculation be done extremely fast because it occurs within the current firing cycle. Auto-advance requires only remembering past cycle(s) and guessing what the next cycle should be. Both methods can be accomplished with discrete components. Newer auto-advance systems use micro-processors to reduce part count and increase accuracy.