The timing value sets when the ignition system triggers. There are a myriad of things that follow that trigger event, but, fundamentally, they contribute to the phase/load/frequency relationship that is eventually transmitted to the propeller. Due to the functional differences between magneto ignitions and electronic ignitions, an electronic ignition can alter that phase/load/frequency relationship in a meaningful way, by potentially altering how quickly and completely the mixture is ignited and combusted, even if the trigger angle is identical to that of the magneto.
This is not a purely theoretical consideration. Test data supports that this can make a difference to propeller loads and that the resulting differences in loads, and whether that difference is of concern, can be specific to the propeller, engine, ignition system combination. Unfortunately, the test data available at this point in time does not support making any more definitive of a statement as applied to EI systems in general; it would not be sufficient to publish a range of timing settings independent of the ignition system specifics. Specific propeller, engine, and electronic ignition system combinations that have completed sufficient testing to be determined to be compatible are listed in the tables in Manual 193, Volume 1. Testing of additional configurations is ongoing, but takes time.
Propellers are subject to vibratory loads created by the engine and are therefore subject to fatigue. These vibratory frequency orders are often high, leading to a large number of accumulated cycles that must be sustained (or avoided with an operating restriction or life limit). The relationship between stress amplitude and frequency (S-N) means that small increases in stress at high frequencies can lead to reductions in component life. It is for this reason that we must be so specific and careful about changes to the system. As engineers, when we say a system is compatible it means that we have determined that the design life is not compromised, that the system (as specified) is safe; we must therefore restrict ourselves to the conclusions for which we have sufficient data. An untested combination may be safe, it may be unsafe, it may be safe if used with an operating restriction, but without the data we don’t know and cannot make a statement of compatibility.
Our compatibility determinations are made with the ignition set up per the manufacturer's recommendations. This means that for magneto ignitions, the timing is set per the type certificate (data plate) for the subject engine; usage of timing settings other than those are, just like other modifications, not determined to be compatible. For electronic ignitions, the EI is set up and operated per the EI manufacturer instructions and predetermined timing profiles. Customized user variations to base timing and/or timing advance profiles are unlikely to be supported due to the significant variation possible, but this is still TBD as we continue testing.
Manual 193 was born in the engineering department and is the result of us here at Hartzell, some of whom are homebuilders ourselves, wanting to communicate helpful information for the experimental community and to promote safety by providing a documented resource of compatible combinations and the factors that alter compatibility. Manual 193, Volume 2, which should be published soon, will attempt to explain a little more of the "why" behind some of these factors, though much of it is paraphrased here as it relates to EI. We intend to revise and add to this manual as we complete more testing. We always welcome feedback on the effectiveness of our communication.