I am sure that one could (and someone will) bench mount a unit and document the timing schedule RPM vs advance vs MAP.
Yes. It's reasonable to assume Lycoming sets the 390D's base timing at 20, but I remain curious about maximum advance at altitude.
For a 390, I wouldn't take the variable version anyway. Might save a little fuel by cruising in the 8 GPH ballpark, but it will still be slower and hotter than running in the low 9's.
Parallel valve is a different story.
Below was yesterday, on the way to lunch, fixed 23 degree timing, peak EGT. 66%, 187 TAS, 9.2 GPH. About 30 sq inches of cowl exit area, roughly half that of a stock RV-8. Cuts drag, but pushes CHT up a little.
Note the difference in operating philosophy. In theory, we can add ignition advance and pick up power, while accepting higher CHT. Or we can use mild ignition timing, but cut cooling airflow, resulting in the same CHT. I choose the latter because it reduces mechanical stress (lower peak cylinder pressure), and cuts drag.
BTW, I have two onboard maps, one fixed, one advancing, selectable with the flip of a switch. At 22/2450 the advancing schedule (below) triggers at about 28 BTDC...but the airplane doesn't go faster.
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