The timing curve of the old original syncro spark ignition was just right..4 BTDC fpr starting, linear advance to 28 or 30 at 4000 rpm... .It was designed by Ed Vollmer and sold by C&H....No one has so far been able to clone it..
RC EXL got close, then screwed it up when DLE wanted it changed...
There is no real need for an advance curve on our toy gassers anyway..28 total works just fine except for starting...And then no retard is needed if you use an electric starter.I bet 99.9% of all model airplane pilots couldn't tell the difference between different timing "curves"