"the problem is that the dominant variable is virtually unknowable"
This is why I think the poster from the superhonda site chose to make some assumptions to simplify the math and drive toward conceptual understanding . . .
I think you are spot on with the implications of long stroke and low RPM. Page back to Figure 1-12 in Taylor's book. I think such engines have a more shallow 'effective burn angle' and as such can tolerate only so much spark lead.
The one thing that I have always had a hard time getting my head around is the presence of auto-ignition versus 'ping'. I don't think that they necessarily go hand in hand. The idea that 'ideal tune' is where the flame-front smoothly consumes the entire charge just as it reaches the piston head seems overly theoretical to me.
In practice I think that smooth combustion consumes MOST of the charge - with the rest auto-igniting. But since auto-ignition pressures are occurring well after TDC (and for just a small percentage of the charge) there is no 'ping' associated with them . . .
Perhaps this is why advance-till-it-pings-under-load is a persistent rule of thumb . . .
