Martyn,
Very nice work! If you think about the kind of things that cause detonation it becomes easy to see why the singles are so susceptible:
Lean fuel mixture
Advanced timing
High compression ratio
Incorrect octane
Lugging the engine
It is not difficult to get a few of these occurring at the same time, particularly in a single. Detonation can cause wildly high pressures in the combustion chamber as we know. What we are shooting for with the timing is to get the peak pressure in the combustion chamber to occur at 12-14 degrees ATDC. This would mean all of the pressure is pushing down on the piston at exactly the right time. If you think about the dyno, I started at 36 degrees and did a run every time I retarded the ignition a few degrees. I ended up at 19 degrees because the dyno was telling me I was making more and more power (you stop and go back to the previous setting when the power drops.) So you are really measuring where this peak pressure is occurring. So, every time you change the ignition, or the number of spark plugs or the porting or the piston, etc., you need to find out when that peak pressure is occurring.
When you look at a hot rodded engine with twin plugs, squish band and 11.3:1 pistons, the squish band and the twin plugs are not as much "hot rod" parts, but they are detonation preventers. They allow you to run the higher pressures in the combustion chamber without the deleterious effects of detonation. These changes often do not add more power, but they allow you to do the other changes that add more power, like the 11.3:1 piston.
If you think of peak pressure occurring at 12 degrees ATDC you can see that as you advance the ignition you are also advancing the place where that peak power occurs. In fact, you can advance it so much that it is occurring BTDC and actually slowing the piston down on its rise to TDC. This is not only robbing power, but it will wear everything out at an accelerated rate, particularly the big end. So, I hope you can eventually find a dyno as it will give a truly modern timing figure.
David