Actually, bore size does affect timing requirements, albeit not very much. The advance in timing is there to allow peak pressure to build in enough time toe power stroke as possible by allowing the mixture that little bit of extra time to burn. That's why higher compression domes (which also have better designs that promote burn) require less advance to make the most of their design.
To make it simple, the more volume that the flame has to encompass, the greater your ignition advance requirements will be for any given powerband, so yes, bore size will affect ignition timing REQUIREMENTS, not the setting. That flame has to burn through the mixture and will generally do so at a constant rate of speed throughout the RPM range unless other factors are altered. That's why the DT200 has 8 degrees of advance at lower RPM ranges and adjusts to 30 at the higher RPM range.
I witnessed a guy adjust ignition advance on a Chevy 350 strapped to a dyno to see what setting provided the most power. IIRC, he made the most power with the advance set at 32ish. I talked to him later on and with Dart heads installed, he made more power at mid 20s that he did at 32. Now, Dart heads make some serious power (or can be made to allow some serious power), so I can assure you that the combustion chamber design was much more efficient.
Another little gem that I'll donate is that you can tell just how efficient a head design is by how much an increase in ignition advance affects power. On a dyno, you can do this by increasing advance until you start to see power fall off and then go back some. If you have the total advance at 18 and a jump to 20 makes a slight increase, but the jump from 20 to 22 falls off quite a bit, you have a pretty good head. However, if a jump from 18 to 20 makes a slight increase and from 20 to 22 makes it only a very, very little bit worse, your head design isn't as good as it can be. Don't get caught up in the numbers, but when you see a broad range (5 degrees or more) where power isn't affected, what's happening is that you are getting into that range where advance is where it needs to be, but pumping losses are starting to take that power back away. This generally occurs when timing is pretty high (40 or more), but it does happen. A Blaster, for instance, is more sensitive to ignition changes with a modified dome than a stock one.
To be honest, an 20% increase in displacement by stroking an engine will require less additional timing than one increased 20% by boring. That's because the extra clearance between the piston and dome that is necessary because of the extra volume doesn't put the furthest gasses as far away as boring the engine.
There are several factors that affect ignition requirements, but they all boil down to combustion chamber volume, combustion chamber contents, combustion chamber geometry, and when the engine wants to see peak power (AKA, RPM).
yesterday was the first run and i go out with a friend here are a pair of pics
