mark, an ignition map should plateu once you have reached peak timing, which is usually midway on in the rpm range.
ignition timing is purely there to start the burn before TDC simply for the fact that it takes a period of time for the whole chamber burn wholistically and release peak cylinder pressure at a certain crank degrees ATDC, which is dependant on bore, chamber shape, fuel,VE, rod/stroke ratio, CR, AFR so on and so forth. At lower rpm the time period in terms of crank degrees revolution is less than at higher rpm, so requires less 'advance time' for the spark to start. Once you reach a certain point it becomes less beneficial to continue advancing, then detrimental as you either hurt torque because you reach peak cylinder pressures too early or rather too close to TDC, or even before it and generate negative work. Then there is the combustion problems such as pinking, knock, det and auto ignition associated with peak cylinder pressures that arrive too early. Its a downward spiral where you might only get a touch of knock, but that will overheat the piston/chamber which can lead to auto-ignition in a short period etc. Everything is linked.
Mapping ignition correctly is a true secret art, AFR any monkey can do. But to understand ignition timing and what your trying to achieve is a subject that can be as large as engine design it self. If you want to have a go you'll want to get a set of det cans minimum so you can listen to any changes in burn noise.
If you want to do it properly you'll need a dyno so you can keep an eye on BSFC, EGT & AFR on every cylinder, burn noise (det cans). And putting the relationship between them is the key to getting things spot on. I'll explain it fully when you are down next, it sorts the men from the boys
