[charlie66]

OK, this is really pretty simple, so let's just put the facts out there:

Mixture burn rate is mainly dependent on fuel type and octane, and cylinder pressure. Simple as that. Increasing octane gives you a slower more controlled burn, and increasing cylinder pressure (more dynamic compression, more boost, etc) speeds it up. There are other factors like chamber design, etc. that can have a smaller effect, but in the interest of simplicity, let's just go with the big ones. Ignition advance is nothing more than the tool used to make the big boom/peak pressure/complete burn/whatever happen at the right point of crankshaft rotation. Most things you read on average/conventional Otto Cycle reciprocating piston engines will tell you that this point occurs 15-20 degrees ATDC.

Every dynamic load and speed state of a given engine will have an ignition timing point that yields maximum brake torque. Also very simple. Advancing the ignition past this point will create pressure spikes, negative torque, and detonation, and retarding it from this point will cause loss of efficiency and excessive exhaust temperatures. Too retarded is mechanically safer than too advanced, but neither is optimal. Running an engine with excessively retarded timing is, well, retarded. There just isn't any reason to do it given modern accurate electronic engine controls, especially those with knock control capabilities.

Retarding timing excessively just to use a higher boost pressure than you normally could for a given dynamic state is counterproductive. No good tuner does this. A book could be written on the how and why, but just think about the things written above and give them careful logical consideration.