The combustion chamber shape (squish etc.) and EFFECTIVE compression ratio + chamber size + mixture ratio define the combustion speed. The ignition and spark plug have a certain amount of flame introduction delay. These factors define the required ignition timing.
With tuned pipes the timing most of the time is later due to very high effective compression ratios.
At high rpm, with associated high squish turbulence action, ignition should be later as well, sometimes as late as 15°.

Low compression, lousy design engines can get away with timing as early as 35°.