Principle, increasing load

With increasing load (air mass/combustion), the pressure, temperature and turbulence in the cylinders will also increase, as will the combustion rate.

Normal ignition timing at 2000 RPM and wide open throttle can be around 5--6 degrees BTDC, compared with 25--28 degrees with a light load at the same speed.

The difference in ignition timing is reflected in the difference in combustion rate between high engine loads and low engine loads. Consequently, the ignition timing must be retarded at high engine loads.