As you speed up the burn in the cylinder by more compression, more squish, more velocity in, more pipe and more turbulence you have to back off where you ignite the mixture. So the more suped up the engine is, the less advance it needs and may need the timing backed off less than stock. True with 4 strokes and 2 strokes.
2 strokes have another dynamic going on however. Low rpm the pipe is off resonance and we don't get the "supercharging" effect. This low pressure range of engine operation can be made more lively by giving it a bit more advance. Once the engine is up in the powerband range however, cylinder pressure is at a maximum and we need the timing backed off for optimum power and the life of the piston. If we fire the flame up early, it has lots of time to heat up and cook the piston. Advanced, the fire starts off slow and picks up pace as we cross Top Dead Center where turbulence kicks in, so often we can do this without preignition or detonation, but it is still applying heat to the piston.
Whereas when we delay (retard)ignition, leave the mixture cool, the cool mixture sucks the heat out of the piston, heating and compressing and stirring the mixture until the last moment when we fire it off. It fires in a controlled flash, is only an instant on the top of the piston, and is out the tube, which heats the header and pipe, raising the rpm band even higher.
Try it, especially with a modified machine.
2 strokes have another dynamic going on however. Low rpm the pipe is off resonance and we don't get the "supercharging" effect. This low pressure range of engine operation can be made more lively by giving it a bit more advance. Once the engine is up in the powerband range however, cylinder pressure is at a maximum and we need the timing backed off for optimum power and the life of the piston. If we fire the flame up early, it has lots of time to heat up and cook the piston. Advanced, the fire starts off slow and picks up pace as we cross Top Dead Center where turbulence kicks in, so often we can do this without preignition or detonation, but it is still applying heat to the piston.
Whereas when we delay (retard)ignition, leave the mixture cool, the cool mixture sucks the heat out of the piston, heating and compressing and stirring the mixture until the last moment when we fire it off. It fires in a controlled flash, is only an instant on the top of the piston, and is out the tube, which heats the header and pipe, raising the rpm band even higher.
Try it, especially with a modified machine.
