change ignition timing

[Mariman]

I love my S-80 2001 2.9, but now I use lower octane petrol (because the price differ a lot in my country), the engine sounds knocking, I know it is because of different ignition timing between low octane and high octane petrol, low octane petrol need an ealier ignition than hi octane.
Is anybody in this forum can help? how to set the ignition timing? thank you.

regards,

Mariman.