1) Use the lowest octane possible that your engine can run without detonation.
2) Octane = flame retardant
3) In theory, water is the best octane, that's why WRC cars use water injection to cool down the intake air.
... true... you only need high octane for high compression engines, so that when the pistons compress the air/fuel mixture it wont explode because of the extreme pressure...
...plz nobody try injecting water, hehe water DOES NOT compress thats how you can hydrolock your engine, fill your cylinder with water, when the piston goes up the water is stronger than the connecting rod and , BAMMMM your engine is gone... hehe