The octane depends on the compression ratio and the boost. The owners manual should be a good guide. If you have done any modifications to the waste gate for more pressure, then a higher octane and timing should really be spot on. The lower octane will give you better bang as it does explode faster, but under higher pressures, a hot spot will probably occur in the combustion chamber, probably a sharp edge on the piston and that will cause the fuel to ignite early or the same time as the spark plug goes and the two flame fronts colliding causes that pinging sound.
You try to use the lowest octane you can for the best power before detonation. Anything higher and the fuel burns slower and not completely all the time. It is a pain sort of but after I finish the car and want to get it as close as possible, I take a twelve mm wrench with me. make a few hard runs somewhere and if it pings, I retard it some, just small movements and if it doesn't ping the I advance it a little bit. You can do this at any drag strip, but you have to wait for openings, so I use any un-used street. I don't need to go crazy with speed, but a stong pull in 2nd and 3rd should give some idea.
Good luck
Russ
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