octane question

[guinness77]

First a little background....
I got my 400ex in 04 and ride HARD every week. After a leakdown test and compression test revealed that the top-end is definitely showing that it's in need of a rebuild, I'm having it done. I'm going for a stock (or near stock) piston size. They might have to hone out .5 mm or so, but it's no biggie to get the right piston. It will run at about 9.1 compression.

Meanwhile... I'm having a Curtis Sparks timing advance key installed. This will advance my spark timing by 6 degrees in the piston stroke.

What this boils down to is that the fuel/air mixture will have MORE TIME to burn. So I have to go with a litte higher octane. My question is what octane range will work? Bone-stock I run premium grade pump gas. I think that's 97 but I might be wrong. [It's been so long since I checked]. If I'm in a money crunch I'll get regular 87 octane pump gas and it's works with just a little knocking. I'm certain that 111 is wayyy too much. What sounds about right to you?

What is the likelyhood of pre-detonation? I heard that will ruin a motor pretty quickly. With that much advance I'm certain pre-detonation is likely.

Any input is welcome.

Thanks!

[GPracer2500]

You should be just fine with premium pump gas.

BTW, the abnormal explosion of air/fuel mixture you're referring to (as oppose to the normal controlled burn) is just called detonation, not pre-detonation.

[guinness77]

OOps. ... detonation. Thanks!