You're right AV8TOR when I used 87 Octane gasoline on my fuel mix for the Ryobi 31 it would pre-ignite and made a metal like sound, then I changed to 91 octane as you suggested and the ignition got better that is why we recommend using 91 octane gasoline.
On my weedeater 25 engine the fuel workrd just fine at 87 octane so I think that each engine brand has it's own requirements as the fuel mix is concerned.