Another thing worth mentioning is that there are several dozen "flavors" of gasoline sold in the US. IIRC there are almost 60 of them for the PRK (People's Republik of Kalifornia) alone. Some municipalities get their own designer formula and some change the formulas with the seasons. Just another thing to partially explain why some folks get pinging on 89 under light load and others get none on 87 even with full throttle.