The magnums (5.2, 5.9) came in big heavy trucks and SUVs and ran just fine on 87 octane. But they had long slow advance curves and knock sensors that were very active. If you’ve played with the timing at all and added a performance style curve you may find that it rattles at 9:1 with iron heads and 87 octane. But you won’t know until you test. The answer to test each fuel and use the lowest octane that the engine doesn’t rattle on is the correct answer.
