My understanding is that unless your engine is experiencing detonation or knocking, there is no reason to purchase a higher octane fuel. It does not improve performance. The only thing I could think of is if the knock sensor is detecting detonation and retarding timing even with a 91 octane fuel? Does the Z actually benefit from a higher octane than recommended in the owners manual?
Is there a difference in the oxygenation of Chevron vs V-Power? I could definitely see that affecting performance. Maybe V-Power doesn't contain ethanol?
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