Some engines will run fine on lower octane fuel without retarding timing to avoid spark knock. Some lower octane fuels might not have the cleaning additives though there are aftermarket cleaners that work fine. Generally, if an engine calls for 89 octane, using 91-93 is a waste of money. And, generally, if an engine calls for 91-93 octane the timing will be retarded if lower octane is used. Depending on how much timing is retarded the performance of the engine can be negatively impacted. The discussion of whether it matters should be apples to apples. If the question is about a 50i it is best to compare to another 50i. Comparisons are also somewhat faulty as they are not on the same road, same way of driving, same temp, same humidity etc. While impractical for most of us consecutive dyno runs with the considered octanes should be accurate.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas