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91 RON is 87 Octane.
Remember. Octane rating is a fuel's resistance to ignition. You don't want it to resist ignition. You want it to resist ignition just enough that it doesn't preignite. These engines are designed and tuned for 87 octane (at best, if not lower).
The higher octane results in incomplete combustion. That's where the carbon buildup and deposts begin. It's impossible to jet or tune for higher octane fuel without also changing physical parameters of the engine. An engine with an 8.5:1 compression ratio and modest spark advance is going to be the same no matter how much fuel you feed it.
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