United States Vs. European & Asian Octane Ratings
In Europe 98-octane gasoline is common and in Japan even 100-octane is readily available at the pumps, but this octane nomenclature is misleading to Americans as foreign octane ratings are derived entirely differently from their own. So, like every other measurement system it seems that everyone else uses a different scale than the United States does, but unlike most other instances where we have had the good sense to create different units of measure(e.g., inch vs centimeter), in this case we all use the same name: Octane.
Japan and Europe use a system called RON or Research Octane Number to determine the octane rating of their gasoline, while stateside we use a system called AKI or Anti-Knock Index to determine gasoline's octane rating. To further complicate things, the AKI system is actually derived from the average of the RON system and another more complicated system referred to as MON or Motor Octane Number.
To recap, The USA's methodologies for measuring gasoline's octane rating are different, but share some common elements. With the commonality of RON in mind a good rule of thumb is:
Multiply the foreign RON Octane rating by 0.95 and you will have the US AKI equivalent
( RON Octane Rating x 0.95 = AKI Octane Rating )
98 RON Octane x 0.95 = 93.1 AKI Octane (US measure)
100 RON Octane x 0.95 = 95 AKI Octane (US measure)
So, as you can see the 93 or 94 octane fuel we are all paying an arm and a leg for is actually quite comparable to the higher octane fuels found in Europe and Japan. The people whom have to worry about low octane rating are our friends out west in places like California that top out at 91 octane.
91 AKI Octane (US measure) = 95.5 RON Octane
100 Octane need from a non-US perspective is still quite high, though, as we are generally unable to easily obtain 95 Octane at the pump. As that number increases, it is more difficult to find it in unleaded variants as well. If your JDM car/engine calls for 100 octane, you'll probably be just fine running it on pump 93, especially if it retains sensors like knock and so forth which will pull timing if it's sensing detonation or incomplete combustion. If you're going to modify the engine, you should always dyno tune your car for best results and proper fueling.
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