Reference: US Octane vs The World

I’ve heard a few times that JDM engines run on the equivalent of 100 US octane. I thought that sounded a little weird so I finally looked it up. Turns out, the information is all over the place. However, it seems as though people were mixing their RONs with their AKIs. As with the metric system, Europe and Japan use RON and the United States uses AKI. And AKI is average of RON and MON.

US 93 is equal to 98 RON.

I was a little skeptical of the claimed 100+ AKI octane rating for E85 as well. Pure ethanol is rated at 100 AKI (actually, just under at 99 AKI or 109 RON) and it would need a gasoline mix with an AKI of at least 100 to keep the octane rating from dropping. Again, the information is all over the place for E85. It appears that people are again using RON in place of AKI. My best guess is that E85 is probably in the 97 AKI range.

Math: 85% of 99 AKI is 84; 15% of 87 AKI is 13, and 84+13 gives us 97. Race E85 probably has a higher octane rating, which complicates matters.

US (AKI)Euro/Japan (RON)MONName
E85 (97)10391E85
939888Premium
919587Premium
899484Midgrade
879182Regular

Sources

https://en.wikipedia.org/wiki/Octane_rating#Anti-Knock_Index_(AKI)or(R+M)/2

https://www.goapr.com/support/fuel_guide/

http://www.pencilgeek.org/2009/05/octane-rating-conversions.html

Click to access e85_driver_training.pdf

https://www.researchgate.net/figure/RON-MON-AKI-and-OS-of-fuels-used-in-this-study_tbl1_318813527

https://www.sciencedirect.com/topics/engineering/research-octane-number

Click to access b13_woebkenberg_2-b.pdf

2 Comments

  1. Peter Spinale says:

    Yeah, I’ve had these arguments. “US gas sucks, why do you put up with 93?!?” Umm not the same number as your 93.. “numberz r numberz!!!” However I’m still annoyed by Arizona’s 91.

    Liked by 1 person

    1. A.A.Smith says:

      We are spoiled with 93 AKI in TN. Although, when I lived in MI I would fill up with 94 AKI at Sunoco. There was no ECU tuning for TBird Turbo Coupes so we just cranked up the fuel pressure and ran higher octane with the extra boost. 😂😂

      Like

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