Ki-84 was designed for 90, and was running on 84, if that. I'd not be surprised if the late war crap was even below 80 octane.
The Japanese fuel in '44 and '45 was complete junk with stuff like pine additives to try to boost it. We had crushed their ability to refine even 87 octane.
The Ki-84's engine was designed for fuel the Japanese couldn't produce anymore. In addition it was designed to use Methanol/Water injection, the lack of which in the US test was used to justify the higher octane American fuel used.