It's all about detonation resistance... when you have a high-specific-output powerplant like the VHR with high compression and timing pretty near MBT from the factory, you need the extra detonation resistance of 91+ octane fuel. With a factory tune, you'd probably be OK running mid-grade but the ECU would pull timing due to a minimal amount of knock. 85 octane would be a serious no-no unless you never rev above 3000 rpm at half throttle.
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'10 PW 370Z 7AT - wife's car - Project Raspberry Cheesecake
'08 Evo X - built motor+trans | BB-X @ 32 psi | CH3OH | self-built+tuned - ??? WHP
'01 Galant V6 - built motor | GT35R | self-built+tuned - 550+ WHP (retired) |