Fuel Selection and Maintenance at Altitude - Colorado

Rabbit

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I've got a 2017 Civc Hatch LX CVT and recently moved to Colorado. Altitude in my area is 5550 ft. Using a standard 3% loss per 1,000 feet, I'm estimating that I loose around 15% of my power.

My question is about fuel. In Colorado, "Regular" fuel is 85 Octane. Not to be confused with E85; 85 Octane. This is because at lower pressure, fuel is more stable and detination less likely - or something like that. I have run 85, 87, and 91 with similar results. There is a feeling of lag with the 85, but no issues with knock. 91 makes the car feel more responsive but this might be in my head. I usually use 87, as that is the Honda recommended octane rating. But there is no * about operating at altitude.

I can actually get 91 octane cheaper than 87 at the local Costco. Costco only has 85 and 91 nearby, and other non-discount stations offer 85, 87, 89.

So, do you think I should run 85 ($2.20), 87 ($2.95), or 91 ($2.75)?

Are there any concerns for the long term health of my civic that you would have while operating at altitude? Let me know your thoughts.
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latole

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Rabbit

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Something I'm considering is installing an aftermarket intake to help improve the uptake of oxygen and help curb some of the decreased performance and lost fuel efficiency.
 

tacthecat

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85 is fine and it'll be difficult to gain back the HP/TQ lost by the change in altitude.
Remember my '68 RoadRunner being a slug when passing through Albuquerque during a cross-country jaunt.
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