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I have a 2019 Accord sport 2.0 and live in a high elevation area. My home and commute to work sits 3900-4300 feet above sea level.

Due to high elevation, gas stations usually offer 85 fuel, then higher ones. Am I harming my engine if I put 85 and remain at high elevation? I know the owner's manual recommends 87+, but 85 should act the same at this elevation right?
 

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I'm in Chinle, AZ, which is Four Corners area - the town here sits at around 5900 ft. elevation I think. And of course, it ranges from 5000 to 7500 ft where I usually travel around in the area. It's odd you're seeing 85 octane at that low an elevation. The lowest I've seen is 86 octane and only start seeing 85 when you get into the 8000 ft. range.

I personally have never fallen for it. If 87 octane is not the lowest grade available, I suck it up and pay the extra for mid-grade, with that being usually 88 octane.

In theory, lower atmospheric pressure leads to lower cylinder pressures and less of a need for the anti-knock additive (octane). In practice, I don't think anyone has definitively produced a great explanation for or against using lower than the manufacturer's recommended octane at high elevation. Using lower octane MIGHT be okay in a NA vehicle, but FI engines are a whole different bird. Under no circumstances would I use less than 87 on this engine (or any turbo engine), no matter the elevation. The ECM has a barometric pressure sensor, so the ECM is fully aware of the atmospheric pressure you're operating at. And unless you know how Honda has programmed the turbo in terms of boost pressure gradients and how it's programmed to change at higher elevation, I'd not mess around with lowering the octane in the fuel. Turbo boost is electronically controlled and can be increased at higher elevation to offset the negative effects of the lower atmospheric pressure. Of course, there comes a point where the added heat from extra boost follows the path of diminishing returns. But I don't know what or where that point is. Honda has done their research and I'm sure they know, but Honda engineers are a lot smarter than I. Point being, using lower than recommended fuel can add an unpredictable variable in there when the ECM is dialing in boost, spark timing, fuel injection, etc. Not only that, but 85 is REALLY low for 3900 ft. elevation.

Is it damaging? Maybe. You have a knock sensor that the ECM monitors and it will retard timing in the event that it detects preignition. So, it will prevent further preignition events from happening, but in order for the ECM to recognize the preignition via the knock sensor and take corrective action, there have already been some preignition events that have occurred. Preignition is a very chaotic and uncontrolled burn with multiple flame fronts colliding within the cylinder (the "ping" you hear), it's hard on the ring lands and burns through the boundary layer at the cylinder walls. I.e., bad stuff is happening and it's stressful on important parts and causes some pretty intense heat transfer to those important parts. If allowed to continue then you bet it's damaging (as in a pre-knock sensor era vehicle).

Why risk it for the few cents / gallon?
 

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I trust my knock sensor to work. I am much more afraid of blowby coating the valves, so I did a PCV mod. Capped the intake off.
 

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I'm with AccordULEV here. I think your elevation is too low for 85 octane to be the gas you use. I buy my gas at an elevation of 5000 ft and the lowest octane they sell here is 87. Yes, I've used 85 octane at 6000 ft and above, but, I probably only fill the tank twice with this octane before I put in a higher octane fuel, such as 88.

- Jack
 

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You can blend. That is put in 85 and a higher octane to achieve at least 87 or some other desired octane.
 
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AlanTheBeastV2
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