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What Octane Should I Use?

13K views 16 replies 10 participants last post by  TSiWRX 
#1 · (Edited)
So, a lot of people on FB in the Ascent Groups have been asking what octane they should use. It's 87 octane (or higher), everywhere on the planet, regardless of climate, and regardless of altitude.

That means even if you live at the highest elevations in the country, you NEED at least 87 octane, or you risk damaging your engine.

Thanks to our turbos and computer and sensors, even at the top of Independence Pass (12,095 feet), our cars can easily get the same oxygen/air levels as it would at sea level. Thus, it requires the same octane fuel as at sea level. 87. Or better.
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I explain why here:
 
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#3 ·
85 and Coors? Lol!!! Watered down all the way. 😁 :ROFLMAO:
 
#7 ·
I'm too lazy to go look yet... Does octane change with altitude? When we were traveling last year (pre-covid) I kept to the 87 or above number even when we were in areas of higher elevation, and regular gas was below 87. Does the 87 from lower altitude stay 87 when moving up? Or should I be buying higher octane if I am planning to go to altitude to cover changes?
 
#9 ·
Always 87 octane, no more than 15% ethanol, no matter where, how high, or what you're doing with a stock tune.
5047
 
#11 ·
I'm not quite sure my point was understood - if I buy 87 octane gas at sea level, and then on the same tank of gas travel to altitude, is it still 87 octane? I.e., does octane number depend on altitude? Or would I need for example 89 sea level octane to have 87 octane when transported to altitude?
 
#12 ·
It is always 87 octane, at any elevation, for any use. Anything above that is optional. Anything below that should never be used.

Octane ratings are not based on altitudes.
 
#14 ·
Thank y'all; that mirrors what I thought I understood through searching - that older carbureted engines could handle lower octane at attitude, in that there was less air, the mixture would then be more fuel rich relatively and less prone to detonation/pinging. So gas could be sold with lower octane at altitude with no real problems. But now, we need 87 or better, no matter the altitude being drive.
 
#15 ·
Naturally aspirated engines can get by with lower octane at altitude because the maximum pressure reached in the cylinder during the compression stroke is less. Higher compression pressure requires higher octane. Turbocharged engines can increase and actively control inlet pressure so are not limited by ambient pressure (within reason).

Modern naturally aspirated engines with closed loop mixture control (all current engines that meet emissions reg) automaticlly adjust the mixture for high elevations and so are not running rich, though they can still use lower octane gas due to the lower pressure.
 
#17 ·
Note that in the 2023+ Owners Manuals, Subaru specifically writes that 91 AKI is recommended for towing to reduce the risk of engine overheat - courtesy of @beanboy:


I read from your thread, @ifly021956 , that your Ascent is a 2021. Currently no-one has yet come forward with anything official from Subaru as to why they made this higher octane recommendation in the '23+ vehicles, only speculation, and of-course, noting that it is Subaru's official recommendation.
 
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