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Running higher octane fuel on a vehicle not designed for it doesn't really do anything except empty one's wallet faster...octane merely burns slower so it serves to reduce pre-ignition (knocking) on engines that have higher compression, etc. The Ascent is engineered for optimal performance on "regular" fuel.

OP, I've read in a number of places that at higher altitudes, the Ascent turbo can perform well with 85 octane fuel which is sold in your area. It's supposedly equivalent to the 87 octane we run down here in the "low lands".
 

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I'm in Salt Lake and we, too, have 85 available. Discussed this with my dealership and they also said no 85. I run 87/88 or 91 at Costco.
 

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I’m kind of surprised to see people running 91 octane in this car... You’re just wasting money unless the car is designed for higher octane, and there’s no evidence that the Ascent is.

Costco in my area offers 85 or 91 and nothing in between. It is a top tier gas. Since I can't put 85 in my car, I put 91 if I'm at Costco. If I'm at any other gas station, I put 87/88 (varies per station). I'm not the type to put in higher octane just because - but Costco tends to be cheaper and top tier and convenient. Sometimes you just have to use what works! :)
 

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For you folks who actually asked Subaru about the 85 octane fuel available in your area, did you specify in your question that you are operating at high altitude?
 

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For you folks who actually asked Subaru about the 85 octane fuel available in your area, did you specify in your question that you are operating at high altitude?

Yes. My dealership is also at high altitude ~ 4500 feet. 85 is "regular" here. I specifically asked this question and the response was something related to the turbo engine but I cannot remember the exact wording from 10 months ago. I just knew I was going to have to choose 87+ for this car.
 

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I only run premium/mid-grade when it's hot out and towing otherwise 87. Helps prevent LSPI which these engines are known for being they are turbo. Sure it will run it will run on 87, but will it run at it's full capacity?? They have specifically designed an oil to combat LSPI which says alot about these small displacement turbo engines and how they run.

Pulled from another site:

Remember this: with gasoline engines on OBD-II design now runs on individual over-the-plug top coil configuration (can control individual cylinder ignition timing and mapping), and pretty much all of them comes with a knock sensor on the block as a feedback mechanism back to the ECU, typical engine ignition mapping is designed to dynamically run the ignition timing to just before the 1st sense of "preignition" knock is being sensed. Given the higher octane rating gasoline resists knocks better than 87, and given the ignition timing is dynamically "Read" and actively adjusted, it is of no surprise that by using higher octane gasoline (resists preignition better) the ignition timing will advance a bit further than that of 87, but again, it will stay within the "Safe" zone where the just-before-knocking sound is being picked up by the knock sensor.

Even newer, all computer controlled german diesel engines are like that nowadays, with adjustable diesel pumps and knock sensor to constantly read and adjust the spray timing and duration to minimise that dreaded "clacking" sound and gives better power output and exhaust control.
 

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With naturally aspirated engines the higher you go, the air density goes down and that allows the lower octane to be used without issues.

With a turbo charged engine, you have the turbo shoving air in until it gets up to the pressure the turbo system is designed for. Therefore it still needs the same octane at altitude as it does at sea level as the pressure inside the engine is the same. When I'm up at Flagstaff (8,000 ft) my boost gauge reads the same as it does here in the Phoenix valley (2,000 ft).

During WWII they made heavy use of turbo and super chargers in the planes to allow them to fly higher without loss of power.
 

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This questions was already asked SoA. They recommend 87 or higher, at any altitude.
 

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Air is less dense at altitude. Air and fuel is what is mixed in the engine to make it go. The theory is, that, if the air is less dense then the fuel needed can have less of an octane rating
most modern cars have sensors to compensate for altitude
car will know how to work with 87 at higher altitude just fine , but using 85 if you go in lower altitude you can end up outside designed specs
 

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From what i understand these new 4 cylinder turbos are always running at the ragged edge and the computer makes a lot more adjustments than a naturally aspirated engine. So i'm thinking when the temps and altitude rise i'm better off with a higher octane to try and reduce Low Speed Pre-Ignition while towing near it's max capacity when engine is running near is it's max. Had performance issues while towing last year in 6% grade in 90 plus temps...it was struggling while trying to pass. I've now moved to mid-grade/premium while towing and think this might help.

As engines are transforming, so are the issues associated with them.
Engines these days are built to work smarter, not harder, to meet rigorous fuel economy and emissions standards without eliminating performance. You may have heard terms like turbochargers and gasoline direct injection (GDI). These technologies are found in over 55 million cars today and have improved horsepower and torque without reducing fuel efficiency.

But with that greater efficiency, newer engines are starting to experience issues that were unheard of in older engines, including Low-Speed Pre-Ignition (LSPI). Sounds complicated, right? The damage caused by the condition could be scarier than the name itself.

Let us break it down for you.

Technically speaking, LSPI is an abnormal combustion event caused by the higher in-cylinder pressures common in modern engines — those that are turbocharged and/or GDI — while operating under low-speed, high-torque conditions. If you’re currently driving a vehicle with one of these engines, pay close attention to what we’re about to tell you.

Imagine you’re sitting idle in your vehicle and press the gas pedal to head down the street. Now, picture the combustion chamber of an engine – where the magic happens. What should be happening is a small explosion from the mixture of fuel and air which causes the pistons to move up and down. In modern engines, the explosion can happen too early. The fuel droplets are released into the combustion chamber and ignited before the piston is in the correct position, potentially causing catastrophic damage.

The good news is there’s a couple of things you can do to help prevent LSPI from occurring. The first is choosing a motor oil that combats the abnormal combustion event. The combustion chamber needs to be lined well with quality oil that can take the extra heat.

Automakers are aware of these issues and have pushed the American Petroleum Institute (API) for new motor oil requirements to address them. In response, API recently published a new classification (“SN PLUS”) which adds LSPI protection to the host of performance requirements listed in the current SN category. While API has not begun granting licenses for “SN PLUS” products yet, Valvoline is leading the industry with a line of passenger car engine oils that exceed the LSPI protection requirements of this new classification.

In addition to choosing a quality oil, be sure you’re using the proper fuel for your vehicle. Check your owner’s manual for the manufacturer’s recommendation for your type of engine.

Bottom line is, your modern engine has great benefits. But preventative maintenance is just as important as with older vehicles. Be sure to follow your manufacturer’s recommended routine maintenance schedule to help keep your ride in great shape.

https://team.valvoline.com/diy/maintenance/what-you-need-know-about-low-speed-pre-ignition-lspi
 

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You’re wasting money on the 91 octane. It’s not a BMW. I live in Santa Fe, elevation 7,000ft. I have been running 87 in both my Outback and Ascent. Just don’t buy the cheap stuff. Now if I could get my old body to agree with this altitude I would be very happy!
 

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DO NOT LISTEN TO Jim in PA on this subject. Jim, even at high altitudes (I live in CO), the Ascent is not designed to operate at octane 85. If you care about your warranty, I strongly recommend sticking with octane 87. Anything above 87 isn’t necessary.

**PS I work for Subaru of America.
*I also drive an Ascent Touring, and know the ins and out of it very well.
 
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