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The Ascent runs great on 87 Octane (minimum), but I am wondering if there is any reason to upgrade to a high octane while towing. This my first turbo so I don't know if there is any cutback in power to prevent preignition since the turbo is running pretty much full time.
 

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As improbable as it seems, based on all of the data that's broadly available (from tuners like Cobb), there really is no power to be gained by using a higher octane. In other words, the computer really isn't pulling any or much timing or enriching the mixture for this engine to cope with 87 octane. It's either a really good engine design that's not prone to preignition or detonation, a really conservative tune from the factory, or (most likely) both.

Until the Ascent, I haven't owned a modern car that didn't benefit from a higher grade of fuel. Most factory tuning I'm familiar with seems to take the approach of running the most spark advance you can and using the knock sensors to keep it from giving too much...and running a higher grade of fuel allows the computer to give more timing to the engine than it does on 87 because of the increased resistance to knock. I can measure this directly in my 2009 Honda Ridgeline with scan tools and I can see it indirectly in my average fuel economy numbers. I've driven that vehicle for about 60k miles now and it just runs better and gets better fuel economy when using 89, 91, or 93 octane. I usually do run 87, but the difference is there.

Back to the Ascent -- I haven't felt the desire to try a higher grade of fuel in it because all information says it won't make a difference. One of the more compelling things I've seen was a dyno run by Cobb with the stock tune and they showed the spark retard due to knock and there just wasn't any.
 

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Some manufacturers like Audi and Mazda rate and publish their turbo engine's horsepower with regular and premium grades of fuel. Maybe it's an attitude/marketing angle, but Subaru doesn't do that so I wouldn't unless I heard detonation. Although if I lived or drove at high altitudes I might normally use higher octane.
 

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Unless you buy a premium fuel tune and what you need to use it from someone like Cobb, there's pretty much no benefit to premium fuel other than "maybe" more detergents with some brands and the benefit of paying more. There are folks who swear they get "more" of various things, but in general, nope.
 

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For the most part, I use 87 when towing this (though I did have a time of using Shell 93, since it has twice the detergents of their 87, and more than anyone else tested)...

7293
 

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Question: Over time will there be any problems when switching grades back and forth?

Been using 87 like Subaru recommends.The only time I mixed grades was on a old Mazda many moons ago,eventually the MAF sensor went bad which Mazdas were well known for back then.
 

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Question: Over time will there be any problems when switching grades back and forth?
Shouldn't be any issues with that...other than some tanks costing more than others. :) :D
 

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