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From experience owning our 2014 Forester XT (including driving cross country at multiple altitudes), even though the engine "can" run regular per Subaru and the owner's manual, you really do not want to. The ECU can detect the lower octane fuel (and likely knock events from the knock sensor) and reduce ignition timing, etc. When this happens, you get less power (obviously) AND worse fuel economy. This information is spelled out in section 7-3 of the 2014 Forester Owner's Manual. The power and fuel mileage reduction is noticeable, even if only running one tank. So, even from a cost perspective (due to MPG drop using regular), it make sense to always run Premium fuel in the Forester XT FA20DIT.
However, Subaru RECOMMENDS using premium fuel in the Turbocharged Forester XT. If Subaru engineering has done all of their testing, tuning, and other evaluation using regular 87 octane fuel AND recommends using a minimum of regular 87 octane in the Ascent, there is a chance that premium fuel will NOT be needed to meet the published power and MPG figures. With how Subaru has been publishing everything thus far, this should be the case. That being said, using premium fuel for additional safety margin to prevent knock (which will cause the ECU to limit optimum performance) is always a good idea. If Subaru says the Ascent will make 260 hp, 277 lb-ft, and get 27 mpg highway with regular fuel, do NOT expect improved horsepower or MPG above the published specifications by using premium fuel.
In the end, we all need to wait and see what is printed in the owner's manual and see how this new engine responds to different fuel grades. Until then, we are all guessing based on previous experience with the FA20DIT and other conjectures.
However, Subaru RECOMMENDS using premium fuel in the Turbocharged Forester XT. If Subaru engineering has done all of their testing, tuning, and other evaluation using regular 87 octane fuel AND recommends using a minimum of regular 87 octane in the Ascent, there is a chance that premium fuel will NOT be needed to meet the published power and MPG figures. With how Subaru has been publishing everything thus far, this should be the case. That being said, using premium fuel for additional safety margin to prevent knock (which will cause the ECU to limit optimum performance) is always a good idea. If Subaru says the Ascent will make 260 hp, 277 lb-ft, and get 27 mpg highway with regular fuel, do NOT expect improved horsepower or MPG above the published specifications by using premium fuel.
In the end, we all need to wait and see what is printed in the owner's manual and see how this new engine responds to different fuel grades. Until then, we are all guessing based on previous experience with the FA20DIT and other conjectures.