UPDATE: A Ford spokesman has explained that while horsepower in the Mustang EcoBoost will be down when using 87 octane fuel, as will be the case with virtually any engine when using a fuel with a lower octane rating than the recommended level (in this case 93), the torque figure is unchanged at 320 pound-feet. Below is the full comment:
“We’ve no notion where this graphic originated, but if real, it may have been produced (as many training materials are), before final power certifications were complete. It is incorrect as the torque figure of 320 lb-ft does not change when using 87 octane fuel in the EcoBoost Mustang (lower octane fuel generally only affects horsepower numbers which occur at higher rpm). The 2.3-liter EcoBoost produces its rated power of 310 horsepower using its recommended fuel (93 octane), and like all modern engines, will proactively adapt to a lower octane fuel mixture. We publish power figures based on our recommended fuels, but peak horsepower would be reduced when using a fuel with a lower-than-recommended octane rating.”
Early reports of the 2015 Ford Mustang EcoBoost hinted at the notion that the power figures would be significantly lower when an owner filled up with 87 octane fuel. When running the good stuff, the EcoBoost engine produces 310 horsepower and 320 pound-feet of torque. Should you decide to fill up with 87, you can expect a power output of 275 horsepower and 300 pound-feet of torque, at least that's according to a Ford training manual obtained by Mustang6G.
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If you're filling up your turbocharged Mustang with 87, you deserve less power as it should be clear that such a motor wants to drink the good stuff. Yes, the car is capable of running on 87 with no issues, but just be ready to be a bit slower. This is why you see disclaimers in car ads, because automakers get their best horsepower ratings when they run higher-test fuel.
Now, this should be even less of a surprise with the EcoBoost Mustang because a Ford Motor Company [NYSE:F] engineer allegedly came out and said to expect a 13% power loss. The actual power loss figures shown above aren't that dramatic. The horsepower figure is down 11.3% and the torque is down 6.3%.
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The moral of the story? Gas is cheap right now, so enjoy the better fuel and make your Mustang happy.