Fuel Tips: 91 vs. 93

By Staff Writer
Aug 21, 2021 | Sunoco, Fuel Tips, Octane | Posted in Features | From the Dec. 2018 issue | Never miss an article

Paid article presented by Sunoco.


The Rule: A lot of enthusiasts autocross in the SCCA Street and Street Touring classes, where the rules require a “fuel which is ‘Federally approved for use on public highways,’ and which does not exceed an octane rating of 93 AKI (Anti-Knock Index = [R+M]/2) with an allowed variance up to +0.9. Fuel may not exceed 15% ethanol (E15).”

The Availability: Those in the western half of the country usually only have access to 91-octane fuel, while 93 octane is fairly common in the eastern half. All things being equal, an engine can be tuned to make more power on 93-octane fuel than 91.

The Issue: At the Tire Rack SCCA Solo Nationals, drivers come in from both coasts–some carrying 93-octane fuel and some 91. Lincoln, Nebraska, the site of the event, lies in 91-octane country–although find93.com, a site dedicated to locating 93-octane fuel, shows a pair of outlets in Lincoln.

The Realty: The Street Touring ranks allow reprogrammed ECUs, and some newer production cars can automatically retune themselves based on what they’re fed. In both cases, the higher-octane fuel would be a plus. The Ford Focus ST owner’s manual lays out the advantage in black and white: 252 horsepower on 93 octane and 243 horsepower on 87 octane. “If this is a linear difference, that would put 91 octane making 249 horsepower,” says Zachary Santner, technical specialist at Sunoco Race Fuel. “How about a 3-horsepower difference for East Coast versus West Coast fuels?”

The Possible Edge: Ethanol may not increase miles per gallon or help a fuel’s shelf life, but it can yield more power, making it a possible way to gain an edge using those same SCCA rules. “The science says that the E15 is worth it because the fuel contains more oxygen,” Santner continues. He can’t say how much extra performance will be seen at the wheels, but tuning to a fuel containing 15 percent alcohol would, at least in theory, deliver more power than one containing 10 percent, the usual standard these days.

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