91 vs. 93 | Fuel Tips

By Staff Writer
Feb 16, 2022 | Sunoco, Fuel Tips, Octane | Posted in Features | From the Dec. 2018 issue | Never miss an article

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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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David Elfering
David Elfering GRM+ Memberand New Reader
11/16/21 8:37 a.m.

Every time I see a site discussion about E85 or even E15 fuel and read through the details, it talks me out of farting with it. It would be interesting to know if there real benefits other than hopeful butt dyno ones. 

frenchyd UltimaDork
11/16/21 9:08 a.m.

In reply to David Elfering :

Then there is the matter of enforcement. 
  How often do you see engines torn down at any event?   How often do you see fuel sampled? Do they even have the ability to determine what's in the fuel?   It's not just alcohol that can give an edge. 
    Maybe because Vintage racing is gentlemen ( and a few ladies) racing each other. There is no prize money or really any advantage to cheating. In fact In 40 plus years I've never seen anybody tear down an engine or sample fuel.  
     I've built small block Chevy engines up to 430 cu in. With parts straight off NASCAR  and WoO shelves.  I've also successfully raced  ( and won) with a Target master engine straight out of the crate.  
  It's a rare entrant who doesn't have at least something of dubious legality. The only question is will that person get caught and lose that $15 plastic cup. 
As far as advantage of E85?  You don't read Hot Rod  or watch U tube do you?  Or even read the articles in GRM ? 

z31maniac MegaDork
11/16/21 10:51 a.m.
David Elfering said:

Every time I see a site discussion about E85 or even E15 fuel and read through the details, it talks me out of farting with it. It would be interesting to know if there real benefits other than hopeful butt dyno ones. 

It's super simple. Most cars for E85, you literally just put in the GM fuel sensor, the rear O2 plug monitors for E content and tune. 

For a BRZ, you can call COunterspace Garage and get a full tune and kit for around $1300 IIRC.

I just used the OFT since I used it on my previous NC. It's a little more involved, but still not difficult for the quick 15-20whp pickup with no other mods. 

Colin Wood
Colin Wood Associate Editor
11/16/21 1:31 p.m.

In reply to David Elfering :

If you are looking for some actual hard data using E85, we were able to squeeze abut 20 extra wheel horsepower in our C5 Corvette project car–which you can read more about here: E85 Ethanol Fuel: How to Corn Your Way to More Horsepower

For additional information, here are some articles discussing E85 with our good friends at Sunoco:

jharry3 GRM+ Memberand Dork
2/16/22 10:15 a.m.


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Ultra 94 Premium Gas - Nothing Less Will Do | Sunoco

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