Mustang Tech: The Odds & Ends of Octane & Your Gasoline

Question: What’s the difference between the various octane ratings for gasoline?

Answer: It isn’t a stretch to say our daily lives depend on gasoline; we rely on it for transportation and, for many AM employees and customers, for recreation as well.  So it would seem logical that we would know a bit about the smelly stuff that supports our modern “get up and go” lifestyles, but gasoline octane is one of the top 10 things we get questions about.


It’s all In the Numbers…


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Depending on where you live, you may see anything from 85 to 93 octanes when you pull up to the pump, but what do those numbers really mean for your Mustang fuel system?  Octane is the measure of a fuel’s resistance to knock, aka detonation.  The number on the pump is known as the “Anti-Knock Index” or AKI number, and is the average of the RON (Research Octane Number) and MON (Motor Octane Number).  The research octane number is obtained by running the fuel in a test engine under “low load”, controlled conditions and is typically a few points higher than MON.  Motor octane is found by running a preheated sample of the same fuel in a similar test engine at higher speeds with variable ignition timing, pushing the fuel to its limits.  These results are compared to mixtures of iso-octane (100 octane) and n-heptane (0 octane), giving them their octane rating.

…but what octane should I use in my Mustang?

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If you’re running a naturally aspirated engine with bolt-on mods that don’t significantly affect compression ratio (like CAI, exhaust, throttle body, pulleys, etc) then the OE specified octane is your best bet. Using a higher octane won’t give you more horsepower or better mpg’s – this is one of the biggest myths out there.  So when is the right time to start splurging on higher octane gas?

  1. If you’re tuned – you can always run a higher octane than you’re tuned for, but never EVER run a lower octane – this can severely damage your engine.
  2. If you’re spraying – nitrous raises compression ratio and combustion chamber temperature (sorry about the confusion, guys). Because of this, lower octane fuels are likely to cause knocking or pinging (detonation).
  3. If you’re blown – don’t laugh! Running a low octane fuel in your turbo- or supercharged motor can cause premature catastrophic failure. (Ok, now you can laugh)

Also – never run leaded gas (like race gas) in a car with catalytic converters.  Tetra-ethyl lead (the additive found in “leaded” and race fuels) is a popular octane booster that was replaced with MTBE and ethanol for environmental and health concerns in the 90’s.  Leaded fuels will result in an untimely death for your cats and your oxygen sensors.

Knowledge is the Key…

Many Mustangs are given early death sentences because of uneducated drivers, but hopefully this will educate and answer an all too common question.  Running low-octane fuel in a car that should be running 91 or 93 will put it in the ground well before its time.  Conversely, spending $40-$50 for a tank of 93 is just wasting money if your owner’s manual specifies 87. Now that you’ve been enlightened in the ways of octane – go share the Gospel of the Gas!

  • Kevin

    My stang is tuned to run Sunoco ultra 94 here in Canada. Do you not have 94 octane in the states?

    • VonBlinden

      I’ve never seen anything higher than 93 except for at the race tracks down here.

      • Will

        Same here, at almost every station I fill up at the highest I see is 93. Although at one point in time I do believe there was one station that did carry 94.

        • Chris

          Sunoco does sell 94 Octane in some locations here in the United States.
          There is also a company called DeltaSonic in Western New York that sells 101 at the pumps. Talk about juicin’ up your octane!

  • Jeramey

    In Michigan and a few other places where there are sunoccos they do have 94!!!

  • Dolnack

    Doesnt gas with higher octane help clean the engine up alittle or is that the ethanol?

    • Chris

      higher octane will allow for a more complete burn of the fuel mixture in your combustion chamber and will help “clean off” any unburned residues in the combustion chamber. Gasoline is found nowhere else in the engine but the combustion chamber(at least it shouldn’t be), so it will clean the combustion chamber, but not the entirety of the engine itself.

      • Heather

        Actually Chris, the cleaning properties of gasoline are due to detergent additives, not octane – this is a common misconception. Higher octane fuels do not necessarily burn more completely, but rather in a more controlled manner – they are less likely to combust from compression (detonation) than lower octanes. A complete burn is more dependent on factors like spark quality than octane.

  • Dyno Don

    How does nitrous increase compression ratio? I though that was combustion chamber volume and piston height.

  • old turlinga

    NOS does not change the compression ratio (as you said that is a function of the size of the chamber and piston top design) but it does increase the combustion pressure and temperature, so a higher octane is recommended to prevent detonation

    • Heather

      You are correct – NOS does not change compression ratio, just combustion chamber temperature and pressure. That was an error on my part, sorry guys!

  • TBI Stang

    Although I see almost everyone say that higher octane doesn’t give you anything better on a non-tuned car, I disagree. No matter what anyone says, I see at least an extra 1-2mpg average on all of my cars along with better response. I could run 87 or 89 in any of my cars for one tank and then run 91 and REALLY notice a difference.

    • Will

      Actually TBI is right. I’ve never really noticed any difference in power, but running 89 or 93 each time that I change the oil, and everyonce in awhile between changes I have gotten better gas mileage and the car always runs smoother.

      • Heather

        It is possible to see a *slight* improvement in mpg or performance by running a higher octane fuel, however 1-2 mpg is not something the average driver would notice. You also have to keep in mind that there are many other factors that determine mpg. In most cases, there will not be a *significant* improvement in mileage or performance from running a higher octane fuel.

  • http://americanmuscle sheldon williams

    better mix it with some 87 octane av gas is to dry and hard on a stock fuel system av gas doesn’t have the lubs that the system needs
    I only run av gas in my 1/4 mile car