What is Octane and why does it matter?

Fuel gauge

You’re driving along when that little indicator lights up, letting you know that it’s time to fill the tank.  You pull up to the pump and have to choose from 3 or 4 different “flavors” of fuel, from the benign “Regular” to the spirited “Super” or “Premium”.  Now the only apparent difference in these fuels, other than price (we’ll talk about detergent properties at another time) seems to be this posted Octane rating, ranging from 87 or so for the regular, to 94 for the Premiums.  What exactly does this Octane rating mean though?  Will using a higher octane fuel give you better performance?  Will using a lower octane ruin your engine?  Before answering these questions, let’s first learn what octane is and why it’s in your fuel.

If you’re not familiar with how your engine works, take a look at How Stuff Works for a good explanation.

Octane is a hydrocarbon in the same family as butane, methane, and propane.  When added to fuel along with another hydrocarbon, heptane or it’s equivalent, the combination causes the fuel to become more resistant to spontaneously igniting under compression in the cylinder.  You can hear this ignition when it happens, your engine will be “knocking”.  Why is this important? Well, your engine is a well-tuned piece of machinery and when the explosions in the cylinder happen out of sequence it can cause serious damage to the internal workings.  Generally speaking, the number on the pump represents the percentage of octane to heptane in the anti-knock mixture.  87 rating is 87/13 octane/heptane, 93 is 93% octane and 7% heptane.  Those of us who have been around awhile will remember the days of leaded fuels.  Tetraethyl lead added to fuel allows for even higher compression than 100% octane, however due to the environmental impact, and the damage that leaded fuel does to the exhaust system on a modern car, it has been banned for use motor vehicles.

91 requirement 90 requirement

Different engines have different compression ratios, usually this means the higher the performance capability, the higher the ratio.  Therefore, higher performance engines will require a higher octane rated fuel to perform safely and optimally.  Usually this will be noted near the gas cap, or on the dashboard by the fuel gauge.  For example our minivan has a 3.8L V6 engine, the compression ratio is 9.6:1, my 1400cc sportbike has a compression ratio of  12.0:1 and a minimum octane requirement of 90.   In any case the manufacturer will have supplied their minimum octane recommendation for your vehicle, and it’s always best to at least match what they specify.  The engineers know best.  Using a fuel rated higher than specified won’t give you any better performance, but using one with a lower rating can lessen your performance, and even worse cause major mechanical problems.  Personally, I only use a brand name premium detergent gas in all of my vehicles, those that need it and those that don’t, but that’s just me.  Whatever you drive, I hope you’re better able to make the best choice for you and your vehicle.

1 Comment

  1. Bruce Sallan (@BruceSallan)

    What does my Volt take? High octane electricity? JUST KIDDING…I don’t have a Volt…

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