Why High Octane Doesn’t Matter (Unless It Does)

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Dimly-Lit Gas StationIf your car calls for 87 octane gas, you only need to put in 87 octane. You won’t get any benefit by putting in a higher octane and it will only cost you more. If your car calls for premium gas, put in premium gas. You can do damage to your car if you put in a lower octane and you’ll understand why in a moment.

Most cars have a four-stroke gasoline engine where the strokes refer to the cylinders moving up and down. One of the strokes is what’s known as the compression stroke. The piston compresses a mixture of air and gasoline before it is ignited by a spark plug. Octane rating of gasoline refers to how much that gas and air can be compressed before it spontaneously ignites, the lower the octane the less it can be compressed before igniting. Premium gas can be compressed far more than regular gas.

Why you should only buy what your car needs. If your car calls for 87 octane gas, that means it’s going to compress that air/fuel mix to a point where 87 won’t ignite on its own. If you pay more and add in 89, you get no benefit because it’ll only compress that mix to the 87 level. You get no added benefit because your engine can’t take advantage of the higher compression ratio.

Why you shouldn’t skimp and buy a lower octane than required. Your engine operates most efficiently when that air/fuel mix explodes when it’s supposed to explode. The whole timing of the engine is fouled up when it explodes early and that’s what happens if you put regular gas into an engine designed with premium in mind. When 87 gas is compressed to 91 levels, it’ll explode prematurely and foul up the timing of the engine (this is known as “knocking”).

Why people think higher octane is better. Because it is better! If all other specs are kept equal, an engine with a higher compression ratio will have greater horsepower. However, you need an engine that is operating with the higher compression ratio. If you have an engine compressing for an 87 octane fuel and you put in 91, nothing changes except you’re a little bit poorer.

Gas prices have come down the last few months (whew!) but you aren’t doing yourself any favors by getting fuel your car isn’t designed to use.

(Photo: riza)

{ 13 comments, please add your thoughts now! }

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13 Responses to “Why High Octane Doesn’t Matter (Unless It Does)”

  1. Miranda says:

    I never really thought about WHY I was using 87 octane, as required by my car. I always did, but we have been considering using premium. Now, though, I know there is no point.

  2. I can recall arguing with a guy in my high school Geography class about the merits of high octane gas. He was *sure* that his car was faster using premium. 🙂

    The only exception to not buying higher octane fuel for a car that calls for 87 is if you’ve got an engine knock, a not-uncommon problem on older cars. Upping the octane level will often fix this problem. Actually, to be more accurate, it just alleviates the symptom (the engine knock) of another problem.

  3. philip says:

    When you pay the premium for the higher octane most companies also add in some other additives with it. These additives can help keep the engine clean and to burn well. It will not make for lower performance either.

    Personally I like to put in a tank of mid-grade fuel once every 8-10 tanks to take advantage of this. If this is completely useless I am not certain but it is from my understanding. Kind of like shell advertises that theirs runs cleaner and does not gunk up as much, it is all about additives that are put in basically the same time they fill the truck that brings it to the stations, otherwise the stuff is all the same.

  4. Fred says:

    Good article. minor note–the piston moves up and down inside the cylinder. The cylinders don’t move relative to the car..

  5. jim says:

    Hahaha good point Fred.

  6. Miss M says:

    It’s another thing to consider when buying a car, whether it requires premium, since that will add to the total cost of ownership. I bought a car that requires premium (turbo), when gas neared $5/gallon for premium I was starting to wish I bought a car that ran on 87.

  7. peter says:

    I have a 97 ford explorer that was starting to idle terribly and almost stall. I kept pondering what repairs would need to be done on it, then i decided to try super unleaded. It now runs and idles great – doesn’t feel like it will stall. For this older car, better idling means paying a bit more at the pump and not paying a shop.

  8. Carla says:

    Its good to know that I’m doing the right thing by putting 91 in my car (that actually requires it). Its an ’07 Acura TL S and I know my SO would kill me if I put anything less in it! I will be glad when we move out of the area and sell it though! Its a great car, but selling it means one less expense for me and pollutant on the road.

  9. Patrick says:

    Back when I was in high school, I had a firebird that required 91 octane. It was really hard to put more expensive gas in my car considering I really didn’t have much money at the time. At least now I have a car that uses lower octane gas and is much more fuel efficient.

  10. Chris says:

    While this is accurate, it is important to consider that you can use lower octane fuels without harming your car if needed.

    Modern cars utilize a computer to retard ignition timing if sensors indicate that the fuel is detonating early (aka knocking). Retarding the ignition timing reduces the tendency to detonate, but also reduces power output and fuel efficiency. So putting a lower octane fuel in your car will actually reduce your efficiency (and probably require you to purchase more fuel per mile driven) but will not harm your engine over the short term.

  11. This post has been featured on the 89th Carnival of Money Stories at Retire at 40.

    I too have never seen the point of putting higher octane in your car. When I first bought it, someone told me this and I’ve kept it with me ever since.

  12. JR says:

    Does the 89% octane fuel produce less carbon after ignition than the 87% octane.. Please reply.

  13. Rebecca says:

    I have a 07 VW Passat wagon and the cap says 93. So my stupid question is one I already know the answer to. Putting a medium 91 in the tank will also be damaging to the engine? LOL. I can see people ready to bite my head off. But I thought that I might get an educated answer from a nice soul=-) Oh, and I was reading a forum where someone said the Shell near them has discount days where they sell premium for the price of regular…I have not found anything of the sort on my internet searches…anyone who knows something about that I would love to hear from you…

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