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High Octane Gas Myth

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Old Fashioned Gas PumpI know a lot of people who believe that higher octane gas is better for your car than lower octane gas, as if the words premium next to the higher ratings actually mean the gasoline is better for your car. It isn’t. If you believe it is, don’t be dismayed, you’ve simply become accustomed to reading the words “premium” next to the label and believing the great marketing machine that helped create over $10B in 4th quarter profits for companies like Exxon-Mobil. The octane rating of gasoline is a measure of how much you can compress the gasoline before it ignites, not how “good” it is.

Cars that require higher octane gas actually need that higher priced gas because the engines compress the fuel more before it ignites it. If you put regular gas in a car that requires premium, the gas will prematurely ignite when it’s being compressed and the engine will give you a knocking sound. This is bad for your car.

Chemists perk up…
For all you burgeoning chemists, gasoline is basically octane and heptane, or hydrocarbon chains that are 8 (octane) or 7 (heptane) carbons long. Octane simply can be compressed better (i.e. without the exploding part, at least at the same levels of pressure) than heptane. An octane rating of 87 means it’s 87% octane.

It’s a “rating”…
The reason why higher octane is more expensive is because it’s harder to refine the gasoline so that it contains more octane. Now let’s add another wrinkle… the octane is actually an octane rating, or it behaves as gasoline with that percentage octane would behave but might not actually have that much octane in it. While that doesn’t really matter, it does mean that the gasoline you use could have a mix of other things in it (still real gas though) to give it properties of a higher octane without actually having more octane. Does it matter? I don’t think so but I write software for a living.

(Photo: ella_marie)

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55 Responses to “High Octane Gas Myth”

  1. FMF says:

    Only a small percentage of cars actually need premium gas — isn’t that correct? I bet the % of premium gas sales is way above that number.

  2. jim says:

    I don’t know what the percentage is but this article in the Washington Post makes it sound like even the cars that say “premium only” can adjust their engine compression ratio for the lower octane gas so that the knocking doesn’t even exist. Now, if you do that, you’re definitely going to experience a lower horsepower because your engine could’ve taken advantage of the higher octane but if you drive a Dodge Neon, going up in grade won’t help.

    This is an interesting quote from the article: Chrysler recommends 91-octane fuel for its high-performance and turbocharged automobiles, such as the SRT models, “but all our products will run on 87 or 89,” Quinnell [spokesman for Chrysler Group Engineering] said. Any loss in performance would come when an engine is pushing to its maximum power and speed, he said, so “if an average driver got into one of our SRT vehicles, they wouldn’t notice any change.”

  3. Nick says:

    I drive a MINI Cooper, and just like other cars manufactured by BMW, it comes with the “Premium Gas Only” sticker. I’ve talked to many other MINI owners about whether our cars really need 91 octane or higher. Here’s what I’ve learned.

    Of course, the MINI will still run with lower octane fuel. In fact, it will run without knocking because it has an anti-knock sensor that will automatically adapt if lower octane fuel is used.

    MINI owners who have used or do use lower octane fuels have reported some or all of the following: lower fuel mileage (lower enough that the gas savings on lower grade isn’t worth it), slight to major loss in horsepower, increased emissions, and occasional stalls when starting from a stop.

    Interestingly enough, I’ve also been told by people who use gas HIGHER than 93 octane that they start seeing similar problems. I’ve never personally seen gas with a 94 octane rating, but the few people I’ve talked to who use it say it has the same effect on their MINI that driving with 89 or 87 does.

    Personally, I stick with premium. Especially with these $2.50+/gallon gas prices, it’s worth it to pay an extra 15 to 20 cents a gallon to get an extra 5 MPG (that’s the average difference I’ve read between regular and premium in a MINI). Several MINI owners also prefer a certain brand of premium gas because it has a detergent formula that helps keep the engine cleaner and running smoother between oil changes–especially useful if you own a MINI which uses synthetic engine oil and your oil change is every 15,000 miles.

    More and more everyday, though, I’m thinking about getting a bike instead–one that runs on nice, 0 octane me-fuel.

  4. jim says:

    When I was doing calculations for my car, I found that on a 30mpg car the cost per mile was approximately 7-8 cents. So that extra 5 MPG is worth approximately 35-40 cents on a 30 mpg car and I’ve never seen a price differential between grades of 35-40 cents.

  5. I’ve always bought 87. Never had a car that needed anything else.

  6. FMF says:

    It would be interesting to know the difference in cost over the lifetime of a car — and over a person’s driving lifetime. I bet it adds up to some pretty big $$$$$.

  7. CK says:

    Just buy a siphon and move next door to someone who parks their SUV outside. ;-)

  8. jim says:

    A friend of mine told me they put screens in gas tanks so that you can’t do that anymore…

  9. fusel says:

    Does anyone know the octane rating for vegoil? I came across this post when researching diesel / vegoil conversions and one of the unanswered questions is how octane rating of vegetable oil will affect the diesel engine.

  10. Cap says:

    oh man, this reminds me of stupid kids driving a Civic.. and pumping in “the best” kind of gas.. and when I tell them they’re wasting money, they shrug and say, “I don’t care, my parents are paying for it.”

    then they proceed to wipe out the credit card and swipe it.

    makes me want to punch them.

  11. Matt says:

    One has to wonder who these parents are, who buy gas for their kids to burn. (I bought my own car when I was 18, and on the rare occasions before then that I was allowed to borrow my mother’s, I was expected to return it with the tank full, and to do so at _my_ expense. The only time I wasn’t paying for the gas was when she was driving.)

  12. FMF says:

    Matt — me too. My parents were kind enough to let me use the car whenever I wanted, but I HAD to buy gas.

  13. Ya know, it’s funny. When there was nothing better to do on a Friday night when I was in high school, we used to drive around “burning gas” (that’s what we actually called it). Of course, we had to pay for the gas ourselves, but it wasn’t all that expensive. I doubt “burning gas” is a popular pastime nowadays (although people do it more than ever when just going about their daily life).

  14. Star Money Articles for the Week of January 30

    My favorite posts this week from the MoneyBlogNetwork members and beyond: Five Cent Nickel discusses how the FTC is cracking down on credit repair scams. Blueprint for Financial Prosperity talks about the myth of high octane gas. Consumerism Commentary…

  15. Weekly Roundup – 02/03/06

    Here are some of the most interesting personal finance articles that I ran across over the past week or so…

  16. Al DiDomenico says:

    My son has a 66 mustang coupe 289, 4barrel with headers, father in law told him to use preimum gas I told him it is a waste of money, Who is right?

  17. Zareth says:

    The facts are simple.
    The higher Octane the more it can be compressed
    Also the larger the explosion.
    Combustion engines are simple, Fuel is injected through the fuel rail into the cylinders and mixed with oxygen. The More oxygen the larger the explosion, The Higher the octane rating, the higher the explosion. The higher the gasoline/air mixture is compressed the higher the displacement. The knocking sound in cars requiring premium gasoline is because for example; in a V4 one cylinder fires and displaces, this causes the other cylinder to close and compress the gasoline and oxygen before it combusts. If the ECU is trying to compress it too much it will explode in the chamber before it is completely compressed and it has to change velocity before its ready causing the chamber that was exploding to have to shut again before its ready. Thats bad.
    But in a regular car that does not need “premium” gasoline, using it will never bad bad for the car. It creates more horsepower, Maybe three to four horsepower at the most using the kind of octane fuel you can get from the gas station. But in a car using 110 octane, such as racing fuel, the horsepower gains would be much greater.
    The problem is you need about a ten horsepower boost to notice any great difference in acceleration or top speed so you really dont notice that its helping, but it is.

    • Dick Moran says:

      Using higher octane fuel than that recomended will only INCREASE the amount of cash you spend. Any horsepower increase would come because of a higher compression engine.

  18. Anonymous says:

    I mistakingly bought 91 octane gas for my Lawnboy two cycle lawn mower,does 91 octane burn hotter than 87 octane gasoline.Some one told me it does and can damage the mower. Is this a fact.

    • Brian says:

      Use AV fuel in small engines and you wont have the jets clogging up on you. Also the fuel won’t go bad on you.

  19. CK says:

    Your mower will be fine.

  20. james says:

    Premium gas does not increase power or fuel economy enough to justify the price. This is documented on other websites, and you can try it yourself and keep track over a few tankfuls.

  21. Tony says:

    Fools. If your engine requires Premium, please, for the sake of the car use premium. If you use a lower octane, the car cannot “change the compression ratio”… this cannot happen, its like asking the cars computer to change the diameter of your wheel rim. The computer cannot change the shape of your engines internals. What it does do is pull ignition timing. It detects the engine is knocking and retards the timing (this is called Knock Retard) What this does is cause the spark plug to ignite earlier than it would have. Ever 2 degrees of ignition timing is equal to 5 HP. On a car that requires premium, like a turbo/supercharged or high compression car, you can lose in excess of 20* of timing, thats 50 HP!!!! thats a noticable drop. Also, ignighting the fuel mixture earlier causes the pistion to push down on the cranksaft earlier than it should, causing excess wear on your connecting rod bearings, which are very expensive to replace (and can cause complete engine failure if left unchecked) not to mention, that extreme cases of KNOCK can cause a lean condition or a “chiped” piston, when a chunck of the piston actualy fragments and beats the inside of the combustion chamber untill it looks like the moon.

    Higher octane will not burn higher unless it contains more ethanol. the more ethanol in your gasoline the hotter it burns, and less HP BTW, reguardless of the “octane” rating of ethanol, the required Air fuel mixture is 5 times that of gasonline. HIgher octane should burn cooler.

    Please use the fuel recomended for your car, and leave that decision to the people designing cars, because honestly, lay-people don’t know anything about whats going on inside their car.

    On any car, the higher octane you run, the more timing advance you can run, so as long as your cars computer can command it, and your fuel can support it, you can get more timing + and more HP. (boosted, high compression cars especialy)

    to the mini owner, use the highest octane you can get… 93 is good.

    personaly, i use 94, although im trying to find some 100 unleaded local… and yes, my car needs it. I also get 34-40 MPG highway.

  22. jim says:

    As a general rule, always read and follow the owners manual.

  23. Mr. Bitter says:

    Unbelieveable, knuckleheads without a clue discussing the viability of high octane fuel

    1. The reason you have only had to use 87 octane gas is because modern cars have lower compression ratios and computers with anti-knock sensors. Today’s cars are designed to run on low octane gas. Not only low octane but lead free gas which requires hardened valve seats and in the case of higher performance cars aluminum heads to reduce cylinder temps so that the crap gas will work. – Guess who is paying for all this technology so we can use unleaded low octane fuel? That’s right consumers, and if you think that performance would not be better with leaded higher octane fuel think again, but the design of the car would be different than the crap we drive today.

    2. Older cars require higher octane due to a lack of computers/knock sensors and higher compression ratios. Repeat they require it…unless you lower the compression ratio by swapping heads, gaskets, changing the stroke, piston, and adding hardened valve seats. Higher octane premium is not a “myth” for these vehicles it is required unless you modify the motor.

    3. Newer cars generally require no more than the 91 or 93 that you see at the premium pump. This is useful for turbocharged, supercharged, and higher compression ratio cars and trucks. Read your owners manual it will set you straight.

    4. If you change the boost levels of a turbocharged or supercharged car or add nitrious to the mix you are increasing cylinder pressures (i.e. compression ratio), and you will need higher octane gas. Ricers that are buying this gas may be modified using one or more of these methods. In some cases the best pump gas won’t work, then you need “racing” gas which is now 100 or better. Sadly this gas is barely better than what used in the family station wagon in the 60′s and 70′s. Engine wear was less prevelant with the old gas because the lead that was in our gas lubricated the valve guides and reduced seat temps and prevented valve wear. TEL (tetra ethyl lead) is the best anti-knock compound bottom line. It is still used in AV Gas and Racing Gas. TEL will destroy your emissions system./catalytic converter. Sodium based repacements for TEL exsist but they don’t work as well as TEL.

    5. We could go to Methanol (octane 112 or better) and make tons orf horsepower and burn clean and efficient . Methanol requires 2x amount of fuel for each gallon of gas to but only costs a buck a gallon. So equivalently cheaper too but then the car companies and Government would have to find another way to rip people off.

    6. Your motor uses the octane that it was designed to use. If you “over” octane a motor there is no damage done the motor will hum along fine but will not derive any extra power from the fuel. If you under octane a motor you can count on having to rebuild the motor long before it should be required and perhaps destroy the motor if it’s a high performance application.

    Don’t argue or make comments about something you know nothing about. I read the crap most if you wrote and wanted to puke.

    There is no “myth: premium fuel is a neccesity in some cases and the consumer is paying for the EPA regs through higher fuel prices and all the crap technology that they throw on our cars to burn the no lead low octane fuel. If you had ever driven a real high performance car or race car then you would know when you really need a higher octane fuel, nothing worse than stomping the pedal only to hear the engine destroying sounds of a piston slapping the cylinder wall “ping”. Maybe you guys should discuss the benefits of nitromethane next that will be a real hoot.

  24. MIKE BISHOP says:

    I have been building street engines, race engines, and alky engines with my father at his race shop since I was 11, my father recently passed away and I remember all the times when folks would ask us about weither or not they should be using the higher octane gas.
    First and formost unless you have a engine that has a compresion ratio of more than 9:5:1 IT DOES NOT MATTER!
    We settled this by taking two identical 2002 Z06 corvettes with nearly identical mileage, draining the gas tanks and filling one with 87 oct and the other with 103 oct, then setting the timing equal making sure both cars had the same plugs, wires, filters, oil.
    We dynoed both several times, each car was nearly identical in HP and Torque.
    As far as ye old leaded gas, the lead was used to cushion the valve seats in the good old days cause they didnt have hardned valve seats.
    I make a nice living with the family business, so far after 35 years of serving our community the only complaints we ever get are from the guys who have us build dragster motors and they just keep wishing we told em to put more money into it.
    As far as the gas argument goes there isnt even a noticible difference, on the engines we see that come into our shop for rebuilds, as a matter of fact we get people all the time who cry because they used premium gas all the time and we show them another identical engine that didnt use premium and theres no difference even in the combustion chambers.
    But thats ok, all you premium junkies keep on payin the gas companys for the expensive stuff and lower those gas prices for the rest of us.

  25. hobby says:

    I have 98 boxster and it’s is recommended to used 93+ I tried 91+ and i couldn’t even drive that thing… lol. somecars are made for higher octane and it won’t run as well as it supese to I lived in AZ for while and no gas station has 93+ I had to buy octane booster every time I fill up when I forget??? car drives with knocks and smoke it’s impossible to drive in public…


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