GAS - High Octane Truth or Myth?
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This is a discussion on GAS - High Octane Truth or Myth? within the Everyday Impreza Talk forums, part of the Community - Meet other Enthusiasts category; 2006 Subaru WRX. I am sitting here debating Octane Fuel ratings with a coworker. I want to open this up ...

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    GAS - High Octane Truth or Myth?

    2006 Subaru WRX.

    I am sitting here debating Octane Fuel ratings with a coworker. I want to open this up to the community...

    The gas door says Premium Fuel Only. Truth or Myth?

    Disadvantages? Hazards? Efficiency?

    Please respond.

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    Registered User rex01752's Avatar
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    Very very true. My brother put reg. octane in my 02 and it ran like crap. It was mis firing i had no throttle response and no boost what so ever. It also threw all 4 misfire codes...it is a bad idea. If it happens on accident go to autozone or some autoparts store and pick up some octane boost that fix it for me.
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    If you know how car engines work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1.

    The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking is BAD. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

    The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.

    The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.

    It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.

    During WWI, it was discovered that you can add a chemical called tetraethyl lead (TEL) to gasoline and significantly improve its octane rating above the octane/heptane combination. Cheaper grades of gasoline could be made usable by adding TEL. This led to the widespread use of "ethyl" or "leaded" gasoline. Unfortunately, the side effects of adding lead to gasoline are:

    Lead clogs a catalytic converter and renders it inoperable within minutes.
    The Earth became covered in a thin layer of lead, and lead is toxic to many living things (including humans).
    When lead was banned, gasoline got more expensive because refineries could not boost the octane ratings of cheaper grades any more. Airplanes are still allowed to use leaded gasoline (known as AvGas), and octane ratings of 100 or more are commonly used in super-high-performance piston airplane engines. In the case of AvGas, 100 is the gasoline's performance rating, not the percentage of actual octane in the gas. The addition of TEL boosts the compression level of the gasoline -- it doesn't add more octane.
    Currently engineers are trying to develop airplane engines that can use unleaded gasoline. Jet engines burn kerosene, by the way.


    Last edited by TheJ; 05-28-2008 at 04:56 PM.
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    TRUE there's NO debate.

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    I imagine this question will continue to come up with gas prices rising so I'm going to go ahead and add this to the NMH FAQ.

    The only possible "myth" out there, is when people think there is some super advantage to putting higher octane fuel in an engine that does not require it.
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    If your coworker has a car that says "premium fuel only", then tell him to put in regular gas and find out what happens.
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    Quote Originally Posted by TheJ View Post
    I imagine this question will continue to come up with gas prices rising so I'm going to go ahead and add this to the NMH FAQ.

    The only possible "myth" out there, is when people think there is some super advantage to putting higher octane fuel in an engine that does not require it.
    you mean recommend it? There are cars that run on 87 fine but the manual recommends (doesn't say require) 91+ which will make a good difference.

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    Quote Originally Posted by infox View Post
    you mean recommend it? There are cars that run on 87 fine but the manual recommends (doesn't say require) 91+ which will make a good difference.
    A lot of cars are capable of running on many octanes of fuel. They have different ignition and fuel maps depending on which octane is being run. The car will try to run the aggressive map and if knock is detected then it will run a less aggressive map.

    The reason it says recommended is because you will get more performance, from the higher octane allowing a more aggresssive map to be run.
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    J, that was a great article

    good find
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    Jet engines burn kerosene, by the way.
    This statement is true in a sense. Commercial Jet engines use a fuel called Jet-A (or AVTUR). It is an Unleaded/paraffin (kerosene) hybrid fuel that is very similar in chemical structure to Diesel fuel.

    Military Jet engines use a fuel called JP-8 (Navy uses JP-5) which is ref. in MIL-DTL-83133F. "Description: Kerosene type turbine fuel which will contain a static dissipator additive, corrosion inhibitor/lubricity improver, and fuel system icing inhibitor, and may contain antioxidant and metal deactivator."

    Having worked around AVGAS (AV 100), Jet-A, and JP-8; I can tell you firsthand that all three have their own unique properties and dangers. Jet-A and AVGAS (though very different) are fairly stable fuels with a relatively low odor to them. JP-8, when exposed to it for extended periods of time (10+ min) can make you feel nauseated, light-headed, or dizzy. The smell remains with you for quite some time (upwards of a day in some cases) and is very oilly.

    /threadjack

    OP, the 87 octane will not destroy the engine, however it will cause decreased performance, decreased mpg, and could result in extra maintenance. Most of these are very nominal.
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    87 Octane

    I own a 2006 WRX limited wagon. I commute 120 miles when I do go to work which is usually 4 times a week. I recently started using 87 octane which contains up to 10% ethanol. This was after always using 92 octane for 42,000 miles. If you live in the Chicago/Milwaukee area you've been living with this wretched gas for well over 10 years. Don't even get me started on the politics of this and all boutique gas blends, thats for another forum. With 92 octane I was averaging a little over 25 mpg. I'm still averaging very close to that, within 1-2 mpg. I can't say for sure if it really has any mileage effect since construction to Chicago changes daily. I can say that I have never heard any pinging and here's why.

    Most all newer engines (within the last 10 years) have knock sensors that actually "listen" to the engine frequency. It is located below the throttle body, near the tranny. It bolts to the block. Knocking will produce a specific frequency that when detected by the knock sensor it sends an electric signal to the ECU/ECM to adjust timing and eliminate the knock. So fear not your WRX is smarter than you think. Run a tank of lower octane and see for yourself. One tank will not damage your engine. If you want more proof research "subaru knock sensor" they're about $80.

    My father is a retired engineer who worked for Mobile. He always told me that there isn't an engine built these days that can't run fine on low octane gas, even with the junk corn in it. He also says that there are enough detergents in any blend of gas you can get to keep an engine clean. So don't buy into the cleaner higher priced gas, it's a lie.

    If it's all about performance for you then use the higher octane gas. After all if you do the math it's not a huge difference. My tank holds 15.2 gallons and if 87 octane is $4 and 92 octane is $4.20 then the the difference between a full tank is $3.04.

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    Quote Originally Posted by jcaputo View Post
    +1 Definitely read this thread! Jcaputo did an excellent study that proves Premium > Regular.

    Always use 91+ and your subie will be happy.
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    Quote Originally Posted by subyman34 View Post
    I own a 2006 WRX limited wagon. I recently started using 87 octane which contains up to 10% ethanol.
    Doesn't ethanol have some type of octane boosting effect, and did you tune your car for that?
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    Yes, ethanol boosts octane, but only 10% won't do a whole lot and they might be telling you the effective octane with the ethanol added (i.e. it was 85 octane, but with 10% ethanol now it's 87). We have 93 octane here in idaho, but I think that is counting the 10% ethanol they add to the mix.

    I run 85% ethanol and that has handled 33psi spikes on my car no problem. Unfortunately your fuel system has to be rediculous to handle it. I am now at 98% duty cycle on 1150cc injectors and two fuel pumps with a base fuel pressure of 50psi, holding out 31psi.
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