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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; How "octane" is calculated varies from place to place. It's possible that you have a better grade gas there in ...

  1. #46
    Administrator RayfieldsWRX's Avatar
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    How "octane" is calculated varies from place to place. It's possible that you have a better grade gas there in Norway, but you can't determine it by that number.
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  3. #47
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    Quote Originally Posted by TheJ View Post

    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.


    93 octane is not 93 percent octane 7 percent heptane. This number in the US is used by determining the anti knock index. Also known as Ron plus Mon divided by 2. Or seen at the pump as r+m/2. Wikipedia.com search octane rating .

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    While the octane thing is not a myth, I have been running regular in my 06 WRX 95% of the time since i bought it in 2009. I now have 127000 miles on it and use it for commuting to work and back. I think the fact that the static compression ratio is low and todays computers are so good at managing timing and knock, you can get away with it for commuting provided you drive with restraint.

    I know people have claimed that they get better mileage with super than regular, I have not seen it. It's the exact same mileage regardless what I use. I average 25 in the winter and 26-29 in the summer. Higher if I ad hear to the speed limits and take care with braking.

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    UnBanned Sinister's Avatar
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    Still wouldn't do it. What's the point of driving a WRX if you're on that onramp, want to gun it, but can't because you decided to save 3 bucks on your tank of gas and fill up with regular
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    Quote Originally Posted by zore View Post
    While the octane thing is not a myth, I have been running regular in my 06 WRX 95% of the time since i bought it in 2009. I now have 127000 miles on it and use it for commuting to work and back. I think the fact that the static compression ratio is low and todays computers are so good at managing timing and knock, you can get away with it for commuting provided you drive with restraint.

    I know people have claimed that they get better mileage with super than regular, I have not seen it. It's the exact same mileage regardless what I use. I average 25 in the winter and 26-29 in the summer. Higher if I ad hear to the speed limits and take care with braking.
    If you read up on how subaru controls knock, you will understand that this is a bad idea. Sure, it is still ticking, and I don't want to freak you out, but pull a plug and with a magnifying glass you will see little metalic beads on the electrodes. That metal used to be part of your combustion chamber. I have seen them when my initial cobb stage II flash had problems, and the DAM was still at 13 for only a short period. Yours is probably lower, and you are doing it a lot. These motors can go 300K plus miles. Yours may not. One really bad knock can detonate your motor too, typically at the rings.

    The ECU always tries to add back timing. It pulls it when it see knock, but it doesn't just leave it out. It keeps trying to put it back assuming you will have loaded the proper gas eventually. So, it is always knocking. It really isn't good and I would recommend you stop doing it.
    "From a little spark may burst a mighty flame." - Dante
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    Quote Originally Posted by mycologist View Post
    If you read up on how subaru controls knock, you will understand that this is a bad idea. Sure, it is still ticking, and I don't want to freak you out, but pull a plug and with a magnifying glass you will see little metalic beads on the electrodes. That metal used to be part of your combustion chamber. I have seen them when my initial cobb stage II flash had problems, and the DAM was still at 13 for only a short period. Yours is probably lower, and you are doing it a lot. These motors can go 300K plus miles. Yours may not. One really bad knock can detonate your motor too, typically at the rings.

    The ECU always tries to add back timing. It pulls it when it see knock, but it doesn't just leave it out. It keeps trying to put it back assuming you will have loaded the proper gas eventually. So, it is always knocking. It really isn't good and I would recommend you stop doing it.
    I guess the proof will be in the pudding when I get around 200k. What is the base compression ratio anyway? 8:1?

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    What's the point of driving a WRX if you're on that onramp, want to gun it, but can't because you decided to save 3 bucks on your tank of gas and fill up with regular
    +1
    People can do as they may with their possessions...I've found best results when I used them "as the engineer intended". Plus there wouldn't much point to an 18G hybrid running low octane gas.

    It's just a few bucks. If that's a lot of money to the driver, he is likely driving the wrong car in the first place.
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    Quote Originally Posted by Sinister View Post
    Still wouldn't do it. What's the point of driving a WRX if you're on that onramp, want to gun it, but can't because you decided to save 3 bucks on your tank of gas and fill up with regular
    3 bucks? When you are shelling out 80 to 100 bucks a week in gas, it turns into more than 3 bucks. And as a side note, I'm no longer 20 something years old so needing to beat on my car off every on ramp and red light just doesn't blow my hair back anymore. I prefer to go fast thru turns, not on strait lines to be honest.

    If I thought I heard any indication of knocking or pinging, I would switch to super faster than I could say, what's that noise? I simply have not heard any of it.

    What i generally hear is a bunch of speculation and not much more than that to be honest but as I said in a previous post, the proof will be in the pudding when the car really starts to get some miles on it. If it turns out badly, I'll be the first one to say that regular is a bad idea.

    I think there's a ton of fear mongering on many of these forums and honestly, rarely any fact to back it up. Would I run regular doing an auto x, drag racing or some other track day activity? Absolutely not. The engineering recommendation are often made in a worst case scenario for people who don't know octane from viscosity. I just think pissing away high octane gas to drone down the highway doing 75 is pointless. I use super when it's required.
    Last edited by zore; 03-30-2012 at 11:43 AM.

  10. #54
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    I don't think it's about age. My father is 60, and still pushes the pedal to the floor on his corvette on the occasional onramp. It's about owning a sports car, and that's why we bought these. If you don't want to accelerate quickly, then why not get a car that handles better, and has less power with better gas mileage?

    I think the issue is... the computer doesn't have any octane detection. It has knock detection. And unfortunately, we have hyper-cast pistons that aren't known for being strong. When you use regular, the computer detects the knock, and retards the timing. It then continues to try to advance the DAM (Dynamic Advance Multiplier) which advances timing, then backs it off every time it detects knock. More than likely you won't be able to hear one, or even 10 knock events... And unfortunately (because of our piston metallurgy) it's possible to crack a ring land with the pressure caused by just one knock (detonation) event. I don't know if you've ever heard this phrase... but: Power doesn't destroy engines... detonation does. The fact it's working for you, is great. I'm glad to hear that you're not purchasing a new engine, but keep in mind that there's a reason why the subaru engineers only recommend 91+ oct for daily use, and only 87 in a pinch.
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  11. #55
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    Quote Originally Posted by zore View Post
    I guess the proof will be in the pudding when I get around 200k. What is the base compression ratio anyway? 8:1?
    depends on the engine. Mine is 8.3:1
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    Quote Originally Posted by Sinister View Post
    I don't think it's about age. My father is 60, and still pushes the pedal to the floor on his corvette on the occasional onramp. It's about owning a sports car, and that's why we bought these. If you don't want to accelerate quickly, then why not get a car that handles better, and has less power with better gas mileage?

    I think the issue is... the computer doesn't have any octane detection. It has knock detection. And unfortunately, we have hyper-cast pistons that aren't known for being strong. When you use regular, the computer detects the knock, and retards the timing. It then continues to try to advance the DAM (Dynamic Advance Multiplier) which advances timing, then backs it off every time it detects knock. More than likely you won't be able to hear one, or even 10 knock events... And unfortunately (because of our piston metallurgy) it's possible to crack a ring land with the pressure caused by just one knock (detonation) event. I don't know if you've ever heard this phrase... but: Power doesn't destroy engines... detonation does. The fact it's working for you, is great. I'm glad to hear that you're not purchasing a new engine, but keep in mind that there's a reason why the subaru engineers only recommend 91+ oct for daily use, and only 87 in a pinch.
    And when will there be a knock? While doing 70 down the highway with no boost? Nope. If I was rolling full boost all the time, regular would be silly and no doubt I'd eventually break something, but I don't so there is no knock, because the WRX is not a high compression ratio engine, it's a turbo engine and there is a difference. After a bit of research, I found out static compression is 8.0 or 8.2 to 1 depending on sources. You aren't going to get detonation unless you are on the boost which cruise control at 70-80 doesn't do.

    Please see my previous post as to why Subaru engineers make recommendations and it is a recommendation. How often do you drivethe recommended speed (yellow sign) thru a turn? Remember I have near 130k on mine already, so many of the arguments are just not going to hold water.

    BUT! If i suddenly go up in a huge cloud of smoke, it will be posted here first.

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    UnBanned Sinister's Avatar
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    how do you get to 70 down the highway? I'm guessing you accelerate? And since you have a turbo... you create boost.

    You're obviously hard headed about the issue, so I'm going to cease to argue with you.

    Either way... you should consider another car if you're that concerned about your gas money, and only drive 70mph on the highway. Our insurance is high, and our gas mileage isn't. Have you thought about a honda or toyota?
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    Quote Originally Posted by zore View Post
    3 bucks? When you are shelling out 80 to 100 bucks a week in gas, it turns into more than 3 bucks. And as a side note, I'm no longer 20 something years old so needing to beat on my car off every on ramp and red light just doesn't blow my hair back anymore.
    I'm older than what you have in your registered age, but still love the feel of properly engineered acceleration, where it's appropriate.
    I'm curious if you've actually done the math about what you're saving. Even at 100 bucks a week, it's still single digits, each week. Easily worth it to buy the gas that the manufacturer intended.

    As Kevin said, you're set in your ways. Cheers, and best of luck. Hopefully I won't be the buyer of your car, some day.
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  15. #59
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    Wouldn't knock events be available via logs anyway? If so, this can be checked to see what happened rather than having to rely on the driver's ears. The car will try to pull as much timing as it can anyway I think.
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    Quote Originally Posted by Sinister View Post
    how do you get to 70 down the highway? I'm guessing you accelerate? And since you have a turbo... you create boost.

    You're obviously hard headed about the issue, so I'm going to cease to argue with you.

    Either way... you should consider another car if you're that concerned about your gas money, and only drive 70mph on the highway. Our insurance is high, and our gas mileage isn't. Have you thought about a honda or toyota?
    So when you accelerate, you full throttle it everywhere you go? So I should buy another car because you don't agree with how I drive it? Typical. I'm not hard headed, I've just got 128k on a car that says you may not know what your talking about. We'll say 3 bucks savings every 12 gallons, that's 20 miles you drive for free. Commute 130 miles a day and tell me you wouldn't like an extra 20miles per tank full. On top of that, I'm not sure what mileage you are getting with yours, but as I stated before, I get 25 in the winter due to the winter blend and 26 to 29 in the summer. I think that's decent.

    The argument is silly, because until I come back with 200-300k miles on the car, it wont matter.


    PS... I change my oil every 5-7k.
    Last edited by zore; 03-30-2012 at 02:11 PM.

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