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  1. #1
    Joe Pentland
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    Yes | No

    89 AKI = 89 Octane???

    Hey all,

    I have been faithfully feeding my Z 93 octane gasoline because I have heard that anything less will cause knocking in the finely tuned engine. However reading the manual leads you to believe that 89 would be OK. Even the gas cap states 89 or above.

    First off, does 89 AKI mean 89 octane? If yes can one get away with 89 as opposed to 93? My BMW enthusiast friends cringe at the thought of using anything less than 93. They would use 102 octane AVGAS if they could!!!

    I don't mind using the 93. However, as prices rise using 89 for a while would ease the pain.

    Thanks



  2. #2
    Lobo
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    Yes | No

    91 Octane is all that's needed.

    The owner's manual for my '01 2.5i recommends 91 octane. I assume the same is true for all other Z3s. (Someone will correct me if I am wrong.) Using anything higher than this is a waste of money and will gain you nothing. On the other hand, the car can handle lower octane fuels by automatically adjusting its timing, thus preventing knocking and the accompanying engine damage. However, the tradeoff is that the engine won't run as efficiently. Some say that you will get fewer miles per dollar with less than 91 octane, but I don't know if anyone has actually measured the effect.

    If I pull into a station with only 89 and 93 octane gas, I fill the tank with a half-and-half mixture (or something close).


  3. #3
    Ambro
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    Yes | No

    I fill up with 89 all the time...no problems

    I have an '01 3.0i, and the owner's manual and gas cap both recommend 91 RON (octane) "Premium grade" gasoline. Since the gas stations around here never carry 91 (only 87, 89, and 93 available...why no 91?!?!), I just fill up with 89 octane.

    Now, let's see if I remember correctly from my mechanical engineering courses, but octane is an additive to gasoline to prevent premature ignition due to compression (as does a diesel engine) before the spark plug even sparks. If this happens, the double flame front created by the compression ignition and spark ignition creates the "knock" and a downgrade of performance. Usually, this isn't a problem for today's modern car, especially with the variable valve timing available in most high-end cars. Even though BMW recommends 91 octane, I usually just get the 89. And as far as I know, I have no engine or performance problems, so using any higher octane gasoline would be pointless and not worth my money.

    Mike


  4. #4
    Vidterry
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    Yes | No

    I think you need to remember a little more...

    Your engine management system will adapt so that you do not have engine knock, and to that extent you are exactly right...no problem. However, in order to do that, one of the results is that you LOSE horsepower. Though I am not in any sense a techie on these things, and would not begin to argue with your engineering background, those who DO know have regularly documented the hp loss. Sense most here fight stridently for every single HP, I just can't imagine willingly giving up any for the sake of a few cents a gallon.

    I'm sure your car is running fine on regular...it is just not running as well as it could, or the way it was intended to run. Your car, your choice...but probably NOT the recommendation to make to others, particularly when prefaced with 'engineering experience.'

    Or do you think it is just a plot between BMW and the oil companies to make us pay for NOTHING?

    Terry


  5. #5
    PerryinVA Z3U4IA
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    Yes | No

    As another ME, you are right except what Vidterry

    says is true. The engine is adaptive to knock and as such the timing changes result in a loss in hp, or if you don't use the hp, a loss in mileage. I regularly see a 1.5-2.5 mpg increase with 93 vs 89, which always pays for the extra cost, reduces the # of fillups, and the extra performance is a bennie. YMMV, but this is true on my 99 2.3.....

  6. #6
    Dane Kane
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    Yes | No

    Don't forget: GAS is a commodity!..it gets stored.

    Some pump gas may be more than a tad optimistic about the octane rating...so 89 might actually be 85...lot's of reasons for it to go down,but none for increases...91 at least allows for some fudge-factors...dk '97 2.8 bg

  7. #7
    RETRO
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    Yes | No

    My Mobil 93 gives me better milage on both BMW's

    87 528e and 2000 Z3 rather than 89 octane. about
    1-2 miles per gallon.(octane) "Premium grade" gasoline. Since the gas stations around here never carry 91 (only 87, 89, and 93 available...why no 91?!?!), I just fill up with 89 octane.

    Now, let's see if I remember correctly from my mechanical engineering courses, but octane is an additive to gasoline to prevent premature ignition due to compression (as does a diesel engine) before the spark plug even sparks. If this happens, the double flame front created by the compression ignition and spark ignition creates the "knock" and a downgrade of performance. Usually, this isn't a problem for today's modern car, especially with the variable valve timing available in most high-end cars. Even though BMW recommends 91 octane, I usually just get the 89. And as far as I know, I have no engine or performance problems, so using any higher octane gasoline would be pointless and not worth my money.

    Mike



  8. #8
    EricS
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    Yes | No

    Octane definitions


    Hey all,

    I have been faithfully feeding my Z 93 octane gasoline because I have heard that anything less will cause knocking in the finely tuned engine. However reading the manual leads you to believe that 89 would be OK. Even the gas cap states 89 or above.

    First off, does 89 AKI mean 89 octane? If yes can one get away with 89 as opposed to 93? My BMW enthusiast friends cringe at the thought of using anything less than 93. They would use 102 octane AVGAS if they could!!!

    I don't mind using the 93. However, as prices rise using 89 for a while would ease the pain.

    Thanks


    (R+M)/2 is how octane is usually measured in the US. R= RON, and M= MON: both of these are other ways of measuring octane. As you can see, the US has chosen to average the two. So, if your car calls for 89 RON and you are putting in 89 at the pump in Iowa, you probably are not putting in 89 RON but something less. I believe that the DME of our cars can take advantage of octane up to the low 90s as measured by R+M/2, so I would suggest, if you want max performance and mileage that you use the highest octane you can get.

    AVgas is highly leaded and not an option. Racing gas may be, but is probably a waste of money unless you have software that can take advantage of very high octane fuel.


  9. #9
    Bart B.
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    Premium gas may give more miles per dollar...

    ...of it and therefore would be the cheapest gas to burn in your car. Gas that delivers 18 miles per dollars worth is cheaper than gas delivering only 17 miles per dollars worth - even though the 18 mp$ gas costs 10 cents a gallon more to buy.

    Higher octane gas typically produces more energy per gallon and that usually means more horsepower per ignition.

    I've found in the last four cars we've owned and the two we now have (Volvo S80 and Z3 coupe) that they actually go more miles on a dollars worth of premium than mid-grade. One needs to drive a few tankfulls with each to see the difference.

    Fuel economy has to have some currency unit in its expression; miles per dollar or cents per mile cost. Fuel efficiency is miles per gallon.


  10. #10
    Bart B.
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    Yes | No

    Premium may be cheapest for your cars...

    ...to use.

    Figure the miles per dollar of gas on both cars with both octanes, then decide.


  11. #11
    Bart B.
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    Yes | No

    OK, but what's MON and RON? Need this...

    ...in order that folks understand the difference. Otherwise, the average of them is meaningless.

    Right?


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