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Thread: Best Fuel Grade to use in 1991 850i

  1. #1
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    Best Fuel Grade to use in 1991 850i

    I have been running 93 (premium/super) but based on a little research, it looks as if 87 (regular unleaded) is all that is required. Am I wrong and what grade fuel do most people use in there 850s?

  2. #2
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    (been a topic before) I used 87 in a '93 850Ci and if I remember correctly, most of the "normal" 850 owners used the same (?). Bill

  3. #3
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    I always use the highest, premium/supreme w/e they have

  4. #4
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    Plenty of previous threads on this to search on.

    My understanding is that the M70 from a 1991 model is a low-compression engine without knock sensors, and as such there is no benefit to be using octane higher than 87 as pre-detonation will occur, and you may in fact incurr worse performance from higher octane fuels?

    I use 87.
    ___________________________________
    1991 Mauritius Blue BMW 850i, J-Spec

  5. #5
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    Sounds like you have more dollars than sense. Higher octane in the M70 benefits the oil company ONLY. As already stated, you are hurting performance as well as the engine itself (pre-detonation).
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  6. #6
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    Ugh this again?

    V12 = 87

    V8 = 89/91

    ...

  7. #7
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    Stock or chipped V12 - 87 is fine.
    TT V12 - 93 ONLY!

    -JRW

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  8. #8
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    agree... regular gas for low compression motors...
    "...i haven't taken leave of my senses Bob, I've come to them..." -- Ebenezer Scrooge

  9. #9
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    This subject may have been discussed many times before but what are the facts?

    Isn't it true that predetonation is the result of too little fuel present during combustion (lean) or too low of on octane of fuel. Too much ignition advance also causes predetonation. I have a hard time believing that using a octane grade that is higher than required would ever cause predetination. JUst call me crazy.

  10. #10
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    If the majority of experienced V12 owners and the original engineers of the car at BMW recommend 87, why is this information not good enough for you?

    It's not like they designed it that way to save money for the buyers of a $100,000 car.

  11. #11
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    Quote Originally Posted by olinjohnston1 View Post
    ugh this again?

    V12 = 87

    v8 = 89/91

    ...

    m70 = 87
    m73 = 91/93

    v8 = 89/91
    '93 850Ci - Mineralweiß Metallic

    2001 740iL - Titansilber
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    ...the price of cool ain't cheap!

  12. #12
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    Quote Originally Posted by olinjohnston1 View Post
    If the majority of experienced V12 owners and the original engineers of the car at BMW recommend 87, why is this information not good enough for you?

    It's not like they designed it that way to save money for the buyers of a $100,000 car.
    I am more than happy to run 87 which was not the point of my last post. I was just trying to clarify a few things relating to predetonation comments which I feel are incorrect. If you have a problem with people asking a basic question, you need to find another play ground to go bully people.

  13. #13
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    Quote Originally Posted by Grant850i View Post
    If you have a problem with people asking a basic question, you need to find another play ground to go bully people.
    Whoa. I wasn't trying to "bully" you. This question comes up a lot; and with all the same answers, that's all...

  14. #14
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    I'm quite glad that you have started this thread Grant.

    I was under the impression that the higher octane fuels were 'better' fuels

    giving more power because of the higher octane rating.

    I'm very glad that this has been covered again because it isn't something I would have even thought to search on.

    Thank you
    Life is for thriving, Not surviving!

  15. #15
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    Quote Originally Posted by bRIZZAd View Post
    My understanding is that the M70 from a 1991 model is a low-compression engine without knock sensors, and as such there is no benefit to be using octane higher than 87 as pre-detonation will occur, and you may in fact incurr worse performance from higher octane fuels?
    A higher octane rating means a higher anti-knock index. In other words, 91 AKI is less prone to pre-detonation than 87 AKI without anti-knock sensors. But the M70 was designed to run on 87 AKI without knock sensors, so even though 91 AKI is superior fuel, it will offer no (or at most very little) performance advantages. Is it wrong to use a higher octane rating than specified for the engine? No, it's just more expensive. In large parts of Europe the lowest octane rating you can get at the pumps is 91 AKI (95 RON).

    Note that engines equipped with knock sensors (not the M70 thus) may actually benefit from using a higher octane rating than specified. It's often specified explicitly in the owner's manual.

  16. #16
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    Quote Originally Posted by Ascension View Post
    I'm quite glad that you have started this thread Grant.

    I was under the impression that the higher octane fuels were 'better' fuels

    giving more power because of the higher octane rating.

    I'm very glad that this has been covered again because it isn't something I would have even thought to search on.

    Thank you
    I am with you as I was not aware of this and 91 here is always about .20-.30 more a gallon.

  17. #17
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    Why when you put performance chip it say to you 91 or higher

  18. #18
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    Wow, I never realized this.......I just purchased a 95' 840ci with the M60 v8.
    So, according to everyone here, the V8 will run great with a minimum of 89 octane fuel? Also, do I need to compensate for the new 10% ethanol mixture added to the fuel these days? Thanks guys for any input on the subject.

  19. #19
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    Quote Originally Posted by Silverghost View Post
    Wow, I never realized this.......I just purchased a 95' 840ci with the M60 v8.
    So, according to everyone here, the V8 will run great with a minimum of 89 octane fuel? Also, do I need to compensate for the new 10% ethanol mixture added to the fuel these days? Thanks guys for any input on the subject.
    M60 Requires 89 minimum, but would benefit slightly from 91. I avoid ethanol, as I have calculated that it provides slightly worse fuel economy.

  20. #20
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    S70 needs minimum of 91. Aren't the fuel requirements spelled out on the fuel cap or on the filler door somewhere?

  21. #21
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    Quote Originally Posted by TxGR8White View Post
    ...As already stated, you are hurting performance as well as the engine itself (pre-detonation).
    Steffen, this is incorrect. Higher octane fuel does NOT hurt neither engine nor performance. Pre detonation is nothing else but an uncontrolled detonation of the air/fuel mixture caused by the temperature of the compressed mixture. The higher the compression the higher the temperature of the compressed air/fuel mixture. High octane fuels can withstand higher temperatures than low octane fuels without detonating by themselves.
    Since the M70 is a low compression engine it is indeed a total waste of money if someone uses anything better than 87 octane fuel. Total waste of money without any benefit to the user.



  22. #22
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  23. #23
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    Quote Originally Posted by olinjohnston1 View Post
    M60 Requires 89 minimum, but would benefit slightly from 91. I avoid ethanol, as I have calculated that it provides slightly worse fuel economy.
    Thanks for the info......Unfortunately, down here in Florida you can't avoid buying any "grade" fuel without the addition of 10% ethanol to the mixture. I guess its better to just use the mid-grade 91 octane for the M60 V8's.

  24. #24
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    Quote Originally Posted by Silverghost View Post
    Thanks for the info......Unfortunately, down here in Florida you can't avoid buying any "grade" fuel without the addition of 10% ethanol to the mixture. I guess its better to just use the mid-grade 91 octane for the M60 V8's.
    Running 10% Ethanol is not a problem at all. I've been running 85% in my E31850, E34-540 as well as E39-520 and E39-523. The only problem has been poor idle for the first few minutes due to the 30% lower energy content of Ethanol compared to gasoline.



  25. #25
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    Correct me if I am wrong, because it has happened before, but I belive some of you are confusing the issue with the term "pre-detonation". So far as I'm aware, there is either pre-ignition or detonation. Both cause the same type of damage to pistons and rings, however they are caused by different conditions.

    Detonation occurs after the sparkplug fires.
    The air/fuel mixture on the opposite side of the combustion chamber of an advancing flame front, spontaneously ignites due to the rise in pressure. This causes the knock we hear. Higher octane will usually help in this case

    Preignition occurs before the plug fires.
    It is usually the result of the air/fuel mixture being set off by a localized hot spot in the chamber such as glowing carbon deposits, incorrect sparkplug heat range, sharp edges on pistons or valve margins, etc., usually because the engine has been run hard for an extended period of time (long up hill towing, etc.)
    This happens before the ignition system fires, and usually while the piston is signifficantly before TDC. Higher octane will not help with this situation.

    Basically, on engines without knock sensors: if you don't hear detonation use the lowest recommended octane rating. If it's really hot outside and you're in the hills towing a trailer, go up one step.

    On engines with knock sensors: if you want to cheap out, or can't find the good stuff, the computer will adjust the timing to save your motor but performance will suffer.


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    M.R.
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