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What about fuel types & gas mileage?

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  • srs_49srs_49 Posts: 1,394
    octane is by-definition a measure of: resistance to detonation. a measure of the resistance to the initial ignition. ...

    Your first statement is correct, but not the second. There is no difference that I know of in the ease with which 87 octane gas ignites in the presence of a spark verses 91 octane.

    So I don't understand why using 87 octane in the BMW would solve the problem she mentioned. Unless it's related to the winter time formulation of the gas where she lives - which is what I think you were getting at.
  • Exactly. My personal opinion is that the explanation given by BMW is, as we used to say, "the sunshine treatment". We park your car for a day, tell you it's all better, and hope you go away.
  • eliaselias Posts: 1,830
    regular fuel fixed the problem with the BMW, if i read the initial post.
    so the techs did in fact fix the problem. if not then its gotta be fuelpump or injector issue?

    assuming i read correctly that the original problem was actually solved by using the supported 87 octane, it can addressed by adding 4 words to the owners manual:
    "premium fuel is recommended, EXCEPT IN FREEZING WEATHER".
    ?

    some time maybe you will encounter non-start situation in frigid weather with whatever engine and if you have lower-octane fuel available, maybe you'll see it work...

    (btw, so i see your point correctly that the octane measures the resistance to pressure-induced ignition, not sparked ignition?!)

    in any case, i'm wishing to go back to a diesel engine, so looking forward to our ongoing different stylings regarding CETANE . never mind all this octane stuff.

    one of the automags years ago (before oxygenated gas became required in USA) put a big v8 crate engine on a bench and tested it with the various octanes, as well as precisely measuring the tiny octane differences in the gasolihe. it was amazing how little difference the octane was, and how little difference in performance actually resulted.
  • MrShift@EdmundsMrShift@Edmunds Posts: 43,641
    edited January 2013
    yes but that's anecdotal, and anecdotes are not evidence. Stalling can be caused by so many factors that we have to be careful, if we wish to be good diagnosticians, --we have to be careful not to confuse causation with correlation.

    chances are pretty good that someone else with a similar problem will not find regular gas to be a solution.

    I suppose it would be fair, at least at this point, to say that using regular gas does not "cure" cold start stalling in an X3, but that it may, under certain circumstances, alleviate it in certain cars?

    The bench test with the big V-8 would be meaningless if the engine didn't have computer controls for timing I think, because all they had to do was set the timing to just short of the ping point on regular fuel---and then yeah, sure, using higher octane wouldn't matter much at all. There isn't any more "power" in high octane gas.
  • eliaselias Posts: 1,830
    edited January 2013
    yeah... it was a racer-type magazine that tested those V8s, i'm sure they did not omit the knock sensors... pretty sure knock sensors are 'standard' on GM V8s for decades.

    anyway, "YMMV" ! :) speaking of BMWs and fuel types, I've been seeing so many X5d lately! must be nice to drive one of those ....
  • I think knock sensors came out in 1981 or so.
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