Re: Octane vs. MPG

["Diana and Mike Kohlbrenner" <kohlbren-m-d@xxxxxxx>]

- -----Original Message-----
From: tgstewart@domain.elided <tgstewart@domain.elided>


>Well, this thread has certainly taken on a life of it's own!
>...and created a lot of misinformation.
>
>Here are some good links:
> ...
>http://www.cs.uu.nl/wais/html/na-dir/autos/gasoline-faq/part3.html

To answer the original question, Sec. 6.13 from part 3:

   "...If you are already using a fuel with an octane rating slightly
    below the optimum, then using a higher octane fuel will
    cause the engine management system to move to the
    optimum settings, possibly resulting in both increased
    power and improved fuel economy."

For example, if you have a car with a knock sensor, you
will likely get better performance and possibly better
economy with a higher octane fuel that doesn't cause
the timing to be retarded like a lower octane fuel will.
Supposedly the knock-sensored VWs officially
"required" low octane fuel, but they definitely woke up
with higher octane, showing that they were apparently
optimized for the good stuff and the timing was being
retarded when the lower octane stuff was used.

This is also one of the principles used in most aftermarket
chips -- advance the timing, change to high octane fuel
and bingo -- more power.  Theoretically, there is a side
effect of better fuel economy, but my guess is that most
folks who have a chip actually USE it, so the economy
gets hidden.

Oh, and my GUESS would be that you will never get
enough of an economy benefit to cancel the increased
price.  I'd love to hear otherwise, because I'd surely
like to get a chip to wake up my 535i without feeling
bad about having to throw an extra $4 into the tank at
every fill up...

Regards,
Mike Kohlbrenner

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