Thanks for this informative post! It explains a lot. My thought was that
there was a need for a knock sensor only when the compression ratio was
high, lets say over 10:1? This sort of explains to me now why stroker motors
eat more gas.

Joe

> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM]On Behalf
> Of Stuart MacMillan
> Sent: Friday, February 25, 2000 9:33 AM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Gasoline Octane rating explained
>
>
> Karl is right, the octane rating boosters are just chemicals that lower
> the detonation temperature of the fuel air mixture so it doesn't
> "explode" all at once before the piston reaches it's power stroke.  The
> energy content is the same, but the power produced in an engine will be
> reduced if there is detonation rather than an even burn.
>
> Fuel does not explode in the cylinder, it actually burns in a wave, with
> the flame front starting at the plug with a spark usually just before
> TDC.  The flame continues to burn through the fuel/air mixture as the
> piston moves down, creating an even expansion of gasses to produce the
> power.  If the fuel detonates all at once (explodes instead of burns
> evenly), particularly before TDC, the result is detonation knock, loss
> of power, and engine damage.
>
> Modern engines have knock sensors to adjust the engine timing to
> eliminate the knock, but the Vanagon does not.  These engines can get
> more power with higher octane (to a degree) because the knock sensors
> retard the timing for lower octane gas, which reduces the power output.
>
> The only benefit to running higher octane gas in a Vanagon is to
> eliminate detonation, or knock.  Use the lowest octane gas that will
> prevent knocking when going up hill in a low gear (remove the engine
> cover and do a test run, but keep your windows down!).  Using anything
> higher will simply line the pockets of the oil companies.
>
> Karl Wolz wrote:
> >
> > I am not a chemical engineer, but my understanding is that the amount of
> > energy in a gallon of gas is the same no matter what the octane.  Higher
> > octane gas simply burns more slowly.  This is required in
> higher compression
> > engines where the gas without octane enhancers would start
> burning all by
> > itself before the spark simply from the existing heat and
> compression.  This
> > is preignition, knocking, or pinging.
> >
> > If someone out there has a more accurate description, or if I'm
> dead wrong
> > here, please let me know (as I know you will!).
> >
>
> --
> Stuart MacMillan
> Manager, Case Program
> 800-909-8244 ext. 8208
>
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>