[Vwdiesel] Altitude compensation

James Hansen jhsg at sasktel.net
Sun Sep 4 01:05:03 EDT 2005

I don't know, but I doubt very much there are high enough compression
engines on the consumer market that REQUIRE high octane rated fuel. Higher
compression gives higher burn temp, and more nitrogen emissions.  Lower
compression gives less. I think the fuel requirements have more to do with
running a more agressive timing advance curve, and leaner mixture to get
better performance.  I would suspect as you do, that lower octane fuel in
most engines would not be as detrimental to mileage in the majority of
cases, regardless of altitude.
  -----Original Message-----
  From: Libbybapa at wmconnect.com [mailto:Libbybapa at wmconnect.com]
  Sent: Saturday, September 03, 2005 8:18 AM
  To: jhsg at sasktel.net; vwdiesel at audifans.com
  Subject: Re: [Vwdiesel] Altitude compensation

  In a message dated 9/2/05 8:54:55 PM US Mountain Standard Time,
jhsg at sasktel.net writes:

    In your passeenger car, there are
    knock sensors that detect the merest hint of preignition, and richen the
    mixture and retard timing to prevent harm.

  That's what I was getting at.  With a high compression motor running on a
given octane rated gas, it will preignite less at higher altitudes than at
lower ones.  Therefore it makes sense to me that someone at see level
putting 87 octane in a car that "requires" 91 would see the knock sensor
retarding the timing with resultant pooor performance and poorer fuel
economy.  That same motor with the same fuel run at 7,000 ft. will not
preignite (or not to the same degree) therefor no relutant "poorer
performance and economy" from the lower fuel grade.  When an engine is not
equipped with a knock sensor, it is common practice here (7,000 ft.) to
advance the ignition timing several degrees.

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