[Vwdiesel] Altitude compensation

Libbybapa@wmconnect.com Libbybapa at wmconnect.com
Sat Sep 3 10:18:00 EDT 2005


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. 

Andrew


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