High altitude, and low octane

AudiBiTurbo at aol.com AudiBiTurbo at aol.com
Thu May 31 09:55:49 EDT 2001

Without going into too much detail, they're partially correct;
Air is less dense at higher altitude and does require less of a matching fuel 
charge per revolution (or lower octane);
That said, most fuel injected engines have a knock sensor and can advance 
timing to take advantage of higher octane fuel.  Of course, it still is 
limited by the incoming air density, but I suspect the higher octane fuels 
would still yield benefits at higher altitudes;
With a turbocharged or turbonormalized vehicle, the incoming air density is 
"maximized" by compressing it and making it denser, which of course makes the 
need for a higher fuel charge.

Certainly with all modern Audis higher anti-knock ratings are preferred for 
most drivers, at any altitude.  Brad's 2.7TT?  Definitely.  Older Audis would 
make less use of the higher octane, and a normally aspirated 4000 might not 
notice a difference between ratings.  A cheap way to test is a g-tech pro, 
but usually most drivers who care can "feel" the difference between octane 
ratings.  Of course, loafing along like granny versus maximum acceleration 
with a full load are different operating conditions (i.e. testing should be 
done with harshest driving conditions you'd experience).

Hope I've helped,
Mark Rosenkrantz
Audi BiTurbo at aol.com

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