[Vwdiesel] Altitude compensation

[James Hansen]

Well, you're talking gas burners.
Injection systems in modern gasoline engines all have oxygen sensors in the
exhaust stream that are used to calibrate the fuel injected quantity so it
runs at the factory determined air/fuel ratio.  Go higher, essentially take
away a bunch of O2, the fuel injected quantity is reduced to keep the engine
running at the same ratio on a stoichiometric basis. Because less air and
fuel, you get less power, but at altitude in a modern car, if you took away
the big hills necessary to get there, your mileage should increase, because
the motor makes less power, and uses less fuel to do so.

The pump gas values expressed in motor octane ratings have nothing to do
with the amount of "power" available in the gasoline for the most part.  I
know that advertising alludes to "performance gasoline" having more power
and that being related to the higher octane rating, but octane rating means
nothing other than the fuel's resistance to knock at higher compression.  If
you run a higher compression motor, you need higher octane fuel so that it
doesn't begin to burn before the spark initiates the combustion, or burn too
quickly once the spark does happen.  For instance, I have to run premium
(not allowed to run race fuel) in my racecar because it has a high
compression motor, a distrubutor with no advance curve- it's locked at 40
degrees, there is no means of varying the timing if it starts to knock, like
road cars have sensors to detect and retard timing. I can't have detonation
occur and eat up an expensive motor.  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.
octane ratings are determined by running the fuel on test engines at the
refinery to determine the resistance to detonation.  The nomenclature comes
from a comparison to years ago when the standard to which gasoline was
compared to was an 8 carbon atom called octane.  A 90% octane blend would
have a certain preignition resistance on the test motor. The fuel blend we
call gasoline was compared to this, and the blend varied to match the
preignition characteristics of the test standard, therefore the tested fuel
was said to have a "motor octane rating" of 90--- meaning it compared to a
90% octane blend running in the test motor.
really, if you build a high compression motor, it needs high octane rated
fuel, but it is the compression that is giving you the horsepower, not the
fuel octane rating.
did I ramble on enough now?  I guess I'll stop before it gets more
confusing...

By the way, a 5 psi variance in compression test values is nothing, don't
fret about that Andrew.
-James



-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com]On
Behalf Of Libbybapa at wmconnect.com
Sent: Friday, September 02, 2005 8:41 PM
To: vwdiesel at audifans.com
Subject: [Vwdiesel] Altitude compensation


When I checked the compression on my TD vanagon all four were within 5 psi
of
each other.  3 were 400 and the fourth was 395.  I was a little concerned,
but the engine ran fantastic, so I've continued to drive it.  My friend's
quantum checked out at 380 on all four.  I recently got thinking about the
fact that
I live at about 7,000 ft.  When the altitude compensation is factored in to
my friend's engine his compression at sea level is 460 psi.  My TD vanagon
engine would read 475-480 psi!  Nothing wrong with that!

I was also thinking about the recent discussion of Passats running on low,
mid or high grade fuel.  Someone mentioned that some people saw no
performance
decrease or mileage decrease with the lower grades, others saw significant
difference.  I got thinking that probably the cars seeing no change were
primarily
at high altitudes.  I'd love to have that member of the group post that idea
to the passat group.  It makes me think that I've been wasteing my money
putting high test in my ABA work van.

Andrew
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