Re: [VAC] Engine Performance

["Don Horn" <donhorn@xxxxxxxxxxxxxx>]

Darrell, 

You've got the right idea, but your explanation is backwards.  The fuel/air
mixture gets richer (i.e., a higher gas to air ratio) as you *increase* your
altitude.  A turbocharger adds more air, effectively leaning it out while
providing more oxygen for combustion.  Fuel injection leans the mixture by
reducing the amount of gas.  With turbo, power loss is minimized.  With fuel
injection you'll lose some power (which happens anyway due to the lack of
air), but the engine runs smoother because the mixture is optimized, and
you'll get better gas mileage.  For a vehicle with a carburetor, adjust it
for the altitude you plan to drive at, and stay there ;>).

If you have a turbocharger, you should use the same octane at all altitudes.
If not, you might as well use a lower than normal rating, as others have
mentioned.  However, using a higher octane won't reduce performance at
altitude, it's just a waste of potential energy (and dollars).

I've flown planes with a mixture control like you mention. As altitude
increases, you lean out the mixture, using the exhaust gas temperature (EGT)
as an indicator that you're getting complete combustion of the gasoline in
the rarified atmosphere.  This is essentially what fuel injection does,
except that it uses the amount of uncombusted oxygen in your exhaust as the
indicator, via the oxygen sensor, instead of the EGT.

The only way I know that a carburetor on a car or truck can be adjusted for
altitude is to change the orifice, although others have suggested there are
systems that will adjust the mixture automatically as you drive.  However,
you *don't* want to apply manual choke at higher altitudes.  As Captain
O'Lary points out, that's the opposite of what you want to do, since it
reduces the air intake, and richens the  mixture still more.

Some relatively inexpensive ways to improve performance at high altitudes
(and at low altitudes as well) are to make sure your air filter is clean and
unobstructed, or get a high-flow air filter like K&N (about $40), and a high
performance muffler can improve airflow (I got one with tailpipe for about
$165 for my Chevy van).  I'll be testing my current setup soon over Raton
pass, crossing Deborah and Jerry's path on my way to the Big Bend area.

Also, make sure your fuel filter is clean.  If your filter is slightly
clogged, it could give you a power drop that's only noticeable on upgrades
and thus appear to be altitude related when it's not.  I had a VW van that
ran fine, but stalled out completely going up extended upgrades (at sea
level), and it turned out to be the fuel filter.   This is a cheap fix if
you think it could be causing a power loss, and it never hurts to have a new
filter, even if it turns out *not* to be the culprit.

I'll stop now...

Don
'63 SOB
'93 Chevy G-20 Van