Carburetors and Altitude

[efi_student]

Never been through the Eisenhower tunnel in Colorado on I-70 I see.  Nor
Wolf Creek Pass.  Both are well over 6,000' above MSL.  My 79 AMX with
its marvelous Stromberg 2 barrel on an I-6 crawled through the
Eisenhower tunnel at 35 mph even with 5 degrees timing advance.  Jetted
down to mains, and flew through at normal driving speeds.  IME
Carburetors have a dismal time compensating for altitude.  I have tuned
both cars and bikes with carburetors for living in Denver after living
in Sacramento.  Sac is about 12' above MSL, Denver is well known as the
mile high city.  Oddly, I needed bigger pilots and smaller mains to work
properly at altitude.  But then again, I was dealing with carburetors of
a lesser design than the Webers.  Mikuni flatslides, Keihin CRs, and the
venerable Stromberg 2bbl are far less advanced.

Lance

-----Original Message-----
From: owner-diy_efi at diy-efi.org [mailto:owner-diy_efi at diy-efi.org] On
Behalf Of Bruce
Sent: Sunday, December 09, 2001 6:01 AM
To: diy_efi at diy-efi.org
Subject: Re: Mass Market EFI (was: Intake manifold construction,
intercoolers)



From: "Jon Davis" <jfdavis at epix.net>
Subject: Re: Mass Market EFI (was: Intake manifold construction,
intercoolers)
> Bruce wrote:
> > > Out of curiosity, Bruce, how do you tune the carbs for things like

> > > temperature, barometric pressure/altitude,
> > They work on Bernolli's Principal, so do those comps as a matter of
> design.
> Why does an airplane have a mixture lever then? AFAIK the carbs in
airplanes
> still work using a venturi, yet they require you to lean the mixture 
> at altitude. As you lean, there is a noticable increase in power 
> (until you
go
> too far). This implies carbs do not compensate for atmospheric 
> pressure variations. Jon Davis

Apples and Oranges.
   Aviation stuff is designed to be as fail safe as possible, ie simple.
I have yet to see a DCOE (that fact has been edited out) in aviation
use, and
I'd doubt that it was properly tuned to begin with.   That and the basic
principal that pilots like to be in total control.
I've also noticed a far number of aircraft that allow mixture control,
but ignore ignition. The few aircraft carbs, that I've looked at were at
best primitive, interesting as far as being able to work inverted, but
that's all.
   Just to get back to EFI:
   There are a number of EFI's that are MAP systems, that other then at
start up never reference to Baro., other then some special cases.   The
correction is only do to the exhuast back pressure change, and depending
on altitude might just be a minor issue, as you climb then it will get
more meaningful,  as far as US roads, I don't think many get over 6K'.
   Go to a high enough altitude, and it's engine design / application
that is the ruling matter. Bruce


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