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Technical Do any carburetors compensate for altitude changes - sea level to Pikes Peak?

Discussion in 'The Hokey Ass Message Board' started by no55mad, Apr 30, 2021.

  1. no55mad
    Joined: Dec 15, 2006
    Posts: 1,892


    Wondering if maybe carbs with metering rods will lean out at high altitudes? Have had experience with a sea level jetted carb barely being able to power a sbc at 7000 feet elevation.
  2. oldiron 440
    Joined: Dec 12, 2018
    Posts: 2,425

    oldiron 440

  3. Gearhead Graphics
    Joined: Oct 4, 2008
    Posts: 3,852

    Gearhead Graphics
    from Denver Co

    computerized ones with injectors.
    Deuces, Tman, Hnstray and 3 others like this.
  4. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 7,966

    Bandit Billy

    loudbang, Deuces and Johnny Gee like this.

  5. Mr48chev
    Joined: Dec 28, 2007
    Posts: 30,056


    I have to go along with that no. Remembering my car stumbling a bit going over the pass on US 50 in Colorado in 1969 and being told by the gal in the cafe at the top to step outside the door and look at the sign above it when I said something about it. At over 10000 ft you couldn't expect one to run perfect.
    Deuces likes this.
  6. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,470


    The 80s Quadrajets do..
  7. 34Phil
    Joined: Sep 12, 2016
    Posts: 319


    Airplanes had to have it. But for cars Holley had a miser or a power carb with a solenoid to control the needle in the early '80's but they went away quickly. Miser could lean and the other enrich from a standard setting. Early computer carbs used this.
  8. Budget36
    Joined: Nov 29, 2014
    Posts: 7,262


    The emission carbs of the 80’s would require a lot of work, but probably doable although outside the scope of the HAMB. If you’ve been there before and had to make carburetor changes you should have a handle on setting up a spare carb and taking it with you if it’s in the budget.
    Deuces, blowby and Elcohaulic like this.
  9. TCTND
    Joined: Dec 27, 2019
    Posts: 257


    "Constant vacuum carbs" such as SU's (british) and, I've forgotten the name, but squarish and once made in Napa are self compensating for altitude.
    irishsteve likes this.
  10. 34Phil
    Joined: Sep 12, 2016
    Posts: 319


    Predator carb
  11. Casey Riley
    Joined: Jun 27, 2018
    Posts: 531

    Casey Riley
    from Minnesota

    Airplane carbs that bring 2 different grades of Avgas do...
  12. Why worry about it?
    In 2013 I drive my rod from northern Illinois to Vancouver to L.A. and back home through Denver. Had an AFB style carb on the 350 at the time.
    Going out we crossed the mountains on I90 in Montana. Coming home we crossed on I70 in Colorado. Not the most extreme altitudes, but well above the 700' we tune for at home.
    The only adjustment was to take a dime and increase the idle speed slightly.
    No problems.
    j3harleys and Driver50x like this.
  13. Truck64
    Joined: Oct 18, 2015
    Posts: 5,105

    from Ioway

    I'd agree if you're just scootin' over the hill it isn't a huge deal If you're going to spend some time at those elevations, or if you live in places like Leadville, then it becomes important. Note that the reverse is not the case at all - engines tuned specifically for high altitude will run lean at low altitude and will likely burn a valve or two.

    It isn't just jetting to think about, the power circuit is maybe even more important. I remember as a little kid flatlander cars and station wagons lined up near the summit of Pike's Peak in a traffic jam and watching a thin but steady stream of pure gasoline dripping out of the tailpipe of the cars in front of us. Power valve or carb power circuit opening points are based on engine manifold vacuum, for example at high altitude a Holley power valve suitable for Iowa will never close under any operating conditions up in the High Country.
    j3harleys likes this.
  14. 34Phil
    Joined: Sep 12, 2016
    Posts: 319


    One jet size per 1000' elevation or 20 degrees Fahrenheit. Brian Wilson in two lane blacktop must have had OCD.
    Deuces likes this.
  15. Hnstray
    Joined: Aug 23, 2009
    Posts: 12,173

    from Quincy, IL

    Common carbureted piston engines used in a high percentage of light aircraft do not automatically compensate for altitude. But they are equipped with a mixture control operated by the pilot to lean or enrich as circumstances require, which is not always altitude related.

    Even the (mechanical) fuel injected engines typically have manual mixture control for the same purpose. Same for turbocharged engines.

    Older piston engine commercial and military engines, I believe, often had some altitude compensating carbs, but they also commonly had Flight Engineers who handled the engine functions in most phases of flight.

    Many aviation people think that it is long overdue that certification standards be amended to include EFI and electronic ignition systems. The Experimental category can legally use those systems.

    Last edited: Apr 30, 2021
  16. 1934coupe
    Joined: Feb 22, 2007
    Posts: 4,651


    I lived in Flagstaff AZ for a while and ran my 57 Chevy LT-1 with Edelbrock C3B and a Holly DPer. I think 720 cfm it ran fine up in Flag and down in Phoenix or Sedona and even drove up to the ski area and down to the Colorado river out on the Havasupi reservation. I never raced it or raved to 7 grand but normal stuff and going through. the gears it ran fine.

  17. jimmy six
    Joined: Mar 21, 2006
    Posts: 10,002

    jimmy six

    Not automotive but the Schiebler on my Indian has a mixture control knob and I can reach down and adjust it richer it leaner a click or 2 when riding..
  18. Kevin Ardinger
    Joined: Aug 31, 2019
    Posts: 351

    Kevin Ardinger

    I believe a Ford variable Venturi would as it is dependent on vacuum as to how the venturis work. I wouldn’t run one though. They were a miserable POS to get right.

    Sent from my iPhone using H.A.M.B.
    tubman and Truck64 like this.
  19. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,526

    from illinois

    Predators ?
  20. Mimilan
    Joined: Jun 13, 2019
    Posts: 946


    Blow through turbochargers with a carb inside a pressure box :D
    The air/fuel ratio will remain constant, but the boost pressure will drop off at altitude.
    Last edited: Apr 30, 2021
    rod1 and Truck64 like this.
  21. greybeard360
    Joined: Feb 28, 2008
    Posts: 1,813


    2150 Motorcraft 2 bbl had a high altitude compensation valve on some models.

    Beanscoot, mkubacak and blowby like this.
  22. Mike VV
    Joined: Sep 28, 2010
    Posts: 2,443

    Mike VV
    from SoCal

    NOPE -

    If you are racing, you will be practicing at different altitudes. You need an 02 sensor for proper information on the setup.
    An average (bottom to the top)...of fuel metering is the best that you can do with a carburetor.

    Been there, done that, back in the old "dirt" surface days..!


    Note - Actually they do, only a VERY small amount. Not nearly enough to make much difference when going to 14,000+ ft..!
    Last edited: Apr 30, 2021
    tractorguy and pprather like this.
  23. mkubacak
    Joined: Jun 20, 2005
    Posts: 199


    This is what I was going to say. I have had a couple on some FSJ's.
  24. impala4speed
    Joined: Jan 31, 2010
    Posts: 282


    I agree! Except it was Dennis Wilson, who is no longer with us. Brian is though, kind of.
    Truck64 and redo32 like this.
  25. carbking
    Joined: Dec 20, 2008
    Posts: 3,161



    If one studies the Carter (Carter introduced metering rod technology in 1929) documentation, one will find that Carter released standard rods, rich rods, and lean rods. Typically:

    Sea level - 4000 feet - standard rod
    4001 - 6000 feet - 1 size lean rod
    6001 - 7000 feet - 2 sizes lean

    While a very few 3 size lean rods were offered, generally the 2 size lean rod was suggested above 7000 feet. The rich rods were typically offered on export carburetors to countries with poorer fuel than the USA (how times have changed! :mad: )

    Holley, Rochester, and Stromberg used fixed jets (at least in H.A.M.B.-friendly carburetors). Typically, a 0.002 inch diameter change per 5000 feet. Thus:

    Sea level - 5000 feet - standard jet
    5001 - 10000 feet - 0.002 smaller diameter jet.
    10001 - up - 0.004 smaller jet.

    We have custom-made and sold lots of lean metering rods for folks that live in altitude areas.

    loudbang, deathrowdave and Hnstray like this.
  26. j3harleys
    Joined: May 12, 2010
    Posts: 912


    I'm sitting here at my kitchen table having my coffee and looking at Pikes Peak, it looks like about 14,115 feet high from here and I can tell you any carburetor that runs good at sea level would have avery hard time up there:D
  27. Truck64
    Joined: Oct 18, 2015
    Posts: 5,105

    from Ioway

    Brian had his own "altitude" issues at one time...
    Deuces and impala4speed like this.
  28. carbking
    Joined: Dec 20, 2008
    Posts: 3,161


    If one does one's homework ahead of time, AND uses Carter AFB carburetors; one can carry the lean rods in the glove box, and change them as needed.

    It takes longer to open the hood, and remove the air cleaner than to change the rods. While I do not recommend this, to prove a point, I changed a set while the engine was running. Very easy calibration change.

    tractorguy, loudbang and moparboy440 like this.
  29. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,155

    seb fontana
    from ct

    impala4speed likes this.
  30. no55mad
    Joined: Dec 15, 2006
    Posts: 1,892


    Great responses! Going to go for the cross country trip, Mich to Cal, 47 Fleetline 2 dr sedan, 350 sbc, Edelbrock carb. Got spoiled the last few trips with computer controlled rigs.
    Last edited: May 3, 2021
    pprather likes this.

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