Landracing Forum
Tech Information => Technical Discussion => Topic started by: Truckedup on May 05, 2013, 07:05:21 AM
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This is a very generalized question for a naturally aspirated engine.......Are higher flowing carbs used a Bonneville(altitude) as compared to sea level for the same engine build?
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This is a very generalized answer...... A quote I overheard years ago, "Never sacrifice velocity for volume"...... I'll add "at any altitude" based on my learning since.
Ed
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the general answer is "yes" because the atmospheric pressure is less. For a more specific answer, you will need to provide more details of your particular motor/carburation/application, and someone will chime in that has more experience with the application. You may also get conflicting advice. My carburetor is way toooooo big - - still runs pretty good.
Tom
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This is an interesting problem. In the calculating phase of the build I sized the Triumph carbs based on cubic feet per minute flow and figured I was pulling in less air at B'ville and I would use smaller carbs. In the final decision, the carbs are sized for adequate performance at sea level. This question is one of several I could not figure out by theory.
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Interesting indeed. I have a 540cc pushrod single. I started off with a 38mm flatslide Kehin. Ran in the low 120s. I was told this carb flowed "excellent". Which it seemed to do. I swapped it out for a 45mm round slide Mikuni that I had used on another 500 single. The difference in speed at Bonneville was an 8 mph improvement. I've always been a fan of the theory that a big hole lets more air/gas in. Which goes against the velocity theory. But I'll stick with the bigger carb.
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I had a GMC with three 275cfm Hollys that I replaced with 500cfm Hollys. Way to big by anybody's measurement. Ran better. Had a 460 Ford with an 800cfm four bbl. Put on an 1100cfm dominator. Ran much better.
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At sea level pressure is 14.7 psia.
Bonneville at 4,200 feet is 12.6 psia.
A smaller carburetor could work.
But the entire intake tract would have to match.
If you ignore any kind of intake tuning, bigger and less restriction is better.
The airflow through the carburetor acts on the venturi to provide a low pressure area to pull the fuel and atomize it.
Too large of a carburetor adversely affects the venturi. Too small restricts the air flow.
To calculate the proper size for that altitude requires knowing the volumetric efficiency of the engine.
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Bigger is usually better but it will have a effect on the venturi Delta/P, so tuning to get your mixture right will probably have to be done.
Years ago while running a different type racing, I found that running a restricter plate motor, required me to up the jet size to correct the tuning.
Rouse