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Old 03-11-2012, 12:55 PM
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Carb CFM question

Does anyone know how carburetor CFM is figures? Is it the size of the venturi boosters, the size of the rods and jets, or is it the size of the little brass tubes and passages inside the carb?

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Old 03-11-2012, 01:13 PM
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Quote:
Originally Posted by ChadGresham
Does anyone know how carburetor CFM is figures? Is it the size of the venturi boosters, the size of the rods and jets, or is it the size of the little brass tubes and passages inside the carb?
I'm pretty sure the airflow is determined by the diameter of the venturi, which is the necked-down area of the carburetor throttle bore. Contrary to what I've seen some other hot rodders say, you cannot change the CFM of the carb by "jetting it up or down". You must physically change the diameter of the venturi to change the CFM. Jet changes will alter the air/fuel ratio from lean to rich or or rich to lean, but will not change the CFM.
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Old 03-11-2012, 04:14 PM
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CFM is buy the size of the venturi, booster & basepllate size of course flowed on a bench of a stanadard of 20.4" Hg.

Watch out...many Holley knock off's making claims they are not even in the ballpark!
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Old 03-11-2012, 07:51 PM
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That is good information, i have some general knowledge on carburetors, but knew i could rely on hotrodders for more specific facts. By the way i have an edelbrock, but saw some of those holley knockoffs when i was shopping around.. Tisk tisk tisk
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Old 03-11-2012, 07:59 PM
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The size and design of the venturi determine cfm, which is the volume of air the carburetor will flow and the flow velocity at WOT, in cubic feet per minute. For maximum performance, each motor has a cfm requirement to get there; on the other hand, each motor is different in the maximum amount it can ingest, there are many variables.
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Old 03-12-2012, 01:52 PM
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If you are trying to build lots of horsepower and go faster most people picK a carb that is TOO SMALL.
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Old 03-12-2012, 07:13 PM
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How bout a 4.3 liter chevy V6 with an edelbrock 600? It actually runs great minus the horrible gas mileage
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Old 03-12-2012, 07:50 PM
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I read in another post somewhere that carb cfm requirements for an engine should be figured "CIDx2" and then round down if you have to, to the cfms that are offered. So a 350 would be 700cfm and a 383 would be 766 rounding down to a 750. Would anyone agree with this selection strategy?
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Old 03-13-2012, 06:55 AM
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Cid x2 + 50cfm =

try that

its not quite that simple but......
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Old 03-13-2012, 07:29 AM
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are yall serious about that equation??
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Old 03-13-2012, 08:05 AM
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cu inch x rpm divided by 3456 x vol efficiency . i use 80% . this works good for street cars. when racing we just went by what ever gave us the best times.
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Old 03-13-2012, 08:19 AM
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Quote:
Originally Posted by ChadGresham
How bout a 4.3 liter chevy V6 with an edelbrock 600? It actually runs great minus the horrible gas mileage
Two 850's...
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Old 03-13-2012, 02:07 PM
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CIDx2 works in a street engine. For instance, my 351W will not turn more than 5000RPM in the forseeable future so the 600CFM carb is only a very slight restriction. Keep in mind though, that with better flowing heads and intake the carb becomes the restriction more quickly. For instance, according to engine simulators my stock head and intake engine will never see more than 1 PSI (or in/hg? I forget which measurement) of vacuum against the carb in that engine's powerband. That's little if any horsepower loss. However, slap on some good 190-205CC heads and suddenly I'm seeing 1.4-1.6 PSI of vacuum, which is costing maybe 5HP up top. Hotter cam, aftermarket dual plane intake, and now I can spin it to 6500RPM+. In that case that SAME carb on the SAME shortblock/engine size is now feeling more than twice as much vacuum against the carb at peak HP as it was with the milder setup. So without touching the shortblock or changing the rest of the vehicle, I've just gone from 'needing' a 600CFM carb to an 850CFM.

Where it gets even trickier is that a single plane intake, or even a dual plane intake with a cut down center divider will require noticeably less carb at a given displacement and RPM because each cylinder can pull from the entire carb.
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Old 03-13-2012, 02:57 PM
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It depend on VE. If the bore and stroke is large enough it will flow so much CFM, then if you get heads that flow 2000CFM and a manifold that flows up 2000CFM and if the engine can flow no more then 600 CFM @ 8000 RPM. Then 1400CFM is a total waste. Remember that an engine is an air pump. So much air is flowing depending on the displacement or bore and stroke at 100% VE. Once you add exhaust system, Transmission, air conditioner system, accessory, the VE will drop. 100% VE is for RACE CARS. The only way you can get greater the 100% VE is forcing more air into an air pump like a Blower or turbo.

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Old 03-13-2012, 03:32 PM
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Try this formula: Peak RPMs divided by 2 (intake cycle every other revolution), times cubic inch, divided by 1728 (cubic inches in one cubic foot), times volumetric efficiancy. This is what I use. My VE is about 110%.

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