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Old 04-15-2018, 01:24 PM   #1
Bob Bidonde
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Default Zenith Carburetor CFM?

My speculation is the Model A needs to breath 100 CFM in stock form. I base this on the engine's volumetric efficiency.

100% Volumetric Efficiency means that the engine can breath its total displacement in one combustion cycle. So for 200 cubic inches, and assuming the "L" head engine has a 50% Volumetric Efficiency, the carburetor needs to be capable of 100 CFM

Does anyone know the CFM of the Model A's Zenith carburetor?
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Old 04-15-2018, 01:43 PM   #2
ursus
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Default Re: Zenith Carburetor CFM?

Read all about it in this link from Vince Falter's excellent website:
http://www.fordgarage.com/pages/zenithBcarb.htm
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Old 04-15-2018, 05:42 PM   #3
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Default Re: Zenith Carburetor CFM?

For a naturally aspirated 4 cycle engine, the cfm consumption may be calculated as follows:
Since 2 revs are required for 1 intake stroke, divide the RPM by 2.

Since we are dealing with cubic feet, divide the cubic inch displacement of the engine by 1728 cubic inches in 1 cubic foot.

The volumetric efficiency can vary widely. 75% - 85% for more modern engines. Model A engine is probably less than 75%, especially with an air filter.

Example:
Use 205 cubic inch displacement, since most Model A engines have been bored.
Use 2400 RPM.
Use 73% vol eff

205/1728 = .1215 ; 2400/2 = 1200

Required CFM = .1215 x 1200 x .73 = 106 cubic feet/min

Compare this with Vince Faller's chart.
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Old 04-16-2018, 07:07 AM   #4
Bob Bidonde
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Default Re: Zenith Carburetor CFM?

BN, we have similar CFMs, and they are in the range of the Number 20 Venturi used in the Model A per the Zenith Company's chart on Vince's site. So I am calling it a 100 CFM carburetor.

I wonder what performance gain would come from using the Model B's Venturi an jet sizes in the Model A's Zenith?
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Old 04-16-2018, 09:36 AM   #5
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Default Re: Zenith Carburetor CFM?

If measuring CFM on a flow bench, It makes one whale of a difference how much vacuum is used to get readings. When I worked in this business about 35 years ago there was no standard among head shops nor was there any standard for manufacturers. Hopefully it is more uniform now, I really don't know. Also it is possible to get better than 100% efficiency but only at a narrow RPM range. its all about getting valve overlap (cam profile) in sync with tune length to produce the greatest air velocity with the straightest and smallest port opening and high attack angle at the underside of the valve. That is why most cars have variable length and volume intake manifolds now with 5-6-7 or 8 speed transmissions now.
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