Carb CFM

Lil Red · Dec 26, 2008

Can someone tell me how to figure the CFM of a Carb? Thanks for any help. Mike

Crutch · Dec 26, 2008

For WOT the formula is below. This if for peak cfm requirement at peak HP. You may cause your idle circuit to stuggle.

Area = (Engine Displacement X rpm X VE) /187200

Area is the size of the venturi, VE is Volumetric Effeciency, which you would have to guess at but for a tractor 80 % or less would probably be a good guess. So enter .80 for 80 %. 187200 is a constant.

3.13 inch Dia or 7.69 area is required for a 600 cid motor turning 3000 rpm at 80% VE.

guido · Dec 27, 2008

Hello Lil Red.
To determine the airflow requirements of an engine (CFM'S.), you need to calculate the approximate amount af air volume per minute in eather cubic feet per minute (cfm) or cubic meters per minute (cmm).
Engine displacement per minute is :
N x A x S x N(CFM)
WHERE N = number of cylinders.
A = piston area in sq. ft. (sq. m.)
S = stroke in ft.

N = cycles per min. in one cylinder/RPM'S.
= rpm's for two-cycle engines.
= rpm.s/2 for a four cycle engine.
Just plug in your engine's data and you can find out its air flow requirement with the above formula. Any questions my e-mail is open.
Hope this helps...........Guido.

Custom King · Dec 27, 2008

So nobody knows the answer?

Charles E. McNelly III · Dec 27, 2008

Hi Mike,
ve = displacement x rpm / 3456 . Holley's equation for sizing of carb(CFM).
Good Luck,
Charles

LW · Dec 27, 2008

Figure the CFM of a carb? Flow bench.

mEl · Dec 28, 2008

If I am interpreting your question correctly the only way to do it is to take the carb to a engine builder who is equipped with a flow bench and have it tested under actual airflow conditions. All the formulae that the guys are giving you are valuable but not what you are looking for in my opinion, these are just mathmatical excercises to help determine the engines requirements, not what the carb will flow in actuality. Tractor carbs are not rated in CFM so we can select as Holleys and others in the auto field, if so the info is not readily available to my knowledge. You can also take your manifold, head and carb and have them checked as an assembly. Hope this is what you are looking for. Happy New Year to all. mEl

Superih · Dec 31, 2008

Good post, but are you sure on the venturi size? That sounds a little big.

According to that equation a stock Farmall M (248 cid, 1600rpm)at 80% VE would require a 1.47" venturi. That is bigger than the entire carb.
Would this calculation just show how inefficent the motor is? Keep in mind a stock carb uses a 0.84" venturi.

thanks

Carb CFM

Lil Red

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Crutch

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guido

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Custom King

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Charles E. McNelly III

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LW

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mEl

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Superih

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