Carb Sizing Discussion
#1
One section in my Holley carb rebuilding and tuning book talks about why in some cases carbs larger than the calculated size work better. I came across this article which discusses the same topic related more specifically to 351C motors. I thought it might be interesting to read for some members. The end of the article has some carb models. I don't know how pertinent that is.


Four Barrel Carburetors

At 6000 rpm a 351 cubic inch motor would theoretically inhale 609 cubic feet of air per minute if the volumetric efficiency were 100%. At 7000 rpm the same motor would inhale 710 cubic feet of air per minute assuming 100% volumetric efficiency. Assuming 90% volumetric efficiency a 351 cubic inch motor will inhale air at the rate of 548 cfm at 6000 rpm or at the rate of 639 cfm at 7000 rpm.

However, as the volumetric efficiency of a motor improves the intake manifold vacuum at wide open throttle shall decrease. The intake manifold pressure of a motor with 100% volumetric efficiency is theoretically equal to atmospheric pressure at wide open throttle. The airflow rating of carburetors is measured at a fixed depression, such as 1.5 inches of mercury in the case of Holley carburetors. If the depression across a Holley carburetor is less than 1.5 inches of mercury at wide open throttle it will not flow the amount of air it is rated at, the motor shall require a carburetor with a larger rating than what we calculated in order to supply adequate airflow at 6000 or 7000 rpm. The reason for this is not because the motor demands more air flow than what we calculated but because the carburetor, which is rated at a depression of 1.5 inches of mercury, flows less air if the depression is less than 1.5 inches of mercury; in other words the flow rating of a carburetor as determined at 1.5 inches of mercury becomes less relevant as the volumetric efficiency of a motor increases.

Both the 351C 2V and the 351C 4V have higher volumetric efficiency than the popular in-line-valve V8s people are more familiar with; at wide open throttle the vacuum in their intake manifolds will drop lower than it does in those other V8s if the carburetor is large enough to allow it. This is the reason larger carburetors are recommended for the Cleveland engine series. If an owner allows the popular literature to influence their choice in carburetors, they shall end up strangling their Cleveland motor with a carburetor that is too small.

On top of that the 351C 4V is capable of operating over an extraordinarily wide power band, certainly wider than any other OHV engine from its era. The first 351C 4V performance manifolds designed by Ford were designed for list #4575 Holley Dominator carburetors (1050 cfm)! Ford’s earliest carburetor recommendations also included the Holley 850 cfm double pumper. The 351 Cleveland engines require carburetors designed for engines having higher volumetric efficiency and in the case of the 351C 4V a wide power band too. The usual carburetor choices for a 351C 2V usually range from 650cfm to 750cfm; for the 351C 4V those choices usually range from 750cfm to 850cfm. None of these carburetors are too big for a 351C street motor, especially if they are equipped with annular booster venturis. With a 351C 4V street motor it is a challenge to find a carburetor that performs well at low rpm while also being large enough to take advantage of the WOT (wide open throttle) volumetric efficiency of that motor.

Annular booster venturis atomize fuel better and provide a stronger fuel metering signal at low air velocity. In other words, annular booster venturis benefit the low rpm and mid-rpm performance of a motor in the same manner as the smaller primary throttle bores of a spread bore carburetor. These attributes make annular booster venturis popular for improving the low rpm operation of performance engines, where they have earned a reputation for improving torque, horsepower and throttle response at low engine speeds. However the improvement in fuel atomization distributes fuel more consistently throughout an intake manifold, resulting in more consistent fuel/air ratio from cylinder to cylinder, therefore annular booster venturis actually improve torque and horsepower across a motor's entire power band; and they improve fuel economy too! The only drawbacks of annular booster venturis include their larger physical size (which reduces the airflow capability of a carburetor by a relatively small amount) and their greater cost of manufacture.

Mechanical secondary/annular booster carburetors featuring street calibration and electric chokes

*Demon Carburetors #1282020 - 650 cfm - Speed Demon, mech. secondary, elec. choke kit #421440
*Demon Carburetors #1402020 - 750 cfm - Speed Demon, mech. secondary, elec. choke kit #421440
*Demon Carburetors #1563020 - 850 cfm - Speed Demon, mech. secondary, elec. choke kit #421440
*Holley #0-9379 - 750 cfm - Competition Series, mech. secondary, choke horn equipped
*Holley #0-9380 – 850 cfm - Competition Series, mech. secondary, choke horn equipped
*Quick Fuel Technologies (QFT) #SS-650-AN – 650 cfm – SS Series, mech. secondary, electric choke
*Quick Fuel Technologies (QFT) #SS-750-AN – 750 cfm – SS Series, mech. secondary, electric choke

Vacuum secondary/ annular booster carburetors featuring street calibration and electric chokes

*Demon Carburetors #1282020VE - 650 cfm - Speed Demon, vac. secondary, electric choke
*Demon Carburetors #1402020VE - 750 cfm - Speed Demon, vac. secondary, electric choke
*Demon Carburetors #1563020VE - 850 cfm - Speed Demon, vac. secondary, electric choke
*Summit Racing #M08600VS - 600 cfm – vac. secondary, electric choke
*Summit Racing #M08750VS - 750 cfm – vac. secondary, electric choke

Vacuum secondary/down-leg booster carburetors featuring street calibration and electric chokes

*Quick Fuel Technologies (QFT) #SS-680-VS - 680 cfm – SS Series, vac. secondary, electric choke
*Quick Fuel Technologies (QFT) #SS-735-VS - 735 cfm – SS Series, vac. secondary, electric choke
*Quick Fuel Technologies (QFT) #SS-780-VS - 780 cfm – SS Series, vac. secondary, electric choke


Regards,
Mike
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#2
That info is really interesting and is, in essence, the same as I was reading about DCOE (side draft) Webers. In that street drivability and choke size might need selections away from 'text book'. This thread gives adational info as to why this is/maybe necessary.

In relation to the flat torque 'curve' of the 351C, I can vouch for that having seen the graphs printed for 'her' '70.
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#3
Some great info on why the Cleveland's struck fear in many a Chevy lovers heart all you had to do was feed a Cleveland and it became a Monster! JTS 71 Mach1
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#4
I liked the article. Since the article focus' on Cleveland motors and a lot of members here run Clevelands I thought it was worth posting. I also thought it may provide some additional knowledge if somebody is considering a new carb.

Regards,
Mike
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#5
Absolutely worth posting!

It gives a good explanation on why the Cleveland motor has different requirements than the "normal" in-line valve train.
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#6
No engine produces 100% VE. The article assumes that the exhaust is also capable of handling the maximum flow. Most street exhausts are compromised.
The article also only addresses VE at max rpm. That is only parameter of engine performance.
Torque is probably most important in a vehicle's performance, and maximum ve is not a good overall indicator of torque capability.
There is no "free lunch": every advantage is "paid for" somewhere, and the 4V Cleveland's extraordinary high rpm flow capabilities is somewhat offset by the mediocre low rpm torque production.
It is well known that a 2V Cleveland with aftermarket 4 barell intake and carb will generally outperform a 4V Cleveland on the street.
Off-idle torque is much stronger on a 2V vs a 4V.

1/4 mile and racetrack use? Different story altogether.
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#7
I was always taught naturally aspirated motors won't achieve 100% VE. I don't know any more and I tend to believe the article. I have seen this same information in three sources. The others being a Holley supertuning and rebuilding manual and Edelbrock's tech section of their website. Edelbrock doesn't go into details but gives a range of percentage values by which a carb size should be increased over the calculated required size. Edelbrock's high end of the range seems a bit high, even to me. But the point would be Edelbrock also recommends carbs slightly larger than the calculated requirement.

I thinks the article discussed using carbs with annular boosters to help recover low RPM performance with larger carbs. I have only used one carb with annular boosters and I will say there is a noticeable improvement in performance throughout the RPM range.

I guess in the end, you really need to consider more than apply the simple mathematical formula that calculates the CFM requirements for your application.

Regards,
Mike
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