What size carb do you need?

Camtron

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There’s an easy way to get in the ballpark of what size carburetor you need for your car.

Engine size x RPM x volumetric efficiency / 3456 = CFM

So for example of our stock engines,

318 x 5000 x .80 / 3456 = 368CFM


For volumetric efficiency, its best to figure a stock factory engine works about at 80% efficiency. A “built” street engine may be in the 90% efficiency range and a full on race engine could actually exceed 100% efficiency.

So maybe we have a 318 with a cam, intake and headers moving the RPMs and volumetric efficiency up; Something like, 318 x 6200 x .90 / 3456 = 513CFM

Anyway, there’s some more to all this, but this basic formula with get you in the ballpark. The down side to this, a lot of people may realize they’re wasting gas and more importantly losing performance by sacrificing port velocity running too big a carb on their smog engine...Another issue, small CFM 4V carbs can cost twice as much as a 650cfm 4V.

This formula and information can be found with a quick, Google search, but with so many visitors here I figured it may be useful to someone later.

Feel free to elaborate/correct or add any additional information that maybe helpful to others.
 

AJ/FormS

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I add the combustion chamber.(*)
So for my 367; the Swept is 752cc and the chamber is 75cc for a total of 827cc which is 50.5 cubes, and times 8 cylinders =404 cid; and

(404 x 6500)/3456 =760cfm and I run a Holley 750DP.

I have run smaller carbs but the engine seemed sluggish.
If I had a bigger carb, I would try it.
But the 750 was almost a bolt-on.
This is a street car, that just happens to go 93 in the Eighth at 3457 pounds and at 930ft elevation.

(*)I add the combustion chamber , cuz when the piston is at the bottom, atmosphere is trying to fill 827cc, and Not just the swept of 752..... lol
And I use 100% because everybody knows that Mopars RULE!.... lol.
 

greymouser7

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So for my 367; the Swept is 752cc and the chamber is 75cc for a total of 827cc which is 50.5 cubes, and times 8 cylinders =404 cid;
Check again AJ, this doesn't compute.
 

AJ/FormS

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I add the combustion chamber.(*)
(*)I add the combustion chamber , cuz when the piston is at the bottom, atmosphere is trying to fill 827cc, and Not just the swept of 752..... lol
And I use 100% because everybody knows that Mopars RULE!.... lol.
That's 404 cubic inches of total chamber + cylinder volume, filled to one atmosphere, when the piston is at BDC.
not 404 cubes of engine displacement.
this makes the formula work.
The regular formula is
(engine cid x rpm)/3456= carb size in cfm. For my engine that would be
(367 x6500)/3456=690cfm
this assumes 100%VE
But my engine is probably closer to 85% or 90 at most, so then
690 x .90=621.
Well, I installed 600 which sucked.
We know that formula doesn't work properly on Mopars.
But when you add the chamber volume to the swept volume, I get 827cc which is 50.5 cubes. And this would represent 50.5 x 8=404 cubic inches of air. So now;
(404 x 6500)/3456=760cfm at 100% VE which is a lot closer to the requirement than 690 .. Now .90 x 760=684 better, but still shy, cuz we all know that a 750 is about right for a 360. And that is what I eventually installed with no regrets.
I hope that makes sense.
 

Aspen500

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When computing my engine, it comes up with about 650cfm. Had a 750 vac sec. on it awhile and it never seemed right no matter how much calibrating I did. Yes, the car has an A/F guage. Switched to a Quick Fuel Brawler 850 mech secondary and, night and day. Almost felt like a whole different car. Anyways, I don't really trust those generic calculators.
 

Camtron

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When computing my engine, it comes up with about 650cfm. Had a 750 vac sec. on it awhile and it never seemed right no matter how much calibrating I did. Yes, the car has an A/F guage. Switched to a Quick Fuel Brawler 850 mech secondary and, night and day. Almost felt like a whole different car. Anyways, I don't really trust those generic calculators.
I figured you’d be something like, 512ci x 6,500rpm x .95efficient / 3456 for something like, 914cfm. Changing the RPM to 6,000 and efficiency to .90 is 800cfm on the nose though. Idk what your specs are, but seems close to me.
 

Ele115

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You have to put the whole formula into the divsor of planck's constant and divide by the birth year of black Jesus squared. That gives you 779 cfm
 
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