What size carb do you need?

[Camtron]

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]

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]

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]

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]

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]

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]

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