Happy Easter all. In my quest to not only learn more about Mopar engines, my own engine and car, and automotive mechanics in general, I've been asking a series of questions over the last few weeks. Honestly, it's mostly to keep my sanity as my work has dried up because if Covid-19. So here is the next question: How accurate is the CFM formula? Here why I ask. After speaking to a handful of professional engine builders, they pretty much agreed on one thing that first time engine builders make mistakes on. They over cam, and go too big on carburetion. Now for my build, my original idea was to go with an original 1970 440HP cast iron intake and 1970 Carter AVS carb which flows 625 CFM. Then I decided to go Eddy RPM intake and a new carb, like a Holley or Eddy in 750-800. Money has dried up because of no work, but I still have to finish paying the engine builder. another $1200 for the new intake and carb will be very difficult to justify right now. Gotta keep the fridge full obviously. So I'm 90% sure I'll stick with what I have for now, as I already own it, and can upgrade later. Now, back to CFM. The formula is:

The formula for calculating how much CFM (cubic feet per minute) your engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency(?%) ÷ 3456.

The volumetric efficiency is a bit of a mystery. If you watch Pete's Garage on YouTube, he explains it far better than I can. I'll link it below.

https://youtu.be/xAW3g-qz0_U

So if my engine is say, 90-95%, then that puts my CFM at 605-635. Now that's at 5200 rpm, which is the number I used as I will rarely be above that. So maybe some input from you guys as to how you figure how big a carb to run for your requirements. Have a great and safe day all.

The formula is accurate for street and mild performance applications. The formula isn't used for professional race engines which only run at wide open throttle.

The formula is a very simple mathematical equation the represents flow. It is 100% correct. The VEs assigned are probably correct enough.

But here is the part people don't understand or struggle with. That formula has nothing to do with what is the best size or best carb for an application or needs of the car owner.

The formula assumes that the 1.5" hg pressure drop that the carb (4bbl) is measured/rated at is the "right" or "best" dP. It seldom, or maybe never is.

In real life, the lower the dP (bigger carb) you can get away with while maintaining good fuel delivery, distribution and atomization, the more power you will make.

To piggy back on what Russ said.......
Imo, for a mostly street build, how “well” the carburetor functions in the ranges where it’s used the majority of the time should be a higher priority than if it makes 5hp less than a “bigger” carb at WOT.

Getting a carb that balances good driveability, fuel economy, and power output is what I aim for with motors that spend most of their time running down the road.

In today’s market, this is where things like the Sniper EFI kits excel....... especially when you implement the timing control function.

the cfm formula is a tool that I don't get lost in, and don't use it. I do think it can keep the novice out of trouble but as one progresses you'll move away from it. if your going to use the cast iron intake then use the avs; they're not bad carbs (you could do much worse).

The formula is only as good as the inputs. The displacement, rpm, and conversion factor (3456) are all hard numbers. The VE is not. Depending on engine speed, cam profile, head capability, etc. the VE can be all over the map. As such the formula is only good for a guideline or an initial guess, nothing more.

I have only used the Holley formula above to get a ballpark size. But most rookies think their engines will attain 100% VE. Most will NOT. Not even close to 100%. I think Smokie said the average VE most street engines attain is around 85% with basic "Hot Rodding" of a hotter cam and headers. YMMV!

My 410CI stroker 5.9 with full ported Eldy heads, warm cam, 10.25:1 CR, shorty 1.75" headers may be around 90% VE and at 6500 RPM MAX says I'll need a 694 CFM. I will be running my 850 CFM ThermoBOG on an Air Gap manifold.

My builds tend to be wilder than most so as such I have found that carbs larger than recommended at 100% VE tend to produce better driveability. But that's just me. I've always said show me the formula where a NA 358 CID engine requires 2, 1250 CFM dominators.

If you're looking for information this chart might be more useful.

Of course everybody has an opinion on carbs. Brands, sizes, secondary actuation, etc. And my opinion on this is that getting the right calibration is much more important than the size. As long as the size is not too far afield. Frankly, I don't feel that the CFM is the most important thing for a street car. A less than optimum CFM rating (within reason) would be less negative than improperly calibrated transitions. And less harmful in some cases.

Most here would probably agree that something in the 600 to 800 range would be appropriate for the OP's 440. But it is best to remember that each carb builder builds multiple versions of each size. In that 600 to 800 range there could easily be 60 or more different standard production carbs available. There are probably more than 25 750s alone.

And of course there are the guys that think that you have to spend thousands on some antiquated plug and pray EFI system. But a properly calibrated carb is very hard to beat for the bucks.

Most guys with mild street cars just want their cars to run good with the carb right out of the box. That can be a tall order and for many the reality is that it ends up coming down to luck. If the car runs good with a particular carb OOTB, it is a good carb. If there are driving or starting troubles OOTB, the carb is junk or it needs an expensive EFI installed to cure it.

In other words, it takes a lot more than getting the CFM in the ballpark for the car to start and run good without a lot of tuning.

As a short-cut, I'll make a recommendation based on my experience. The new Street Demons are the best out of the box carb I have ever installed on mild street engines. There are 2 sizes, 625 and 750 with the primary throttle bores being the difference in the 2. And there are 3 versions of each. The only difference between the 3 versions is the material and finish of the center sections. All come with an electric choke.

Either size will work okay for a 440 in my humble opinion. I would lean one way or the other with enough info on the combo and the driver's preferences, but either one should be okay for most mild street 440 combos. And the black composite center would be the best version. At only $300 or so, they are a bargain.

https://www.youtube.com/watch?v=oAhdCRixqqw

Too many stories where the carb was perfect ROTB. Unless some tuning was attempted how would one know????

All the HP 440 from 1967 to 1971 came with 750 CFM carbs, the lo po 440 came with smaller carbs
I've played with a bunch of 440 motors with stock strokes and stock iron heads, all of them went faster and quicker at the race track with more cfm and fuel, one year(1991) I started my old bracket 440 powered Duster off with a 600 Holley D.P. on a old eddy TM7 intake, that combination ran11.70s in the 1/4 mile, I ended up switching to a Holley 750 and then a Holley 950 HP and finally swap the intake and carb to another racer for his Mopar M1 intake made for the Holley Dominator carb and his Holley Dominator 1050 CFM carb that ran 11.20 in my Duster and around 7 MPH faster than the 600 carb. ran
I helped a friend do similar testing going from a 440 Torker intake with a Holley 750 D.P. to a cast iron six pack intake with stock 440 carbs and it picked up .15 ET and a little under 2.0 MPH in the 1/4 mile. His next swap was from the six pack set up to a tunnel ram with two 600 CFM vacuum Holley carbs. made his car go another .15 ET quicker an 1.6 MPH faster in the 1/4 mile
My message is if you want power don't go small on the carb
IHTHs

The formula does something useful - for a single plane manifold with minimal plenum volume.
Any other type (dual plane, divided single plane, tunnel ram, individual throat) is different. Your iron manifold and the Edelbrock RPM are both dual plane, and the carburetor predicted by the formula is too small.
Regardless of what Holley says, a 750 CFM carburetor will flow 750 if the engine draws 1.5 Hg" @ WOT. E.g., a 440" engine @ 6,000 RPM and 100% efficiency calculates to 764 CFM, so that carburetor is a good (but not only, best, or fastest) choice.
If the engine is smaller with lower WOT vacuum, that same carburetor will flow less.
If the engine is bigger with higher WOT vacuum, it will flow more.

edelbrock use to publish a formula in their catalog some years back. they said do the cfm calculation and then multiply by 110% for a single plane and 125% for a dual plane. they no longer publish this and i'll bet it's because their carbs, multi-carb packages may not fit into their formula. anyhow, I think the edelbrock formula is better than the holley for the average guy doing a single 4bbl set-up. as for me I've never gotten out of the venturi area thing and only treat cfm ratings as a manipulated tool.

13 carbs of differing brands/styles/sizes all tested back to back to back.....etc, with no other changes on a 383 with unported 906’s with an RPM intake and headers: