Carburetor cfm calculator

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Dan the man

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I came across a couple of formulas for calculating carburetor cfm. And for a 360 with a max rpm of 5,500 they all showed a cfm rating of just under 600 cfm. Even with a mild built 360 which for the most part won't see 5,500 very often isn't a 600 cfm carburetor too small or is it? I don't want to over carburate but at the same time I don't want to under carburate either. Is it true that the smaller carburetor will provide better throttle response and low to midrange torque? I'm trying to learn as much as I can so when I start the modifications the package will all work together.
 
Those calculators leave a LOT off the table. I use them as an ABSOLUTE minimum in regards to carburetor choices. In other words, About the only way I'd use the exact recommendation off a calculator would be if I was building "whatever" engine to get the best possible fuel economy. I never do that.
 
Rustyratrod thinks the same as I. If your just cruising around, that would be a fine cfm amount. I’m running a Edelbrock 600 on 2 different 360’s right now. One is a 5.9 with bolt on parts only. Just a cruiser. It does see WOT every now and again. But it mostly is just driven here and there.

The other 360 is in the wife’s car and it is seriously under carbed. It does need another 150 cfm (750) for getting all the power even though she mostly just drives it. Her car is a high compression aluminum headed hot rod with a small ish cam @ 224@050 on the intake.

You can feel the limitation on her engine. (At WOT on the big end) While the carb will be changed out later, it’s still a good driver with excellent throttle response. Sometimes the smaller primary is a restriction that can be felt. I experienced this with a TQ on a dead stock (‘78) 400 with only headers and a intake change. I had at my disposal 2 TQ carbs. An electric choke small version was used first. Then I switched it to a early year big primary unit. There was a big difference in the power return with no mileage loss.

This is why you’ll see a 750 on top a 360 instead of a 600/650. A lot has to do with the rest of the build as well. Things that give way to a larger carb are things like increase cam, stall & gear.

See if you can get both size carbs as a loaner from a friend and try them out. Then you can decide which is best for you. Even though you didn’t list any parts for what a mild 360 is, a 600 would not be a bad choice. A larger carb will probably deliver more power up top, BUT! a lot is how often you will be there and do you need that last 10 hp?
 
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rumblefish360 said:
Rustyratrod thinks the same as I. If your just cruising around, that would be a fine cfm amount. I’m running a Edelbrock 600 on 2 different 360’s right now. One is a 5.9 with bolt on parts only. Just a cruiser. It does see WOT every now and again. But it mostly is just driven here and there.

The other 360 is in the wife’s car and it is seriously under carbed. It does need another 150 cfm (750) for getting all the power even though she mostly just drives it. Her car is a high compression aluminum headed hot rod with a small ish cam @ 224@050 on the intake.

You can feel the limitation on her engine. (At WOT on the big end) While the carb will be changed out later, it’s still a good driver with excellent throttle response. Sometimes the smaller primary is a restriction that can be felt. I experienced this with a TQ on a dead stock (‘78) 400 with only headers and a intake change. I had at my disposal 2 TQ carbs. An electric choke small version was used first. Then I switched it to a early year big primary unit. There was a big difference in the power return with no mileage loss.

This is why you’ll see a 750 on top a 360 instead of a 600/650. A lot has to do with the rest of the build as well. Things that give way to a larger carb are things like increase cam, stall & gear.

See if you can get both size carbs as a loaner from a friend and try them out. Then you can decide which is best for you. Even though you didn’t list any parts for what a mild 360 is, a 600 would not be a bad choice. A larger carb will probably deliver more power up top, BUT! a lot is how often you will be there and do you need that last 10 hp?
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IMO, the reason those calculators don't work is two fold. First, none of them take into consideration anything to do with what the combination is. Secondly and this might be the bigger one, they also don't take into consideration WHAT KIND of carburetor.

You know as well as I do an 850 AIN'T an 850. Case and point. An 850 DP Holley compared to an 850 Thermoquad.

An 850 Thermoquad can be tuned to run on a hot slant 6. That same carburetor can also be tuned to run on a hot 440.

Conversely, the 850 DP Holley will be way too much for an slant 6, while can haul the mail on a 440.

This is one reason the factories used carburetors like the Thermoquad and the Quadrajet. They are basically the same carburetors in a different body. Now all the Mopar chest thumpers will come out in force to disagree, but they're wrong. lol

They are both hybrids. Mechanical secondary carburetors with an air valve controlling the secondary barrels. They can be used on MANY different engine sizes from 318 to 440 with very little tuning differences.

As such, those calculators don't count for much when using that style carburetor. The Carter AFB and AVS and the new little Street Demons fall into the same category. They are all variable CFM up to their maximum through secondary door adjustments. Even vacuum secondary Holleys are similar with their changeable secondary springs and or adjustable secondary diaphragms. It's just hard for a calculator to accurately pinpoint, since it does not know your combination or what style carburetor.

If any of that made sense. lol
 
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It does to me! When I use a square bore carb, it is ether sized small for a driver. Driven like a normal car. Or it is sized for the “Hot Rod” barreling down the road at WOT.

With a spreadbore, as long as the primary side is large enough…. It’s good for both.
 
rumblefish360 said:
It does to me! When I use a square bore carb, it is ether sized small for a driver. Driven like a normal car. Or it is sized for the “Hot Rod” barreling down the road at WOT.

With a spreadbore, as long as the primary side is large enough…. It’s good for both.
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I believe that's right in line with what the factory engineers thought. And I agree.
 
Carb selection and worries of being too small or too big and how to size has been around for decades, but now with so many sizes and styles you gotta decide on more than just cfm. Pure street use:
The old but still relevant from 1978 “Mopar Performance” book gets it right IMO:
A39AE61F-49C4-44BB-A85C-04AD1042179E.jpeg
CB002C7A-8275-49FC-B9B7-A26E0097C630.jpeg


Keeping the above in mind then you could move on to formulas and calculations (and if you know the specs for the rest of the “package”) see what cfm you come up with.
As far as calculations go I like Vizards carb selection method which uses a correction factor.
104062AB-FCA6-4AD3-AD6B-74A177DB4599.jpeg
14C8A0F1-0352-4CC9-B6DA-015F56F4836D.jpeg


Figure where your peak power rpm is and Then run the calculation with the conversion factor:
18D15BF0-C93F-4721-9065-A6403D3EF56D.jpeg


Depending on your cams duration @.050” and cylinder heads, for example you might have a correction factor of about 1.01 so:

CFM= 360 x 5500 x 1.01
—————————
3456
Running the calc yields approx. 580 CFM

Take the various calculations and compare, and with those numbers use the Mopar Performance along others way of thinking and err on the side of conservative. Unless you’re drag racing and need ever last ounce of upper rpm power......

For what you have, looks like you’d be using:
Holley: 600-650
Edelbrock: 600-650
Street Demon: 625

And call it great!
 
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12many said:
Carb selection and worries of being too small or too big and how to size has been around for decades, but now with so many sizes and styles you gotta decide on more than just cfm. Pure street use:
The old but still relevant from 1978 “Mopar Performance” book gets it right IMO:View attachment 1715827462View attachment 1715827463

Keeping the above in mind then you could move on to formulas and calculations (and if you know the specs for the rest of the “package”) see what cfm you come up with.
As far as calculations go I like Vizards carb selection method which uses a correction factor. View attachment 1715827466View attachment 1715827465

Figure where your peak power rpm is and Then run the calculation with the conversion factor:
View attachment 1715827477

Depending on your cams duration @.050” and cylinder heads, for example you might have a correction factor of about 1.01 so:

CFM= 360 x 5500 x 1.01
—————————
3456
Running the calc yields approx. 580 CFM

Take the various calculations and compare, and with those numbers use the Mopar Performance along others way of thinking and err on the side of conservative. Unless you’re drag racing and need ever last ounce of upper rpm power......

For what you have, looks like you’d be using:
Holley: 600-650
Edelbrock: 600-650
Street Demon: 625

And call it great!
Click to expand...
Only problem is, with what's available today with carburetors, that information is dated. Look at all the new style Holleys and Holley knock offs like the Quick Fuels and such. That MP information was written long before we had so many different adjustments like the idle and high speed air bleeds, the idle fuel restrictors, the fuel curve restrictors and on and on. That gives a LOT of leeway and tuneability that we otherwise would not have and they did not have when those articles were written. ...at least they were not available to the general public. Professional race teams knew about those mods, but they were not mainstream yet.
 
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I've done so many carb swaps, and even done them at the dragway. 340's/360's, even in stock or next to stock form, have always done better with a 750 vs 600.
 
RustyRatRod said:
Only problem is, with what's available today with carburetors, that information is dated. Look at all the new style Holleys and Holley knock offs like the Quick Fuels and such. That MP information was written long before we had so many different adjustments like the idle and high speed air bleeds, the idle fuel restrictors, the fuel curve restrictors and on and on. That gives a LOT of leeway and tuneability that we otherwise would not have and they did not have when those articles were written. ...at least they were not available to the general public. Professional race teams knew about those mods, but they were not mainstream yet.
Click to expand...
My long winded point I was tying to make is erring on the side of conservative, as that’s how I roll with everything :)
 
Here’s a brief (I’ll try to keep it that way!) example of what I’ve experienced with my vehicle. I used the examples I showed above as my guide when deciding on a carb. I’ll spare others the details of my setup but just say that using the formulas I came up with basically a 750 CFM carb as ideal.
Started doing some strip runs and the first time at the 1/8 strip after a few runs I was in the 8.02 et at about 86 Mph range. I immediately had a fellow Mopar guy stop by asking about details of my engine. Told him I had a 750, he said I should try an 850. Well that wasn’t happening anytime soon. At another track my 60’ were about 1.75 or so. Had another guy say I should try a 50cc pump. Not possible as I was running a Street Demon 750. Anyway, so over a few years and countless test n tunes without changing actual parts, only tuning incessantly I was able to whittle all my times down with the same carb to the 7.60@89 Mph and about 1.64 60’ range. Then I swapped intakes and installed a 750 DP and gained more.
Now with my head and cam changes and further tuning I’m still running the 750 Holley DP. Running new calculations with my new parts yields an ideal cfm of 830. I’m running 7.20@94 in the 1/8 and 11.44@114 in the 1/4 atm on the 750, but now I’m in the midst of tuning and more whittling away. Maybe some day I’ll up to an 850. The 750 runs pretty sweet on the street as well, is dialed in for street/strip so now I can refocus on other stuff to gain. But if and when I upgrade the carb I won’t go over 850 as again, I err on the conservative side!

Brief? :rolleyes:
 
318willrun said:
I've done so many carb swaps, and even done them at the dragway. 340's/360's, even in stock or next to stock form, have always done better with a 750 vs 600.
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How would you describe the drivability between the 2 carbs? Which one did you like better? Pros & cons & opinions between them?
 
Had a holley 770 on my 340 swinger ran great. However always felt it was lacking at low RPM driving, around the neighborhood and such...built a 600dp to see what would happen, so much "cleaner" at low RPM, off idle is great but I definitely lost my WOT performance....it falls flat much quicker now....been thinking of giving the eddy AVS2 a try on my 383 car and if I like it then put on on the 340....that's a big maybe though!
 
You're set up screams for a Carter Thermo quad IMO then my second choice would be a 625 or 650 Carter avs. Both used by the factory in similar setups and for good reason.
 
rumblefish360 said:
How would you describe the drivability between the 2 carbs? Which one did you like better? Pros & cons & opinions between them?
Click to expand...
Noticed no difference putting around town, idling at a stop sign, or cruising down the highway. But under WOT the 750 always offered more, and not on just one project, but all of them. Even when my 360 had stock heads, stock exhaust manifolds, 2.45 gears, with the stock converter, the 750 DP still ran the better time and HERE IS THE KEY - the better 60 ft time as well compared to the 600 edelbrock. Both carbs worked flawlessly without a hiccup. Both ran great, one just performaned a little better.
 
This was a nice little write-up in Mopar action this month. I also have run everything from 600 to 1,000 CFM packages on my 340 but but a dialed in thermoquad for the street it's the s*** ski

IMG_20211128_073157874.jpg
 
when i first put the 440 into my old 65 dart i borrowed a 750 vs holley from a buddy. after a couple of weeks he got his car ready for the road so it had to go back and all i could lay my hands on was a 600 vs holley. the dart was noticeably more responsive but on the open road it was just as noticeable that it lost power above 5000 rpm. considering how often i would be driving above 5k i left it with the 600 and liked the driveability. if i was going to the strip fairly regularly then i'd have got another 750 (or bigger) but that wasn't on the cards. plus i liked the better economy too as it meant i could use the dart for work a few times a week too.
neil.
 
[1] Unless I missed it, I did see the term VE [ volumetric efficiency ] mentioned. It becomes most important when selecting WOT carb airflow, no matter which 'formula' you use. A production engine at peak HP rpm might only have 75% VE. So if the formula calculates 600 cfm, the theoretical ideal carb cfm would be 450 cfm. At the other end of the scale, a full blown race engine might have a VE of 105% at peak HP rpm, so 630 cfm would be the ideal rating.
[2] Theoretical. So many other things to consider when selecting a carb, other than cfm. Booster types, pri/sec bore sizes etc. Plenty of examples of smaller cfm carbs making more HP than large cfm carbs because of better atomisation & distribution. Not all about size.
[3] TQs & QJs. A 750 QJ was used on the Aussie 253 Holden V8; it was also used on the 1967 Pontiac OHC in line six, which was 230 ci. This is the versatility of the air valve type carb. Many people think that these carbs on a smaller engine do not fully open the AV or the sec t/blades. They do. It is the pressure drop at WOT that changes with engine size.
 
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