Carburetor cfm calculator

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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:
Briefer than a lot! And yours is real drag strip experience....although not everybody tunes a flyin brick. lol
 
Run what ever size/cfm carb you want.

Can you run a smaller carburetor and still turn rpm's, sure. You'll just effect the torque output. You can cruise a 600 on a 440 or 500 on a 360 etc... if you want some more umph for passing but ultimately focus on gas milage...use a smaller 4 brl than optimum for max power. 750 or 390 lol you decide. 800 something is a wash for low rpm drivesblility on a 340/360. 950 have smaller venturis...
 
The problem is the way carbs are rated. That rpm x cid / 3456 = cfm formula is that it says your engine moves 550 cfm, the problem basically any carb you put on becomes a 550 cfm carb but at different vacuum level so say a 390 cfm 4 bbl will flow the 550 cfm but at a higher vacuum level aka more restrictive a 950 cfm carb will flow 550 cfm but at a lower vacuum level less restrictive.

You don't need a formula we already know what works, if your trying to get the best power or best power and drive ability etc.. sometimes you got to try a few to dial it right in. But generally 600-650 cfm mild 400-350 hp and under, a 750 cfm 350 + hp but even under 350 hp a 750 cfm can be made to work etc..
 
Back in 1982, '71 340 Cuda street/strip, stock motor+ 484/284 cam, ally intake, small hdrs, 750DP, 3500stall, 3.55's/8x26" slicks, best was 13.28@101+, =301fwhp for weight mph, changed to an 850DP picked up almost 2 10ths. Put that carb on my mates ford Cortina with a tuned rover V8 (215ci) and 4spd who said that carb was too big for my motor and it went 2 10ths quicker over his 600. Forget the calcs if you want performance.
 
Forget the calcs if you want performance.
AKA - Strip Performance-
100% true.
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.
Dan, what is the purpose of the car and it’s performance?
Have you any build plan ideas for it?
Any idea on the cam? Headers or not, etc….
 
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.
A spreadbore 650 pumper on a TQ intake can put an A-body in the upper-mid 12's w/a moderately aggressive cam without any "supertuning".
With the right setup, bigger carbs actually give a better throttle response, the larger primaries give You more per throttle angle. It's when You're WOT that low velocity can hurt "response", but with VS/Air-valve Secondary carbs, that should be a non issue. If You have a loose enough converter, You can go "too big" on a pumper & will no doubt pick up a bit of power, but a fairly wasteful way to do it.
 
AKA - Strip Performance-
100% true.

Dan, what is the purpose of the car and it’s performance?
Have you any build plan ideas for it?
Any idea on the cam? Headers or not, etc….
Just a cruiser. I'm going to be using headers, haven't picked out a cam yet
 
We all know a 750 is the best all around performance carb for a 340, 360, 383,4" sb's, mild street strip 440's etc and we also know they'll eat more cfm the more they're hopped up and rpm'd.

If its not hoped up...then they dont necessarily want that much.
A 318 ,stock cam n the rest..I'd use a 500 eddy. Stock 440 mild cam..Carter 625 is what I'd use. Set it and forget it.
The bigger carb is great, above 4500 rpm is when the 750 tops the 650. But if you have to have a shift kit...you will need to shift manually to even see that rpm range.
That's apart of why they ran them on 340's to 440's . mild shift kits hit at what 5000 rpm maybe , strip 5800 or so.. you get a valve body plate or complete and you'll 6800 rpm
I just wonder if the question is being pondered one dimensional.
"I can run 800, 950,1050 on mine" what are we talking about? A cruiser.
 
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I noticed you mentioned a ET gain, which is great. But is this an everyday driver?
Yes it is and it probably won't see much over 4,500 rpm's for the most part. I thought about a 650 eddy and a performer intake or would the rpm performer intake be a better option? This car will not see any track time, probably some stop light to stop light fun once in awhile. My goal is for the car to have good throttle response and decent low to midrange torque. I want the car to be able to get out of it's own way and be able to pass other cars with out difficulty
 
I thought Uncle Tony said to double the engines displacement and thats your CFM 318 times two is 636 equals cfm (round down to 600) :)

340 times two equals 680 (round up to 700, I run a 750 eddy on mine)
 
For a streeter;
Your engine is max airflow limited by; the cam, the heads, and the ability of the exhaust-system to get rid of spent gasses. This adds up to VE aka volumetric efficiency.
If your carb is sized at least adequately for what the engine can thruput then a bigger carb is not gonna give you much; see Bewys post #25 I think it was.
On the street, where rpm may seldom exceed 4000rpm. I see no good reason to be carrying around a whopper-cfm carb for the once a month blast.
This calculator;
(rpm x cid)/3456 times VE = carb size in cfm
actually does work pretty good for a streeter; You just gotta be honest about your VE.

This formula has a basis in reality. The 3456 is not some random number plucked from a hat, but is a "constant" that takes into account your displacement, your rpm, the number of cylinder-fillings per revolution, and converts it to cubic feet, to spit out a number that at any particular rpm represents what your engine should be able to physically ingest at STP (standard pressure and temperature).
The problem is that your engine is rated in CID by it's swept volume, and not it's Total volume.
For instance;
say you have a 367 like me, which calculates to a swept volume of 752cc per cylinder. But the total chamber volume is another 75cc in my case, so when the piston is at the bottom of the intake stroke, atmosphere wants to fill it with 827cc .. ok,
but at 4.04 bore, this is 827 is actually equivalent to a stroke of 3.92 with a combustion chamber of zero. So then, doing the math, my 367 at 100% VE would be, for the purposes of this formula be.....404 cubic inches. And so;
(404 x 6400)/3456 time 100% VE=750 cfm.. Whereas at 367 it comes to just
(367 x6400)/3456 times 100% VE=680cfm
That's what the atmosphere sees. Whether or not at 6400 it can do it is depend on the VE of your total engine combo.

Now for Dan, estimating a top rpm not exceeding 4000, and a compression ratio of 8/1; lets plug this in.
A stock 360 has a CID determined to be 737.2cc and
At 8/1 she would have a Total Combustion space of 737.2/7=105.3cc for a grand total of 842.5cc. At 4.00 bore this represents a stroke of 4.09inches. so then the displacement becomes 411. and
(411 x 4000)/3456 times 100% VE= 476cfm NOT
(360 x 4000)/3456 x 1.00= 417
Of course at 4000 with the right induction/combo, the engine is much more likely to be able to operate at 100% VE, so 476 should be pretty accurate. In any case the next closest size is a 500, just right.
Or is it?
Not all carbs use the same rating system. There are about eight ways to screw up.
Carter's system yields higher numbers than Holly's system.
A two barrel is rated differently than a 4bbl. If you install 3 of 320cfm 2bbls onto a 6-pack intake, you will not get a 960. If the engine cannot pull more than 750, then that is what you get. You may get more power from the 6-pack, but it won't be from more airflow.
The mighty 340 Thermoquad is an 800 only by Carter's rating system.
In any case; for Dan, any carb rated at 500 or more will get the job done.
But get this;
(411 x 4000)/3456 times 100% VE= 476cfm as we just calculated. and I suggested that a 500 would be just right. Ok but this assumes the engine can pull the intake vacuum down to the level, that was used to establish the rating of the carb.
If the engine only pulls 70% thru that 500, then it is no longer a 500; but is whatever it is.
This is what makes a spreadbore so great on so many vehicles; the smaller primaries can do the work maybe 80 or 90 per cent of the time, below say 3000 rpm ........
And in my opinion for Dan's application; a lil too small is better than a lil too big.
Some of you guys probably missed the First post. or the subsequent one which said;
Yes it is and it probably won't see much over 4,500 rpm's for the most part. I thought about a 650 eddy and a performer intake or would the rpm performer intake be a better option? This car will not see any track time, probably some stop light to stop light fun once in awhile. My goal is for the car to have good throttle response and decent low to midrange torque. I want the car to be able to get out of it's own way and be able to pass other cars with out difficulty
 
The question shouldn't what cfm you need since basically all carbs will only flow the cfm you need, carb ratings are basically meaningless, but what vacuum level am I shooting for, With a race only the question is what's the fastest, street strip whats the fastest with acceptable driveability, with daily driver street what's the largest I can go without sacrificing driveability. Sure you could slap on a small 2 bbl or 4 bbl and drivability will be fine but your giving up hp for nothing whole point swapping from a 2 bbl to 4 bbl in the first place on a V8 at any power level I've never heard any one complain the a 600-650 cfm being too big. A 750 cfm is a mixed bag but I feel most the are unsatisfied probable was more from poor tuning then over carbution.
 
OP, I guess you could just collect a few 4 barrel carbs and swap 'em around see which one is better :)

a 600,625,650,750 etc....( for those that dont wanna do math) LOL!
 
After all, the primary side of the 750 is just a modestly sized 2bbl .................. equaling about
(750/2) X .67=251 when rated as a 2bbl.

LOVE IT! Bingo! And then at the same time, I still use a 600. I always gained mileage on the direct 2bbl. to 4bbl. swap. Even with a modest cam installed afterwards, it still a good size.
Yes it is and it probably won't see much over 4,500 rpm's for the most part. I thought about a 650 eddy and a performer intake or would the rpm performer intake be a better option? This car will not see any track time, probably some stop light to stop light fun once in awhile. My goal is for the car to have good throttle response and decent low to midrange torque. I want the car to be able to get out of it's own way and be able to pass other cars with out difficulty

I think I stated this before in one of your multiple threads going on but here it is again anyway, even if I didn’t.

On my 5.9, I am using a 600 AFB, RPM intake (AG only for a Magnum engine) Hooker Super Comp headers @ 1-3/4 into a 2-1/2 exhaust. Works just fine. The stock cam in that engine falls off @ 4500.

You said your looking at a Edelbrock 650, which means your looking at there AVS. Correct? Go for it. At the level your building this engine so far, I highly doubt you’ll see a difference between the Performer and the RPM.
 
OP, I guess you could just collect a few 4 barrel carbs and swap 'em around see which one is better :)

a 600,625,650,750 etc....( for those that dont wanna do math) LOL!

Math has nothing to do with it really, rpm x cid / 3456 = cfm x VE% = final cfm use that formula for what heads you need and or headers ??
 
The question shouldn't what cfm you need since basically all carbs will only flow the cfm you need, carb ratings are basically meaningless,
Following this advice and thought train will get you into trouble every time.


but what vacuum level am I shooting for,
This as well. Even more so if you don’t fully understand what your getting into.


With a race only the question is what's the fastest, street strip whats the fastest with acceptable driveability, with daily driver street what's the largest I can go without sacrificing driveability. Sure you could slap on a small 2 bbl or 4 bbl and drivability will be fine but your giving up hp for nothing
Considering the OP’s statement of driver with occasionally horsing around with his buddies. This is the least of his problem but helps create one for him. Your boxing him in between two of his statements.

whole point swapping from a 2 bbl to 4 bbl in the first place on a V8 at any power level I've never heard any one complain the a 600-650 cfm being too big. A 750 cfm is a mixed bag but I feel most the are unsatisfied probable was more from poor tuning then over carbution.
So what is it your really saying to him here. This becomes a contradiction for him.
 
Math has nothing to do with it really, rpm x cid / 3456 = cfm x VE% = final cfm use that formula for what heads you need and or headers ??
Math has everything to do with it when you science it out. Which is not always the best way to enhance an engine. There is still much left up in the air about his build. Making use of that formula is trouble. Not knowing the VE of his engine is also a problem even though it can easily fall into a low percentage, the parts used remain to be seen.
 
My GTS has a Quick Fuel 1050cfm annular 4150 style carburetor on it. This is a 9.5:1 451 with stock exhaust manifolds etc. with a Comp xe294h cam with 1.6's. It is every bit as snappy as the 750 for cruising around and if I'm going to launch it that's 4k and up so at that point it's a wash IMO. It took idle feed restrictor, transfer slot, air bleed, power valve and jet changes to get it right. It's pretty close in calibration to my old school 830cfm DP other than the main jets.
 
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.

You can run well into the 12's with 600 carb on a 360. They're cheap and plentiful just chose the one that you feel comfortable tuning. Once you get used to what you have you can do what 12many pointed out and begin to really extract what your combo has instead of changing parts to find more hp.

Just find an early series Carter 625 or Holley 600 and get it rebuilt by some one who knows what they are doing. All these new carbs leave a lot to be desired when it comes to the Tslot and booster venturi relationship not to mention bleeding/emulsion. A TQ would be even better.
 
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