carb cfm ratings

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AJ/FormS

AJ/FormS

68 Formua-S fastback clone 367/A833/GVod/3.55s
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It seems I might be losing it
Does anyone know how carbs are cfm rated and how 2 bbls are rated differently than 4 bbls and what the conversion factor might be.
I am specifically interested in converting a 230 cfm 2bbl to 4 bbl equivalency and specifically interested in re-rating a 650cfm 4 bbl to a 2bbl equivalency.
My brain has temporarily misplaced this information.
Many thanks to any helps
 
2 barrels are rated using 3.0 Hg and 4 barrels rated using 1.5, if that helps any.

I think that is right.......it might be backwards, but I think that's correct.
 
4v carbs are tested at 20.4 inches of water. I can't recall what a 2v is flowed at. RRR's numbers are correct, I just can't think of the 2v number in inches of water.
 
I cant remembered the conversion formula but a 500 2bbl is equal to a 390 4bbl.
A 390 2bbl is just front half of a 650 4bbl and 500 2bbl is just front half of a 750 4bbl.
If that helps.

So my guess would be the 230 2bbl would be 180 4bbl and a 650 4bbl would be about 800 cfm 2bbl.



But really what cfm your engine uses is what cfm your carb will be no matter what you run. like that 230 2bbl will pull a 1000 cfms if you make to but will eat a lot of hp and might not function to well.
 
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273 said:
How Big A Carb Do You Need?

This Article has the formula plus to me the proper way to pick your carb size.
Click to expand...
I don't think he needs to figure out a size 4bbl. But simply the conversion math for figuring the differences between a 2 & 4. How to figure the size of one in relation to the other.

But I like your thoughts on it in your posts.
 
rumblefish360 said:
I don't think he needs to figure out a size 4bbl. But simply the conversion math for figuring the differences between a 2 & 4. How to figure the size of one in relation to the other.

But I like your thoughts on it in your posts.
Click to expand...

Yeah that's it. IE how much air will a 230cfm 2bbl flow when flowed the way a 4bbl is tested/rated.
 
I pretty sure the last formula can do that.

Basically the formula worked out if I did it right to go 2bbl to 4bbl divide by 1.41 and the other way around divide by .71

I got that your 230 cfm 2bbl if rated like a 4 bbl would 163 cfm close to my 180 guess and a 650 would be 915 cfm.


Can I ask why you need to know ?
Is it to see if a 650 is to big for /6 or something?
 
Comments in the post
273 said:
I cant remembered the conversion formula but a 500 2bbl is equal to a 390 4bbl.
A 390 2bbl is just front half of a 650 4bbl and 500 2bbl is just front half of a 750 4bbl.
If that helps.
So, if that is true,
and the 500s are rated as 2bbls, then One would expect half a 750 to flow 375 when rated as 4 bbl. Since it is rated at 500,this is 500/375=1.33 times as much. That would lead me to believe that a 230 would also flow 1.33 times as much, becoming a 307cfm.
But in regards to the 390/650 it muddies things up even more. Half of 650 is 325. But 390/325= 1.2, and now my 230 is just 230 x 1.2=276cfm
You see what my dilemma is?



So my guess would be the 230 2bbl would be 180 4bbl and a 650 4bbl would be about 800 cfm 2bbl.



But really what cfm your engine uses is what cfm your carb will be no matter what you run. like that 230 2bbl will pull a 1000 cfms if you make to but will eat a lot of hp and might not function to well.
Click to expand...
 
I imagine there's some rounding when the factorys come up the cfm ratings.

I'm just saying the barrel size is the same between the Holley 4bbl and 2bbl theres got to be some relation between them and there cfm.

So if 2 500 cfm 2 BBL (1000 cfm) holley's equals one 750. Uses the formula from the article 1000 ÷ 1.41 = 709 pretty close especially with factory rounding.

The 650 one is way off but maybe I was wrong and it's actually the 600 cfm which would be closer to right.

And the 500 cfm 2bbl equaling 390 4bbl was from my dyno program I might of rounded it so it was easy to remember the ball park difference.

I still think the 1.41 and .71 are close enough for whatever your figuring out.

So the 230 would rate 163 at 1.5"
And a 650 would rate 915 at 3"
 
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Since the actual formula hasn't shown up yet, I do remember this being discussed a few times at Moparts. A search there perhaps?
 
rumblefish360 said:
Since the actual formula hasn't shown up yet, I do remember this being discussed a few times at Moparts. A search there perhaps?
Click to expand...
Screenshot_20170502-092708.png


But instead of engine cfm you use the carb cfm that you have eg. 230 cfm and desired vacuum would be 4bbl 1.5 and rated would be the 2bbl 3 which would equal what a 230 cfm 2 BBL would be rated at 1.5" (4bbl rating) 163 cfm.

Actually I'm wrong you would have put 3" at desired and 1.5" at to get something resembling a right answer.
 
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But some how I came up with the right answer with that formula lol.
Screenshot_20170502-094436.png


I'm sure if my divide by 1.41 is right then the other is right with divide by 0.71 to go from 1.5" to 3".

Actually to simplify it you can use 1.414 both ways

2bbl to 4bbl ÷ 1.414 or 3" to 1.5"
4bbl to 2bbl x 1.414 or 1.5" to 3"
 
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