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What Carburetor for Factory 4-bbl?

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Ken_Parkman View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Feb/13/2020 at 9:17pm
OK by me.

Probably a really good idea, cause I read my chart wrong. And maybe you should not believe me!

Sorry about that.

The 4300 is actually bigger than a 600 Edelbrock, my mistake was reading the line with a stub stack. But they are very very close, along with the 625 BG. The 600 Holley 1850 is noticeably bigger though than all 3 of those.

I've decided carb flow numbers are a bit of a marketing game - tracked a huge variation in 750 carbs, even within the same manufacturer.

Edited by Ken_Parkman - Feb/13/2020 at 9:27pm
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PHAT69AMX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Feb/13/2020 at 9:49pm
And this is maybe a little 'beyond' the original posted question,
but what brand and size carburetor to use can be a loaded question...
imho best for an individual to learn a little about the 'marketing claims' and their shortfalls,
then make an individual informed decision...  I focus on Butterfly and Venturi Diameters more that claimed CFM Ratings.
This is the part saved as text from the now defunct other forum, then after that is an image file of the chart Ken Parkman compiled:

Carb flow test
02/09/08 at 22:39:45

For various reasons there seems to be a lot of carburators
around here right now, so I decided to put them
all on the flow bench for comparison.
My flowbench cannot effectively measure a carb cause to do it right
 you need to have a bench that will suck the roof off the shop and
 has a direct power supply from the generating station.
But what I can do is run the bench wide open and then put on different carbs
 and measure the pressure drop, and then I can compare one to another.
 I could check an individual barrel, but that takes fixtures
 I don't have and can't be bothered making. 
As you know there is way more to carbs than flow - a point that
 has really been hammered home to me with some dyno and track testing.
 Also as you know factory ratings are pretty much meaningless
 cause they are marketing more than anything. So this is an effort
 to see how much some manufacturers are exagerating. Do not use this
 data to say one will make more or less power; it's only for interest. 
So here is the list of different carbs and their ranking
 flow wise, smallest to largest: 
Rank/Carb/part number/rating/comments 
 1   Edelbrock   1405   600 
 2   Holley      1850   600   Significantly more flow than Edelbrock 600 
 3   Q-jet       ?      ?     Secondary air flap has a stop to limit opening 
 4   Holley      4777-2 650 
 5   Edelbrock   1407   750   Slightly more than 650 Holley 
 6   Edelbrock   1813   800   Big improvement over 750 Edelbrock 
 7   Q-jet       ?      ?     Secondary air flap opened noticably more than above Q-jet 
 8   Holley      4780   800   Same dimensionally as a 750,
                              but this carb had a slight TB mismatch. 
 9   Holley      3310   780   Factory GM original 3310, down leg booster 
10   Holley      3310-2 750   Strait booster than above, flows a little better but close 
11   Pro-Systems XC     ?     Built for a mild 327 Chev, noticably more flow than above Holleys 
12   Holley      4781   850   No choke plate 
13   Holley      80514  1000  Annular booster carb, very small flow difference from 850 
14   Pro-Systems XE     ?     Down leg booster, very significantly more flow than 1000 Holley 
A couple of comments: 
The Edelbrocks flow significantly less than the same rated Holley 
Holley plays games with flow ratings.
Yes a 750 Holley flows more than a Holley 800.
I've heard it before, but this seems to confirm
 a 850 holley is a lot bigger than a 950.
 Dimensionally the 850 is bigger.
 The 850 with no choke and a stubstack was essentially identical
 to the annular booster 1000. 
Stubstacks noticably improve the flow. 
A fancy cnc milled 2" spacer slightly improves the flow. 
The Q-jets were flowed with an adapter that slightly restricted the flow,
 but calibrating with a holley says the resriction was not much. 
I'm going to try to get a few more carbs to add to this list,
 including a factory AFB and Motorcraft.

Re: Carb flow test
Reply #6 - 02/10/08 at 17:10:59

Trying to find some stock AMC stuff, just don't have any here!
 I hope to score a stock AFB and a 4300 to test, I'll post if it happens. 
What Reagam said is important. A high flow carb is no good
 if it can't properly atomize fuel at a low delta pressure.
 Only take this data as an interesting comparison. 
Of course a really good carb is both high flow and
 can properly control the mixture at a wide rpm and load range.
 A simple flow test cannot tell this.


Re: Carb flow test
Reply #8 - 02/16/08 at 18:22:16

Those were 2 different stubstacks, both K & N.
 One is very old (been through a carb fire)
 and has been modified to better fit the 850,
 which seems to have a different height air horn
 then the stubstack  was designed for.
 The other is an almost new one made from
 a different plastic and fit the 750 very well.
 Both noticably improved flow,
 but clearly the bigger 850 really liked it, the flow was well up. 
The spacer is a really trick cnc milled 4 hole
 translating to an open with a nicely faired 'bullet' in the center.
 But now that I am carefully looking I find it has slightly small bores
 on the 4 hole part. I'm gonna fix that and try again. 
Just scored a couple of Demons, a good prepped dyno 750 holley,
 and a stock AFB. Hope to have a Motorcraft
 and a few more Holleys tomorrow. Will post info 


Re: Carb flow test
Reply #14 - 04/28/08 at 02:32:16

I FINALLY managed to score some stock AMC carbs
 and get all the data together. I've had 29 carbs
 accross the flow bench and it is an interesting exercise. 
I've learned carb flow numbers are a game.
Do not assume a carb is bigger because it it rated higher.
The highest flow 750 carb was 150 cfm better the lowest flow 750. 
A surprise is the stock Motorcraft 4300.
I scored a OWA4-S, and it's surprisingly good flow wise.
It looks crappy, but flows a tiny bit better
 (not enough to give a number)
 than a Demon 625 or an Edelbrock 600, both of which are pretty much the same.
 A Holly 600 is about 40 cfm better than the BG and the Ed.
 The stock AMC AFB (tested # 4664) is about 40 cfm less
 than the sam BG and Ed Carbs.  The Ed 600 is an AFB and is basically
 the same carb as the stock AFB. It has the same bore's and venturi's,
 but the Ed has 1/2 throttle shafts and a little aerodynamic fairing
 to account for the 40 more cfm.
 The 4350 Motorcraft is the same as the stock AFB. 
I scaled the carbs on a 0-100 scale, and here are the small carbs 
Edelbrock 750   (1407) - 71 
Holley 700      (4778) - 71
Holley 650      (4777) - 69 
Rochester Q-jet (xxxx) - 67 (small one)
Holley 600      (1850) - 65 
Motorcraft 4300 (xxxx) - 61 (OWA4-S)
Barry Grant 625 (xxxx) - 61 (Road Demon)
Edelbrock 600   (1405) - 61 
Carter AFB      (4664) - 57 
Motorcraft      (4350) - 57






-> Today, Wed, Feb 20, 2019
-> post on Billd Forum to Ken Parkman asking for missing text



And this is the image, hope it is legible, have yet to take time to type it up:

Edited by PHAT69AMX - Feb/13/2020 at 9:59pm
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javelinjason View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote javelinjason Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2020 at 7:45am
This is all far and above my original question, but it has turned out to be a great discussion! If this were more of a performance build, I'm sure we would have went with a different CFM rated carburetor. Maybe for the next one! 
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PHAT69AMX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Feb/14/2020 at 2:13pm
It you just want a new bolt on carburetor that works - the Summit Brand Crabs people say are good.
They are just a rebranded Holley design from the 80's, and are low cost.
If you are ok with rebuilding and tuning a carb, get a used holley list 1850 for $50 off MarketPlace.
Getting pieces and parts and 'input', ha, for the 1850 Holley will be easy.
All also available for the MotorCraft AutoLite but less readily, maybe more costly,
a new float for that is high dollar from what I understand, and the Dual Inlet Needle set-up
can be tricky to get correctly set-up, some web sources show blocking off the 2nd Inlet Needle.
For the Spacer, finding stock will be less than easy.
Aftermarket carbs most likely will already have the needed hose connections
that were in the stock spacer since the stock MotoCraft AutoLite lacked them.
Square Flange 4-hole Carb Spacer of a material, brand, height, of your choice,
some folks even make them out of wood at home.   
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