Autolite 4100

wild70stang · Jan 4, 2008

I have realized that there is such a thing as too big of a carburetor lol, so I have been looking into buying a lower cfm carb to fit my 302 small block with ford 4bbl intake. I like what I have read from this website and others, but I have found that the 480 cfm models arnt equipt with a tranny kickdown since I have an automatic in my car. Does anyone know of any autolites with a tranny kickdown??? If you do I would like the code so I could possibly order one. Also I know im going to get better gas mileage with a 480 vs my currrent Carter AFB 625 cfm.

rbohm · Jan 4, 2008

find one from a mid 60's tbird with the 390. the cfm is the same as is the jetting oddly enough.

htwheelz67 · Jan 4, 2008

you can also buy a smaller road demon or speed demon carb with annular venturi's, that is what makes the 4100 so good is the annular venturies.

Blu-Steel · Jan 7, 2008

You can install a Ford kickdown on an Autolite 4100. It's a really good carb and I run one on my 66, but mine is a manual. Contact ponycarburetors and they can give you some tips. I bought mine from ebay and got a good deal on it and am very happy with it. FOr simplicity and reliability the 4100 is hard to beat.

D.Hearne · Jan 7, 2008

Well I normally don't recommend Carters (or Edelbrocks) but a 625 cfm Carter isn't too much carb. Maybe you just need it tuned better.

bnickel · Jan 7, 2008

the 4100 from a 67 mustang, flacon, fairlane, etc, with a 289 and auto trans will be the 480 cfm version with the auto kickdown. i have to agree though that the 625 carter is probably not too much carb for a 302, it's probably on the high side of what will wrok best but it shouldn't be too much carb.

Blu-Steel · Jan 8, 2008

Depending on how your engine is set up 625 cfm may or may not be too much. The key is Volumetric Efficiency and your engine RPM range. If you are running a high RPM setup (cam, heads, intake) then 625 would be about right, but if you are running a street setup and the engine sees duty under about 6000 RPM, the 480 cfm would be about right.
I pulled this off of pony carburetor’s web site, (http://www.ponycarburetors.com/default.asp?page=calculatingcfm) but you can find this formula in most good automotive publications and catalogs (it’s also on in the back of Edelbrock’s catalog for example).

"Calculating CFM

Actual Air/Fuel Mixture Intake = Volumetric Efficiency
Theoretical Air/Fuel Mixture

Engine/Carburetor Sizing Relationship

The larger the engine, the more air it's capable of "gulping". In math terms, it looks like this:

Cubic In. Displacement/2 X Maximum RPM/1728 = Maximum CFM

An Example:

289 Cubic Inches/2 x 8000 RPM/1728 = 669 CFM

This is still not the final answer for sizing a carburetor. The volumetric efficiency must be taken into consideration.

In our example a 289 @ 8000 RPM can use 669 CFM
however
660 CFM x .90 volumetric efficiency = 602 CFM
669 CFM x .85 volumetric efficiency = 568 CFM

Our example is based on a Hi-Po 289. Ford installed a 600 CFM Autolite 4100 on this engine. Did Ford engineers know what they were doing? You Bet!
A standard "A" code or "D" code 289 4 barrel with the factory valve train will not run much above 6000 RPM.
6000 RPM requires 501 CFM x .85 volumetric efficiency = 426 CFM. Ford installed a 480 CFM carburetor!"

For a 302 with max RPM of 6000 the numbers work out like this:
.90 VE = 471 cfm
.85 VE = 446 cfm

Hope this helps.

htwheelz67 · Jan 8, 2008

carters and edelbrocks usually give great mileage as do some demons, I think if a carb is tuned to a proper A/F ratio if your using a 500 or 600 cfm isn't going to matter as much as the engine combination, state of tune and overall engine condition.

One of the best options to get better mileage fuel system wise is install efi, even the old holley analog projection systems which bolt on any 4bbl aftermarket intake, when I first tried one (analog) I picked up about 20 hp and TQ and my mileage went up 4 mpg's.

D.Hearne · Jan 9, 2008

Blu-Steel said:
Depending on how your engine is set up 625 cfm may or may not be too much. The key is Volumetric Efficiency and your engine RPM range. If you are running a high RPM setup (cam, heads, intake) then 625 would be about right, but if you are running a street setup and the engine sees duty under about 6000 RPM, the 480 cfm would be about right.
I pulled this off of pony carburetor’s web site, (http://www.ponycarburetors.com/default.asp?page=calculatingcfm) but you can find this formula in most good automotive publications and catalogs (it’s also on in the back of Edelbrock’s catalog for example).

"Calculating CFM

Actual Air/Fuel Mixture Intake = Volumetric Efficiency
Theoretical Air/Fuel Mixture

Engine/Carburetor Sizing Relationship

The larger the engine, the more air it's capable of "gulping". In math terms, it looks like this:

Cubic In. Displacement/2 X Maximum RPM/1728 = Maximum CFM

An Example:

289 Cubic Inches/2 x 8000 RPM/1728 = 669 CFM

This is still not the final answer for sizing a carburetor. The volumetric efficiency must be taken into consideration.

In our example a 289 @ 8000 RPM can use 669 CFM
however
660 CFM x .90 volumetric efficiency = 602 CFM
669 CFM x .85 volumetric efficiency = 568 CFM

Our example is based on a Hi-Po 289. Ford installed a 600 CFM Autolite 4100 on this engine. Did Ford engineers know what they were doing? You Bet!
A standard "A" code or "D" code 289 4 barrel with the factory valve train will not run much above 6000 RPM.
6000 RPM requires 501 CFM x .85 volumetric efficiency = 426 CFM. Ford installed a 480 CFM carburetor!"

For a 302 with max RPM of 6000 the numbers work out like this:
.90 VE = 471 cfm
.85 VE = 446 cfm

Hope this helps.

Only problem with all that and the formula is, it has no correlation with the actual engine operation or the cfm rating of carbs. It's just a formula for picking a carb size, nothing else. For a Holley carb to flow 600 cfms, at WOT, it will have to exert a 1-1/2" hg pull on the bottom of the carb at WOT. Putting a different size engine or different engine build under it changes everything. I don't know what vacuum pull the Carter's were rated with, but I've heard they tend to be smaller carbs than a similarly sized Holley.

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