Venturi Size: Holley 500 cfm vs Autolite 480 cfm

[66 Fastback]

Since we have been discussing venturi sizes in the little six forum, consider this:
Maybe the Holley 500 just has venturi’s that are too big for these engines. A 200 cu in engine at 5000 rpm with 100% volumetric efficiency requires 289 cfm of air.
When Mike R did the carb swap out at Pony Carbs, the Holley 500 cfm made 177.7 HP. The Autolite 4 bbl 480 cfm made 211 HP and had a very flat torque curve. The Holley had venturi diameters of 1.43â€￾. That translates into a total area of 3.21 sq in. The Autolite had venturi diameters of 1.00 and 1.08. That gives a total venturi area of 3.40 sq in. That is nearly the same area between the two carburetors. But, you have divided the flow between 4 venturis instead of two larger ones. Based on the 289 cfm of air consumption, each bbl in the Holley has to provide good fuel metering over a range of air velocities of 0 to 6,480 ft per min. On average, each bbl in the Autolite 4 bbl is handling a range of air velocities from 0 to 3,060 ft per min. Now this is just an assumption, but I suspect that the 4 smaller venturis do a better job of metering the fuel probably because they are operating over a smaller range of air velocities. I suspect the larger primaries of the Holley are not metering fuel as accurately. The Holley will have a larger range of pressure differentials associated with the large range of air flow. However, fluid flow through an orifice or jet is not a linear relationship to the pressure. It is a logarithmic function. The jets in the Holley are being asked to try and deliver fuel at the same AFR over a wide range of air flow based on a pressure differential that is changing logarithmically.
Doug

 

[wsa111]

Doug, the x-factor between both of these carbs is the area taken up by the venturi booster.
The autolite annular discharge booster probably takes up twice the area of the holley downleg booster.
In reality the holley probably has more actual venturi area than the autolite.
On these small engines the lower air flow through the booster definately favors the annular booster for better fuel atomization.
On a much larger engine the downleg booster would flow more & because of higher air velocity would produce more top end power.
Each booster has it downfalls & advantages. Bill

 

[Bort62]

On these small engines the lower air flow through the booster definately favors the annular booster for better fuel atomization.

This.


Yes, the holley 500 is probably "too big" in the sense that the signal through the venturi's is not very good.

Perhaps I am missing something, but since, as you stated, the cross sectional venturi area of the two carbs is essentially the same, the velocity is going to be the same. (at a given flow rate.)

I'm not sure where you came up w/ the autolite only having half the velocity...

 

[wsa111]

What i'm saying in a carb using the large annular discharge booster, the total venturi area is made less due to the more restrictive annular booster.
To get equal flow the venturi diameter would have to be larger to compensate for the larger annular nozzles.
Most of the professional carb shops who offer say a 850 cfm carb, the carb using the annular nozzles has a larger venturi diameter. Bill

 

[Bort62]

What i'm saying in a carb using the large annular discharge booster, the total venturi area is made less due to the more restrictive annular booster.
To get equal flow the venturi diameter would have to be larger to compensate for the larger annular nozzles.
Most of the professional carb shops who offer say a 850 cfm carb, the carb using the annular nozzles has a larger venturi diameter. Bill

Yeha, I was responding to the OP - not you.

I agree with what you are saying.

 

[66 Fastback]

The Holley has 2 venturies that have nearly the same area as the Autolite with 4 venturies. The total air flow through the carbs are nearly the same but the flow is split among 2 venturies in the Holley and 4 venturies in the Autolite. That is where I am coming up with half the flow rate per venturi.


I would imagine since each venturi on the 4 bbl is smaller, the percentage of cross sectional area occupied by its boosters would be larger. Does the Autolite have 4 boosters?
Doug

 

[MustangSix]

Apples and oranges on the advertised flow number as well. 4bbls and two bbls are rated at different pressure drops.

 

[66 Fastback]

The difference in the standard or datum used to measure the flow rating does not come into play in this case. The engine is going to demand a given amount of air at a given load and rpm. Since the flow capacity of each carb exceeds the requirments of the engine, each carb should be delivering the same amount of air unless there is an appreciable restriction in the carbs.

Changing the pressure differential at which 4 bbl or 2 bbl carbs are measured at does not alter the actual physical flow capacity of the carb. It is like measuring gas flow through an orifice meter in standard cubic feet at 14. 65 psia versus using a presure base of 14.71 psia. Changing the standard at which I measure the volume does not physically alter the flow capacity of the orifice meter hardware.
Doug

 

[Anonymous]

I know nothing of formulas and such....

All I can add is real world data,

500 cfm Holley 2 Bbl,

60' 2.22 1/4 mile 15.28 @ 88.94

600 cfm Holley 4 Bbl,

60' 2.08 1/4 mile 14.69 @ 90.98 mph

 

[mach1 mark]

mustang-geezer, put on an autolite 480 cfm 4v and see what your times are. I'm curious to see if the autolite is really better, as all the info claims.

 

[66 Fastback]

Intuitively, I would have thought that a 4 bbl is over kill. You would think a 2 bbl would be better on a smaller engine. But after doing the math and seeing your track results and Mraley's dyno results, I think the 4 bbl carbs may be better because the 2 bbl is operating over such a large range of velocity in comparison to the 4 bbl carbs. Maybe I need to start some controversy and figure out a way to mount a 4 bbl on my 2 into 1 adaptor on the stock manifold openning.
Doug

 

[Gene Fiore]

Maybe I need to start some controversy and figure out a way to mount a 4 bbl on my 2 into 1 adaptor on the stock manifold openning.
Doug

Oh brother, now that is really asking for it!

 

[wsa111]

Gene, i hope you can test not only the holley 350, but also the 500.

I feel with the larger engine the 250 it will handle the 500 with ease. Bill

 

[willowbilly3]

The Holley has 2 venturies that have nearly the same area as the Autolite with 4 venturies. The total air flow through the carbs are nearly the same but the flow is split among 2 venturies in the Holley and 4 venturies in the Autolite. That is where I am coming up with half the flow rate per venturi.


I would imagine since each venturi on the 4 bbl is smaller, the percentage of cross sectional area occupied by its boosters would be larger. Does the Autolite have 4 boosters?
Doug

I think one difference is the port velocities at lower speeds through the primarys of the 4 bbl. versus the 2 bbl. You have the air moving faster at lower speeds.

 

[rancheroman]

For what it's worth, I land speed race a 4 cyl.sohc ford engine (140 cu.in.)with a 600 cfm afb 4 barrel carb, mounted on a dual plane 4 barrel intake manifold with a 4.5 " spacer under the carb. I have tried numerous 2 barrel carbs, holley 350/500 cfm, ford autolite 108,114,121,123,133, 2 barrel carbs and a ford autolite 480 cfm 4 barrel carb. The 600 cfm afb has always given me the fastest time. This combination defies all theories as it starts and idles at 1100 rpm, has good throttle response, and could be a fairgound cruiser. I think the reason this combination is so 'driveable and performs so well is the xti distributorless ignition that I run. My point is, you can dump as much gas into an engine and buzz it to the moon, and it will perform well as long as you have a good ignition to fire it. Thats my 2 cents worth. Paul.
p.s. I did have a 170 pushrod 6 cyl. with an agrie head prior to the sohc.