AFB Venturies why the difference?

[Noel G]

Hi, re-tuning my 318 after a manifold swap and fixing a vac leak. Can anyone tell me why the difference in these Carter AFB (pre Eddy) primary venturies? (came off different carbys but both are P/N 9635SA. Cheers. (sorry about the poor focus - the price of a camera that thinks it's smarter than the guy using it).

 

[Dubob]

I'm no expert but my gut says the one with the single hole is an earlier version and the one with emulsifying holes is an updated later version even with the carbs being the same. But that's a guess. Someone who knows will chime in.

 

[jos51700]

I was going to say something similar, except to add that one is likely an emissions carb part.

 

[Demonic]

Improved emulsion in the second venturi tube.

 

[Noel G]

Improved emulsion in the second venturi tube.

which one (if either) is better for bottom end torque?

 

[Mattax]

but both are P/N 9635SA

I'm going to guess one of those has been 'rebuilt' and the cluster swapped in the process. This is the type of thing that occurs with 'rebuilding' shops. They work in batches and stuff that's similar gets mixed up.

I don't know which is a better match for your 318. Concentrating the holes at the bottom could slow initiation of the main circuit and delay the air correction 'til higher rpm. Might do that on a smaller venturi's carb (relative to engine displacement and rpm). This assumes the main air bleed sizes are the same in both clusters.

Do a search for here for AFB or clusters and @Hysteric
He seems to have dug into these more than most.

 

[Mattax]

This doesn't refer to your pictures but has a general explantion. Emphasis added by me.

Tuner wrote: "The difference is the larger holes in the well tubes make for bigger bubbles in the nozzle stream and that tends to make the fuel discharge in larger globs instead of a smooth flow. A tube similar to the one with large holes under the 340 was used in some Pontiac AFBs but they only had .026” in the main air bleed tube and also had a fairly large surface in the exhaust heat exchange area in the floor of the plenum. On the other hand, some Chevrolet AFBs had tubes with no holes at all except for a kill bleed above float level. Many AFBs of all OE manufacturers had no tube in the secondary cluster. Remember, in AFBs the air comes into the well outside the tube and exits the well up the tube with the fuel."

One takeaway is the large holes will work best with a heated intake manifold floor.

 

[yellow rose]

This doesn't refer to your pictures but has a general explantion. Emphasis added by me.

Tuner wrote: "The difference is the larger holes in the well tubes make for bigger bubbles in the nozzle stream and that tends to make the fuel discharge in larger globs instead of a smooth flow. A tube similar to the one with large holes under the 340 was used in some Pontiac AFBs but they only had .026” in the main air bleed tube and also had a fairly large surface in the exhaust heat exchange area in the floor of the plenum. On the other hand, some Chevrolet AFBs had tubes with no holes at all except for a kill bleed above float level. Many AFBs of all OE manufacturers had no tube in the secondary cluster. Remember, in AFBs the air comes into the well outside the tube and exits the well up the tube with the fuel."

One takeaway is the large holes will work best with a heated intake manifold floor.

Makes me wonder...if the change is emulsion above was due to the heated intake, or the heated intake came because of the emulsion.

Thinking out loud...I know the OEM’s always worried about cold start emissions and driveablity, so the heat would have been there first, and the bigger emulsion came about because the fuel was over atomized (if that’s a word and a thing) and sending bigger globs of fuel into the intake helped not get all the fuel converted to a vapor into intake manifold.

Hmmmmmmmm

 

[Hysteric]

This doesn't refer to your pictures but has a general explantion. Emphasis added by me.

Tuner wrote: "The difference is the larger holes in the well tubes make for bigger bubbles in the nozzle stream and that tends to make the fuel discharge in larger globs instead of a smooth flow. A tube similar to the one with large holes under the 340 was used in some Pontiac AFBs but they only had .026” in the main air bleed tube and also had a fairly large surface in the exhaust heat exchange area in the floor of the plenum. On the other hand, some Chevrolet AFBs had tubes with no holes at all except for a kill bleed above float level. Many AFBs of all OE manufacturers had no tube in the secondary cluster. Remember, in AFBs the air comes into the well outside the tube and exits the well up the tube with the fuel."

One takeaway is the large holes will work best with a heated intake manifold floor.

Hey Mattax. Hope your doing well in the midst of this crisis.

The difference between the 2 boosters is the first one is the booster Carter used in their 625 performance carbs and some other carbs the other booster from memory came from a Carter chevy carb and was later used by federal Mogul in both the 625 and 750 carbs that were die cast. The second booster is also the one that Edelbrock uses in both their 750's and 600's. In my view the first booster emulsion design is far superior to the second. As a matter of fact I use both the front and rear boosters from an early performance carter 625 in my Edelbrock 600 that ran 12.60's. The Carter carbs that I have that were used on Chrysler's share the same emulsion design as the first booster.

Something to think about from the late and great "Shrinker" on emulsion. By the way the Weber tube emulsion design is similar to Carters.

Q:
What I was trying to get at: Is emulsifying the fuel in the center of a fuel well superior to the Holley method from the side? Make me think answers mean more to me than the straight out answer.

A:
Superior in what way? Consider that air bubbles lower the density, So therefore that lower density fuel 'floats' on top of the normal fuel. that causes the lower density fuel to have a higher surface level than normal density fuel. Once the surface level is up to the height of the outlet tube it can flow to the booster. Thats when the engine starts to run on that fuel. At low air flow levels thats all that happens. Your using air to make the surface of the fuel closer to the outlet. At higher flows through the carby the main jet restriction causes sufficient reduction of the liquid level in the well to fully uncover some bleeds. When its like that the air flow through the bleeds blows the fuel along its path. The amount of blow etc is controlled by the MAB and the sizing of the various E-bleeds. Now also think about what happens when the fuel flow into the bowl causes a lowering of the level. That means the pressure on the entrance to the main jet is changed, so the flow rate of the jet alters and that alters the level of fuel in the well. Now what happens if you change the main jet size? Same thing isnt it.

Ok so now put all the emulsion holes on a tube like the Weber plates. If you make them in the same arrangement as a Holley then it will do very similar things. But because you have a tube you have lots of places to put the E-bleeds, you can place them all at one level if you wish and that allows you to have lots of fine bubbles at one place, you can have lots of air but have it as fine bubbles. If you try to do that with the Holley design you can only have one hole and it has to be big to pass the air and then you only get one bubble of air. You get a different density change if the bubbles are one single one or lots of tiny ones. Lots of tiny ones creates lower density. Big bubbles travel faster though. do a experiment with an aerator on a faucet outlet at the kitchen sink. When your filling a glass with non aerated water the big bubbles float up quickly. When you fill it with an aerator nozzle the tiny small bubbles take longer to float up. BUT small bubbles just alter the density and viscocity, where as big bubbles can form a block of air moving through a tube. So when you have a block of air exiting the booster where is the fuel for that parcel of metered air? What does that cylinder run on then?
So now what do you do with the carby, will a large bottom air bleed get you a big enough droplet to blow fuel faster to the booster at high CFM flows? Will tiny bubbles help it start the mains flow differently at low CFM flows?
So answer this question, Which bubble structure will create more even fuel distribution exiting from the carby base?

I would use the first booster.