Fuel Atomization by a Carb

[NormS]

I've seen it written that it is possible for a carb to atomize fuel too well. I'm not so sure this is true. My experience with some billet body carbs with annular discharge venturis, leads me to believe that a carb that has better fuel atomization just needs closer control of the A/F ratios at the top end of the curve. I found that the engines that these carbs were run on, definitely did not tolerate a rich A/F ratio after the rpm's got a little past peak torque.

A carb that atomizes fuel well,is basically increasing the amount of burnable fuel that gets to cylinders, compared to a carb that has less atomization. The "delivery A/F ratio", calculated from air flow and fuel flow measurements, may be the same for 2 carbs. But with the carb that atomizes fuel more completely, the effective A/F ratio in the cylinders, which I call the "burning A/F ratio", will be richer because there is more fuel ready to burn.

I found that a richer top end mixture, with one of those annular venturi carbs, resulted in a rather dramatic loss of hp in the top area of the power curve. Leaning the top end A/F ratios brought back the power, and ended up making slightly more power on less fuel, compared to a similar size carb with more conventional venturi boosters. The more conventional carb was more forgiving about A/F ratio control, made a little less power compared to the annular venturi carb, and required more fuel to make its best power.

What has been your experience with this fuel atomization issue?

[Alan Roehrich]

Not sure what you're seeing is really related to atomization. Most applications for annular discharge venturis are to increase throttle response on applications where the carburetor size has a negative effect on throttle response at lower RPM. The annular discharge increases the signal, pulling more fuel quicker. But often at the cost of air flow restriction.

Most gasoline engines do not like a rich mixture past peak torque. In fact, often the fuel curve that makes the most average power and goes fastest at the drag strip is 12.7:1 or so up to peak torque RPM, and then progressively leaner all the way up to well over 13.5:1 as you approach and exceed peak HP RPM. Annular discharge makes that difficult, but not impossible. There are people experimenting with annular discharge boosters that do not create any restriction, and tuning the fuel curve to compensate for the increased signal and flow.

[Walter R. Malik]

I've seen it written that it is possible for a carb to atomize fuel too well. I'm not so sure this is true. My experience with some billet body carbs with annular discharge venturis, leads me to believe that a carb that has better fuel atomization just needs closer control of the A/F ratios at the top end of the curve. I found that the engines that these carbs were run on, definitely did not tolerate a rich A/F ratio after the rpm's got a little past peak torque.

A carb that atomizes fuel well,is basically increasing the amount of burnable fuel that gets to cylinders, compared to a carb that has less atomization. The "delivery A/F ratio", calculated from air flow and fuel flow measurements, may be the same for 2 carbs. But with the carb that atomizes fuel more completely, the effective A/F ratio in the cylinders, which I call the "burning A/F ratio", will be richer because there is more fuel ready to burn.

I found that a richer top end mixture, with one of those annular venturi carbs, resulted in a rather dramatic loss of hp in the top area of the power curve. Leaning the top end A/F ratios brought back the power, and ended up making slightly more power on less fuel, compared to a similar size carb with more conventional venturi boosters. The more conventional carb was more forgiving about A/F ratio control, made a little less power compared to the annular venturi carb, and required more fuel to make its best power.

What has been your experience with this fuel atomization issue?

I don't know about all of that however, an annular booster inherently provides smaller droplets of fuel even if the amount of emulsion is the same and has more signal at low air speed and less signal at higher air speeds than a regular type booster. Its only drawback seems to be that carbs with that type booster have less total airflow for that venturi diameter there so, a larger carb can be beneficial in most instances.

[NormS]

I'm talking about annular discharge venturis, not annular discharge boosters. The ones I used had 24 discharge holes in the venturis themselves.

[ClassAct]

I'm talking about annular discharge venturis, not annular discharge boosters. The ones I used had 24 discharge holes in the venturis themselves.

Forgive my ignorance, but isn't an annular booster an annular Venturi?

[modok]

What spray patter works best depends on the intake manifold.

is it something like an airgap single plane?

[Mike Laws]

I'm talking about annular discharge venturis, not annular discharge boosters. The ones I used had 24 discharge holes in the venturis themselves.

Forgive my ignorance, but isn't an annular booster an annular Venturi?

An annular venturi carburetor has no booster. There are several discharge holes located peripherally on the I.D. of each venturi.

[bigfoot584]

Is this the type of carb your talking about, no booster at all the main venturi
is the booster in this case. This example is a #6425 Holley 650 2 barrel.

030.jpg

[cjperformance]

Is this the type of carb your talking about, no booster at all the main venturi
is the booster in this case. This example is a #6425 Holley 650 2 barrel.
030.jpg

Whats the part number on the metering block on that one.
I have one thats missing the metering block, for ages I've meant to try out an 850 or 1050 block on it.

[bigfoot584]

Whats the part number on the metering block on that one.
I have one thats missing the metering block, for ages I've meant to try out an 850 or 1050 block on it.
[/quote]

Here's what mine has, when I pulled the block I was surprised it would
be metering block limited being it has the accel. transfer tube in it,
usually see that on the emission type carbs.

0-101.jpg

0-102.jpg

[Truckedup]

The Quadrajet has no boosters in the secondaries, Yes? How does that affect atomization?

[bigfoot584]

The Quadrajet has no boosters in the secondaries, Yes? How does that affect atomization?

They may not have had a booster we know it, but it was a tube exiting to
the center of the venturi acting similarly to a booster, something similar to
below.

031.jpg

[Truckedup]

The Quadrajet has no boosters in the secondaries, Yes? How does that affect atomization?

They may not have had a booster we know it, but it was a tube exiting to
the center of the venturi acting similarly to a booster, something similar to
below.

031.jpg

Ok, I don't see how a simple tube would atomize as well as a booster....Perhaps at higher velocities it works ?

[ClassAct]

Thanks for the answers. I've never seen that Holley 2 barrel carb before. I didn't think about the Venturi having having hole in it.

[bigfoot584]

They were sold in the early '70's, I'd used one on a dirt track car at the time
the rule stated any two barrel carburetor and that was the largest available
at the time, they must not of sold well or were trouble some cause mid to
late 70's you didn't see anymore and I don't believe Holley ever used that
venturi design again.