Divorced idle circuits

[swampbuggy]

I remember divorced idle circuits being talked about a while back here on Speedtalk. So i have some questions to be answered please. 1. On a race quality 4150 platform WHAT is the definition of divorced idle circuits ? 2. What was the reason for developing this feature ?
3. In what application (type of service, racing, driving, etc.) would divorced idle circuits be most likely employed ?
4. What are the good traits of divorced idle circuits ?
5. Are there ANY negative aspects to D.I.C.'s ?

Thanks in advance for any and all responses ! Mark H.

[Walter R. Malik]

Those type 4 barrel carbs were used by Chrysler Corp in 1969 and 1970. Others used some form of that, I am sure.
The idle system was not interfaced with any other circuit as the idle fuel had its own separated circuit and the off idle slot was a different circuit altogether.

The metering block in those carbs were the forerunners to the cast 3 circuit Holley blocks; although the circuits were rearranged for the later usage.

[NormS]

Interdependent idle circuits are what the vast majority of automotive carbs use. In this configuration, the idle circuits draw their fuel from the fuel that has already passed through the main metering jets. As the throttle is opened, the increased airflow past the venturi boosters increases the booster signal(vacuum) pulling fuel through the main jets. This increased vacuum on the main metering circuits also causes the idle and transition circuits to taper off in their delivery of fuel. This circuit configuration is handy in controlling the low rpm/light load/part throttle cruise mixture...a lean mixture which is generally needed for good fuel mileage and low emissions.
To compensate for this tapering off effect at higher loads and rpm's, these carbs typically have power enrichment feature that adds more fuel to the main metering circuit fuel flow, basically helping make more power and decreasing the chances of detonation of the fuel/air mixture in the cylinders at higher cylinder pressures.
Divorced idle circuits do not have this tapering off effect, because they are drawing their idle and transition circuit fuel directly from the fuel bowl. The light load/part throttle mixture, with carbs designed like this, is generally richer than a carb with the interdependent circuits. Because the idle and transition circuits are not influenced by the main metering circuits in a divorced circuit carb, some of these carbs do not require power enrichment circuits to add extra fuel under higher load situations. A divorced circuit carb can more easily flow more fuel under high load situations, because of the lack of influence that the main metering circuits over the idle and transition circuits.
The vintage tractor updraft and sidedraft carbs typically had divorced circuits, and only a few of them had accelerator pump circuits... the need for accel pumps was lower due the high manifold vacuum that those engines had, and the lack of the tapering off effect. These carbs were simpler and cheaper to build without power enrichment and accel pump circuits, and were built in an era where fuel economy wasn't as big a deal as it is now. Norm/CFS

[swampbuggy]

Warpspeed----did the Cup or Xfinity carbs have divorced idle circuits ?? What should i do on my carb, if you don't mind Mark H.

[modok]

Divorced idle mainly gives a greater degree of freedom to use odd jetting combinations, and the idle jet is feeding pretty much all the time too.
In a normal system the idle jet may effectively become an air bleed at WOT. Becha never thought of that eh??

[swampbuggy]

No---never thought about that, but you may be on to something there ???? Mark H. and THANKS Norm for a long reply !!

[Walter R. Malik]

Divorced idle circuits do not have this tapering off effect, because they are drawing their idle and transition circuit fuel directly from the fuel bowl. The light load/part throttle mixture, with carbs designed like this, is generally richer than a carb with the interdependent circuits. Because the idle and transition circuits are not influenced by the main metering circuits in a divorced circuit carb, some of these carbs do not require power enrichment circuits to add extra fuel under higher load situations. A divorced circuit carb can more easily flow more fuel under high load situations, because of the lack of influence that the main metering circuits over the idle and transition circuits.
Norm/CFS

In a divorced IDLE circuit, the transition part, (off idle slot or holes), is a completely different circuit and NOT in any way connected with that IDLE circuit. Although that transition circuit does supply SOME fuel at idle.

[NormS]

The term "divorced idle circuit" is not actually an accurate name for this circuit configuration. There are 2 ways the divorced circuitry can be set up. In the conventional 2 circuit Holley metering block, an idle circuit and an off idle/part throttle transition circuit both pull from the same "idle feed restrictor". Changing the access to fuel, for these 2 circuits, from the main metering well to the fuel bowl divorces them from the main metering circuit, but not from each other.
To have totally divorced circuits, each circuit would need its own "jet", its own up and down wells in the metering block, and its own air bleed. This would require a "3 circuit" metering block, different than the 3 circuit metering blocks that Holley uses on some of the Dominators.
The more commonly used configuration is the one described in the first paragraph.

[pamotorman]

these idle circuits allowed air into the idle mixture because the father out you opened the adjustment the leaner the idle got

[NormS]

On some of the Holley street carbs, they did have metering blocks configured to where the idle mixture needles were actually externally adjustable air bleeds. None of the Holleys, that I have ever seen, which are used for racing applications, have that "reverse idle" configuration. That reverse idle setup usually did not give enough control of both idle and off idle mixture, for an engine with much more than an RV cam.

[Walter R. Malik]

The term "divorced idle circuit" is not actually an accurate name for this circuit configuration. There are 2 ways the divorced circuitry can be set up. In the conventional 2 circuit Holley metering block, an idle circuit and an off idle/part throttle transition circuit both pull from the same "idle feed restrictor". Changing the access to fuel, for these 2 circuits, from the main metering well to the fuel bowl divorces them from the main metering circuit, but not from each other.
To have totally divorced circuits, each circuit would need its own "jet", its own up and down wells in the metering block, and its own air bleed. This would require a "3 circuit" metering block, different than the 3 circuit metering blocks that Holley uses on some of the Dominators.
The more commonly used configuration is the one described in the first paragraph.

The Chrysler Corporation, Holley Carburetors from 1969 and 1970 do have "3 circuit" metering blocks, (ie List 5160 and several others), as I have already said here in an earlier post. They have completely "DIVORCED IDLE CIRCUITS", which is a very accurate description. They have their own separated idle fuel restrictions which pull fuel directly from the fuel bowl not connected in any way to any other circuit ... they have their own separated air bleed, too.
This is NOT near the same as those REVERSE idle circuits.
Maybe you might know something about competition fuel systems but, must know very little about what Holley manufactured for the O.E.M. manufacturers.

Those original 3 circuit Dominator blocks were manufactured from the casting dies for those Chrysler Corporation blocks then machined to rearrange the 3 circuits slightly different. That original idle circuit became the Dominator intermediate circuit.

[NormS]

What Holley made for Chrysler back in '69-'70 matters little to me. I've seen a few of those over the years, and intentionally never used them for any performance application. Some of the stuff that Holley did for the OEM's back then, had no reason to be applied to racing engines. Maybe the OEM's found those features to be advantageous to their mass production factory built engines, but those same features seldom did anything positive for racing engines.
In fact, I found that, in some cases, eliminating those odd features in those OEM specific Holleys, made them run better on those factory stock engines. It appears that some of these odd features designed into those carbs, were designed by engineers who were evidently trying to justify their existence, by coming up with something different. Some of it worked, some of it didn't, little of it had any place on a performance engine.

[Tuner]

This thread is like the parable of the BLIND MEN AND AN ELEPHANT, the story of a group of blind men who have never come across an elephant a carburetor before and who learn and conceptualize what the elephant carburetor is like by touching it. Each blind man feels a different part of the elephant carburetor, but only one part, such as the trunk or side or ear or tail or tusk (.... venturi, air bleed, emulsion bleed, booster, circuit, etc....). They then describe the elephant carburetor based on their partial experience and their descriptions are in complete disagreement on what an elephant a carburetor is. In some versions, they come to suspect that the other person is dishonest and they come to blows. The moral of the parable is that humans have a tendency to project their partial experiences as the whole truth, ignore other people's partial experiences, and one should consider that one may be partially right and may have partial information.

Substitute carburetor for elephant, swap venturi or bleeds or idle circuit for trunk or tusk or tail, and this thread becomes a carburetor parable unfolding before our very eyes.


I.

IT was six men of Carburetorstan
To learning much inclined,
Who went to see the Carburetor
(Though all of them were blind),
That each by observation
Might satisfy his mind.

II.

The First approached the Carburetor,
And happening to peer
Down its round and flowing throat,
At once began to bawl:
"God bless me!—but the Carburetor
Is very much too small!"

III.

The Second, drilling large the bleeds,
Cried: "Ho!—what have we here
Emulsion bleeds so round and small and few?”
But to him it isn't very clear
That more and larger emulsion bleeds cause more fuel to spew
And smoke comes out the rear!

IV.

The Third approached the Carburetor,
And happening to take
The squirming trunk within his hands,
Thus boldly up and spake:
"I see," quoth he, "the Carburetor
Is very like a snake!"

V.

The Fourth reached out his eager hand,
And felt about the knee.
"What most this wondrous beast is like
Is mighty plain," quoth he;
"'T is clear enough the Carburetor
Is very like a tree!"

VI.

The Fifth, who chanced to touch the ear,
Said: "E'en the blindest man
Can tell what this resembles most;
Deny the fact who can,
This marvel of an Carburetor
Is very like a fan!"

VII.

The Sixth no sooner had begun
About the beast to grope,
Than, seizing on the swinging tail
That fell within his scope,
"I see," quoth he, "the Carburetor
Is very like a rope!"

VIII.

And so these men of Carburetorstan
Disputed loud and long,
Each in his own opinion
Exceeding stiff and strong,
Though each was partly in the right,
And all were in the wrong!

MORAL.

So, oft in theologic wars
The disputants, I ween,
Rail on in utter ignorance
Of what each other mean,
And prate about an Elephant
Not one of them has seen!

[NormS]

So Tuner, are you the only one who isn't blind?

[Walter R. Malik]

What Holley made for Chrysler back in '69-'70 matters little to me. I've seen a few of those over the years, and intentionally never used them for any performance application. Some of the stuff that Holley did for the OEM's back then, had no reason to be applied to racing engines. Maybe the OEM's found those features to be advantageous to their mass production factory built engines, but those same features seldom did anything positive for racing engines.
In fact, I found that, in some cases, eliminating those odd features in those OEM specific Holleys, made them run better on those factory stock engines. It appears that some of these odd features designed into those carbs, were designed by engineers who were evidently trying to justify their existence, by coming up with something different. Some of it worked, some of it didn't, little of it had any place on a performance engine.

Your response is typical and foreseen.

You probably never have had any personal experience, used or witnessed the use of a performance carburetor having a true "divorced Idle 3rd circuit". In such a case, the off-idle and tip-in fuel curve control can be more precise for less fuel consumption; especially with the 830 cfm Holley NASCAR carb or any carb being used in a 12/24 hour racing application.

You possibly don't know how to perform this easy conversion on a high performance Holley carb using those Mopar metering blocks., if you were asked to do it.

THEREFORE, it is almost certain you can't envision it would be worth anything in any high performance carburetor setting. You just know it can not be of any value there.

Sure thing ...!

To most, it probably isn't.

AND ...TUNER:
Not seen ...?
In the late 70's and 80's, there are more than 50 or so high performance Holley 4150 carbs out there which I personally have made this modification. All of them showing said results.