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Old 01-13-2011, 11:33 AM
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Carb emulsion question

Carb experts,:
I'm trying to learn about tuning Holleys and am stumbling on the emulsion system.
I understand the basic idea is that it adds air to "shape" the fuel curve, but that's about it.
I have a Proform block w/3 holes in it, a billet Holley style clone block w/5 holes and a stock Holley block w/2 holes.
My first question is are more holes better or does it not really make a difference? I've seen internet posts going both ways.
Second question is, are the main wells in those Holley style metering blocks close in size , shape and volume or are they different, and will that make a diffrence to the engine?
Short of cutting them open on a band saw I can't tell. Any input or opinion would be greatly appreciated.
Thanks
BF

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Old 01-13-2011, 01:09 PM
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I believe Holley has a division that will rebuild what you have, so i would think that you could call them and ask for advise.

I wanted to get one of those center sections too. Just got sidetracked with other projects.

E
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Old 01-13-2011, 02:15 PM
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Quote:
Originally Posted by bfitz241
Carb experts,:
I'm trying to learn about tuning Holleys and am stumbling on the emulsion system.
I understand the basic idea is that it adds air to "shape" the fuel curve, but that's about it.
I have a Proform block w/3 holes in it, a billet Holley style clone block w/5 holes and a stock Holley block w/2 holes.
My first question is are more holes better or does it not really make a difference? I've seen internet posts going both ways.
Second question is, are the main wells in those Holley style metering blocks close in size , shape and volume or are they different, and will that make a diffrence to the engine?
Short of cutting them open on a band saw I can't tell. Any input or opinion would be greatly appreciated.
Thanks
BF
The big thing about the emulsion system is to keep the mixture from becoming excessively rich under high vacuum conditions at the idle end and excessive richness under high venturi flow conditions around WOT. The more holes the more sensitive the system becomes to managing smaller changes. The 2 hole is fine for street applications. The 3 hole is a little improvement to that. The 5 hole is strictly a performance piece where the expectations of high RPMs calls for more capability in amount as well as better, or more discrete, control.

Bogie
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Old 01-13-2011, 09:51 PM
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OK. Thanks, that gives me something to think about. Let me ask this, can you use the emulsion circuit to lean down the main circuit? I ask because I have a 750 Holley w/annular boosters, I don't know what brand booster, but they must have a tremendous signal because as soon as the main circuit starts , it goes rich. Real rich. And I am becoming concerned because I keep going to smaller jets and don't want to get too small.
So far, idle mixture and cruise are fine.
Thanks for the help.
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Old 01-13-2011, 10:42 PM
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Jets meter fuel, the emulsion holes meter air, so the more area of the emulsion holes (regardless of the number of holes), the leaner the overall fuel curve will be. And vice versa.

If it were me, I'd probably look for a good book on Holley high performance tuning. One such book that has some info on emulsion tubes, etc. (there may be other better books on this), is:

Holley Carburetors, Manifolds & Fuel Injection: How to Select, Install, Tune, Repair and Modify Holley Fuel System Components for Street and Racing
by Mike Urich; Bill Fisher

In your case, are you certain that the power valve isn't coming in too early?
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Old 01-14-2011, 12:22 PM
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I've got that Holley book by the way...

As to the powervalve, it opens at 4", checked it with my vacuum pump, and the rich condition occurs with much more vacuum then that, at about 2200...

Now that I think about it, maybe I should look at leaning the startup of the mains, instead of the whole main circuit....I need to graph the whole air/fuel map .Now I need the snow to melt...

If I wanted to lean the startup of the main circuit, would that be a larger emulsion bleed toward the top of the metering block? Or would that delay the start of the main circuit....I wish I could go test this, I hate winter
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Old 01-14-2011, 04:17 PM
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Enlarging the uppermost holes would lean the beginning of the curve.

This was something I had bookmarked previously, it may be of some use to you. It's better than me stumbling around trying to explain it:
Quote:
In the thread “750 Holley carb help” Klaus made this statement, “On carbs it's very important that the correct two-phase flow gets established during emulsion. Otherwise you will see RPM dependency of AFR.” Thank you Klaus, but forgive me if I see your remark as a profound understatement. Incorrect two-phase flow is at the root of all this aggravation. People who have drill bits but don’t know why to use them have been molesting innocent carburetors for a long time. Now some of them are in charge of the manufacture of new carbs and they think they have improved them by using larger drill bits to make the air bleed and “emulsion” orifices. I guess the guys that engineered the original carburetors on the old muscle cars were pretty stupid or they would have “improved the emulsion” 40 or 50 years ago when they had their chance. After all, they had the awesome power of the single-point ignition system at their disposal, they shouldn’t have been afraid of a little soot.

It is well documented that introducing air into the main well encourages low signal flow and can encourage or discourage high signal flow. The natural characteristic of a plain jet and nozzle (no air) is to get richer as airflow increases. The purpose of the air bleed system is to modify that behavior to accomplish a constant (or the desired) air/fuel ratio over as wide a range of airflows as possible. The particular ratios for power and cruise are realized by the selection of jet and rod or jet and auxiliary jet (power valve channel). The purpose of air bleeds is not to emulsify but to accomplish the correct fuel delivery. Emulsion is just a beneficial side effect.

What I’m going on about here is Klaus’ remark about “correct two-phase flow”. That is the description of a fluid flow that is made up of a liquid and a gas flowing together in the same conduit. As the ratio of gas to liquid increases (more gas, less liquid), at some point the gas bubbles coalesce from many small ones into a few big ones and the flow starts to “slug” and become erratic. The carburetor nozzle spits like a garden hose with air in it when there is too much “emulsion” air.


An emulsion of air and fuel has reduced density, surface tension and viscosity compared to fuel alone. This increases the flow of fuel considerably, particularly in low-pressure difference operation, at low throttle openings or lower engine speeds. Just how much of an increase (richer) is dependant upon where and how much air is introduced into the fuel flow.

Mainly, what must be understood is that because the fuel discharge nozzle connects the venturi to the main well, whatever the low pressure (vacuum) is in the venturi, it is also the pressure in the main well. The air bleed is in the carb air horn or somewhere else where it is exposed to essentially atmospheric pressure, which is higher than the venturi pressure. This pressure difference causes air from the air bleed to flow through the emulsion system into the main well and to the nozzle. The flow of air can have very high velocities, approaching sonic in some orifices. The airflow literally blows the fuel toward and through the nozzle. A larger main air bleed will admit more air to the emulsion system and that can increase or decrease fuel flow to the engine. The size, number and location of the other air holes in the emulsion system, the size of the main well flow area, the size of the nozzle and the specific pressure difference at the moment are the determining factors. The ratios of air volume to fuel volume to flow area, with the air volume's expansion with the venturi velocity induced pressure reduction being the key. The bubbles expand as the pressure drop increases with airflow. Suck on an empty balloon to experience the effect.

The fuel flow through the main jet is the result of the pressure difference between the atmospheric pressure in the float bowl and the venturi air velocity induced vacuum acting on the nozzle and the main well. The venturi vacuum in the well is reduced (the pressure is raised) by the "air leak" from the air bleed. This reduces the pressure difference that causes the flow through the main jet. If the air bleed were big enough, the pressure in the well would be the same as in the float bowl and no fuel would flow. Think about drinking through a soda straw with a hole in it above liquid level. Bigger hole, less soda. Suck harder, not much more soda. Big enough hole, no soda. This is the means by which the emulsion system can "lean it out on the top end". Incidentally, the vacuum that lifts water up a soda straw is in the most sensitive operating range for emulsion systems.

It is in the lowest range of throttle opening, at the start of main system flow, that the effect of adjusting the introduced emulsion air (and it's effect in increasing the main fuel flow) is most critical. Small changes can have large and sometimes unexpected or counter-intuitive consequences. The goal is to seamlessly blend the rising main flow with the declining idle/transition system fuel delivery to accomplish smooth engine operation during opening of the throttle in all conditions, whether from curb idle or any higher engine speed. The high speed and load mixture correction is usually easily accomplished, in comparison.

The vertical location of the bleeds entering the main well influences the fuel flow in the following ways.

1: Orifices above float level or between the well and the nozzle allow bled air to raise the pressure (reduce the vacuum) in the nozzle and above the fuel in the well. That delays the initial start of fuel flow from the nozzle to a higher air flow through the venturi and is used to control the point in the early throttle opening where the main starts.

2: Orifices at float level increase low range (early throttle opening) fuel flow by carrying fuel with the airflow to the nozzle.

3: Orifices below float level increase fuel flow by the effect of lowering the level of fuel in the well to the hole(s) admitting air. This is like raising the float level a similar amount (increases the effect of gravity in the pressure difference across the main jet) and also adds to the airflow carrying fuel to the nozzle. Locating the orifices at different vertical positions influences this effect’s progression.

4: The "emulsion holes" influence is greatest at low flows and the "main air bleed" has most influence at high flows.

In the first three cases above, once fuel flow is established it is greater than it would be with fewer or smaller holes. Visualize wind blowing spray off of the top of water waves. It doesn’t take much pressure difference to cause the velocity of the airflow through the bleed orifices to have significant velocity in the orifice, even approaching sonic (1100 F.P.S.) if the orifices are small. The phenomena of critical flow is what limits the total air flow through an orifice and allows tuning by changing bleed size.

Essentially, the emulsion effect will richen the low flow and the air bleed size, main well and nozzle restrictions will control the increase or reduction of high flow. Again, the desired air/fuel ratio is the primary purpose of the bleed system. "Improved emulsion" is an oxymoron if the modification of air bleeds to "improve emulsion" results in an incorrect air/fuel ratio in some range of engine operation. Correct proportioning of all the different bleeds (and, of course, the idle, transition and power circuits) will give the correct air/fuel ratios over the total range of speeds and loads and a flat air/fuel ratio characteristic at wide open throttle.
Be aware that at "1", "2" and "3" above, the orifices referred to are the bleeds- not the emulsion holes.

The consensus is that adding more, not necessarily larger holes will be more of a benefit. Obviously in the case of the aftermarket and HP metering blocks, the number of emulsion holes is set by the manufacturer. In that case, size should be changed rather than number. The number changes CAN be made, but are more often seen w/an emulsion tube set-up, like seen in some factory and modified Q-jet secondaries.
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Old 01-15-2011, 11:00 AM
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Quote:
Originally Posted by bfitz241
OK. Thanks, that gives me something to think about. Let me ask this, can you use the emulsion circuit to lean down the main circuit? I ask because I have a 750 Holley w/annular boosters, I don't know what brand booster, but they must have a tremendous signal because as soon as the main circuit starts , it goes rich. Real rich. And I am becoming concerned because I keep going to smaller jets and don't want to get too small.
So far, idle mixture and cruise are fine.
Thanks for the help.
Yes you can do this, but if the correction jets are not a replaceable type the only thing you can do is to drill them to larger size. The problem with that is going back to a smaller size is very difficult to pull off in case you get it wrong.

The problem with annular boosters is just what you're seeing. These work a lot better on lower revving engines where there is too much cam and/or too much port and valve size for the power band the engine operates in most of the time. Or that it's a basic high out put grocery getter type engine where fuel mileage is considered important; an example that comes to mind would be the 300-330 horse Ford 390's of the 1960s. These used a small 4 bbl (about 450cfm) with annular boosters in an attempt to provide good gas mileage with a high power but not high performance engine.

The Holley has two events going on when main metering comes on. One certainly is main metering thru the replaceable jets. But in the back ground is the power enrichment circuit. However, something you have to look out for before I get into how and when that works is whether or not it's there. Many carbs set up for competition block this circuit off and just use monster size main jets. But if you have a vacuum valve managed power circuit there's four things to look at.

-First, whether a backfire damaged the power valve's diaphragm in which case it leaks fuel into the power circuit all the time which makes for a really rich mixture.

-Second, whether or not its gasket is there and is properly positioned if it is, also whether the valve body is torqued to spec. Or if the metering plate has a crack in this area.

- Third, what vacuum opens the valve. The valve opens with diminishing vacuum. A big cam lowers manifold vacuum, if the valve open is calibrated for a mild high vacuum producing cam when teamed with a long duration or high lift at the valve situation or fast ramping such as caused by high ratio rockers, any one or more of these could trip the valve open earlier than you'd expect. The opening vacuum is stamped on the side of the power valve. General place to start with selecting the opening point is 10 inches of vacuum lower than that pulled on a gauge at idle. This isn't a gospel position, just a place to start tuning from.

-Fourth, is the power jet size. These are pressed-in brass jets just inside the main meter block behind the power valve. They are replaceable with care of removal and installation.

-Fifth, I guess would be if the metering plate has the block off plug for the power valve. In this case the mains will be very large to cover the flow for both circuits. This is done for race tuning where the carb is only expected to be good at or around WOT, so this potential failure point of the power valve is just eliminated and all the needed go juice comes from the main jets. If you're trying to run a carb like this on the street it is way better to restore the power system and lean down the mains.

Another issue can be the idle position on the throttle blades, this is more a problem with large cams and/or high ratio rockers, where to get idle the throttle plates are open into or beyond the transition slot. This will turn on main metering much sooner than normal. The typical solution is to provide additional idle air by drill a small hole in the primary throttle blades or adjusting the secondary throttle stop screw so that they are cracked open a tiny amount.

The intake also has an effect, I wouldn't try to run annular boosters on a 180 degree intake as these provide much stronger venturi signal sooner than an open plenum type intake and will often lead to tuning problems with annular boosters unless your a really good carb tuner with the parts and tools to handle what you got into.

Bogie
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