Reading this on Maxperformance website. Maybe it helps, maybe it causes more confusion.
Tom Vought (Holley Engineer) and Tuner:
Today I am going to throw some SERIOUS CARB THEORY at you.
If you are not interested in that area of carb tuning, see you tomorrow. Tom V.
Tuner: “On carbs it's very important that the correct two-phase flow gets established during emulsion. Otherwise you will see RPM dependency of AFR.” 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.
Basically TUNER is saying that after the mid 80s the people from Holley went other places and the BASIC Knowledge was lost. Marvin Beniot (later Quick Fuel Owner), Chuck Gulledge, Louis Lucas, Steve White, Marty Brown, Jeff Komar, were all old school Holley carb people. The new guys that went to The south (with Holley Aftermarket, basically were unable to do the job and the company went banklrupt the FIRST TIME. (They went bankrupt several times). TV
TUNER: 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.
This is a very true statement That is why some CARB PEOPLE call the Air Bleeds "AIR CORRECTORS" TV
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.
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.
A little simpler, the airbleeds and emulsion are used to set the fuel curve in a Holley. The 3rd e-hole will make it a little more active at the low end, and possibly a little lean up top depending on the main air bleed used. If you look at the 2 circuit conversion thread, you can set your 8082 metering up at the starting point in the thread even though it is already a 2 circuit carb. After that you HAVE to adjust the metering for your engine as to what it wants to get the best performance, but in most cases it will be close. Your iron head 468 is no different than some of the other combo's here, everything from milder small blocks to some killer engine., Unless the combination is bad, poor cam design or the ICL is way off, timing is way off, plugs are too cold... etc, it should be close enough to start with. Learn to properly read the plugs and invest in a wideband O2 if you can, it will help you find the most out of it.
Tom V.
ps I did not feel like typing all of this stuff in my words when TUNERS were most likely would say it better. Tom V.
