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Induction, carb sizes, fuel types and their effects.

General engine tech -- Drag Racing to Circle Track

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Re: Induction, carb sizes, fuel types and their effects.

Post by Troy Patterson »

It might let more air into the emulsion well overall or more at specific points. To many variables there to make a specific statement.

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Re: Induction, carb sizes, fuel types and their effects.

Post by jmarkaudio »

This is a copy from a post by Tuner on the Innovate website on emulsion.

Tuner says,
"Why didn't you ask a simple question?

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.

Now, do you have any easy questions? "
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Re: Induction, carb sizes, fuel types and their effects.

Post by David Redszus »

And with more from "Tuner's Tips", here is another thing to consider with fuel. You can have two fuels with the same distillation curve or components that boil at the same temp, but have one take more BTU of energy to vaporize an equal volume than another. This means more heat from the engine or combustion chamber to vaporize and burn, or a little more time to do so. Points to ponder....
For most fuels, alcohols excepted, if the distillation curves are the same, the BTUs necessary to evaporate the fuels will be the same. That is because the Heat of Evaporation for most hydrocarbons are very, very similar.

The same holds true for Heat of Combustion, if we consider the stoich value of the fuels. That is, if we consider the BTUs per pound of air burned, all normal fuels (alcohols excepted) will be within about 1% of each other.

Regarding distillation curves, it might be well to remember that light fractions do indeed evaporate more easily but they do not contain the high octane properties that are resident in the heavier fractions. Therefore, in order to obtain the true octane from a fuel, it must be fully evaporated. If not, then we need to run richer.

One issue to ponder is why does the A/F ratio need to change with RPM? Being more efficient is a result, not a process. In fact, we might ask, why do we need to run richer than stoich at all? We aren't really burning it all.
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Re: Induction, carb sizes, fuel types and their effects.

Post by Troy Patterson »

Lots of variables such as combustion efficiency, the degree of scavenging of induction charge during overlap, the amount of time fuel is exposed to heat, atomization, cooling of the combustion chamber between cycles and so forth affect the requisite air / fuel ratio at any given rpm.

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Re: Induction, carb sizes, fuel types and their effects.

Post by jim_ss409 »

This thread has me taking a second look at fuel distillation curves. I've been running pump gas in my low compression bracket race car but I think I might do better with a race fuel that vaporizes at lower temperatures.
I can't find any real specifications for the pump gas but I had considered filling a few jugs with winter gas just to see if it made any difference.
If I approach the starting line on a cool morning without thoroughly warming up the engine I get a poor 60' time. I know pumping raw fuel into a cold tunnel ram isn't great for combustion but I'd like to try it with a more volatile fuel.
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Re: Induction, carb sizes, fuel types and their effects.

Post by Troy Patterson »

jim_ss409 wrote:This thread has me taking a second look at fuel distillation curves. I've been running pump gas in my low compression bracket race car but I think I might do better with a race fuel that vaporizes at lower temperatures.
I can't find any real specifications for the pump gas but I had considered filling a few jugs with winter gas just to see if it made any difference.
If I approach the starting line on a cool morning without thoroughly warming up the engine I get a poor 60' time. I know pumping raw fuel into a cold tunnel ram isn't great for combustion but I'd like to try it with a more volatile fuel.
Annular boosters atomize the fuel more allowing the fuel to absorb more heat - sort of like altering the distillation curve.

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Re: Induction, carb sizes, fuel types and their effects.

Post by BrazilianZ28Camaro »

jim_ss409 wrote:This thread has me taking a second look at fuel distillation curves. I've been running pump gas in my low compression bracket race car but I think I might do better with a race fuel that vaporizes at lower temperatures.
I can't find any real specifications for the pump gas but I had considered filling a few jugs with winter gas just to see if it made any difference.
If I approach the starting line on a cool morning without thoroughly warming up the engine I get a poor 60' time. I know pumping raw fuel into a cold tunnel ram isn't great for combustion but I'd like to try it with a more volatile fuel.
If U haven't enough CR, the race fuel will make less power.

Probably a little rich AFR and pump shot could do good things when you race with the engine "cold" :roll:
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.40 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
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Re: Induction, carb sizes, fuel types and their effects.

Post by jmarkaudio »

Troy Patterson wrote:
jim_ss409 wrote:This thread has me taking a second look at fuel distillation curves. I've been running pump gas in my low compression bracket race car but I think I might do better with a race fuel that vaporizes at lower temperatures.
I can't find any real specifications for the pump gas but I had considered filling a few jugs with winter gas just to see if it made any difference.
If I approach the starting line on a cool morning without thoroughly warming up the engine I get a poor 60' time. I know pumping raw fuel into a cold tunnel ram isn't great for combustion but I'd like to try it with a more volatile fuel.
Annular boosters atomize the fuel more allowing the fuel to absorb more heat - sort of like altering the distillation curve.

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These are some of the things I am talking about. Annular boosters do atomize fuel better, but going from downleg to annular with no other changes will pose more of a restriction, making the carb smaller. So it's a tradeoff, but in some cases may be necessary. In the case of the fuel, the low compression makes it necessary to warm up the engine, finding a fuel with better vaporization qualities may help as long as it doesn't put the engine too close to detonating. Finding the right heat range plug and timing play into it as well. Little improvements add up when looking for power.
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Re: Induction, carb sizes, fuel types and their effects.

Post by Troy Patterson »

jmarkaudio wrote:
Troy Patterson wrote:
jim_ss409 wrote:This thread has me taking a second look at fuel distillation curves. I've been running pump gas in my low compression bracket race car but I think I might do better with a race fuel that vaporizes at lower temperatures.
I can't find any real specifications for the pump gas but I had considered filling a few jugs with winter gas just to see if it made any difference.
If I approach the starting line on a cool morning without thoroughly warming up the engine I get a poor 60' time. I know pumping raw fuel into a cold tunnel ram isn't great for combustion but I'd like to try it with a more volatile fuel.
Annular boosters atomize the fuel more allowing the fuel to absorb more heat - sort of like altering the distillation curve.

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These are some of the things I am talking about. Annular boosters do atomize fuel better, but going from downleg to annular with no other changes will pose more of a restriction, making the carb smaller. So it's a tradeoff, but in some cases may be necessary. In the case of the fuel, the low compression makes it necessary to warm up the engine, finding a fuel with better vaporization qualities may help as long as it doesn't put the engine too close to detonating. Finding the right heat range plug and timing play into it as well. Little improvements add up when looking for power.
The annulars aren't vaporizing the fuel per say, but atomizing it more finely - which allows more vaporization possible with a given amount of heat present. Whether or not a given engine benefits will depend on the application.

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Re: Induction, carb sizes, fuel types and their effects.

Post by RednGold86Z »

David Redszus wrote: Regarding distillation curves, it might be well to remember that light fractions do indeed evaporate more easily but they do not contain the high octane properties that are resident in the heavier fractions. Therefore, in order to obtain the true octane from a fuel, it must be fully evaporated. If not, then we need to run richer.
Really?? Lighter molecules tend to be more stable and have higher octane from what I've studied. The long chains tend to break easily and cause knock. Maybe an example of this would be CNG or LPG having pretty high Octane ratings, and stale gas that has been open to atmosphere for awhile has terrible octane.
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Re: Induction, carb sizes, fuel types and their effects.

Post by MPac »

I got the millon dollar answer!! it will blow your mind! Buy a REALLY REALLY big supercharger and find a hemi that is around 800 ci spend 50 dollars a gallon on some nitro put it together and there you go! about 8000 hp for the granny in the big ass boniville or the fat guy in the geo metro lol or go to california and steal john forces its priceless and fool proof! :D
matters not whether you win or lose; what matters is whether I win or lose
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Re: Induction, carb sizes, fuel types and their effects.

Post by David Redszus »

RednGold86Z wrote:
David Redszus wrote: Regarding distillation curves, it might be well to remember that light fractions do indeed evaporate more easily but they do not contain the high octane properties that are resident in the heavier fractions. Therefore, in order to obtain the true octane from a fuel, it must be fully evaporated. If not, then we need to run richer.
Really?? Lighter molecules tend to be more stable and have higher octane from what I've studied. The long chains tend to break easily and cause knock. Maybe an example of this would be CNG or LPG having pretty high Octane ratings, and stale gas that has been open to atmosphere for awhile has terrible octane.
I'm sorry to say that you have been given some bad information.

Lighter fuel molecules always have lower octane values. Lighter fuel molecule often have lower boiling points.

Fuel hydrocarbons can be classified as alkanes, aromatics, olefins, and oxygenates. They can then be sub-classed by carbon number and carbon chain formation.

There are NO alkanes with octane values above 100MON that have carbon numbers below 7. There is only one C7 isomer, isoheptane, (2,2,3 trimethly butane) with high octane (101MON); Boiling point is 178F, SpG .695.
Two others are C8 isomers, isooctanes, with octanes of 100MON, BP 211F to 230F.

Among the aromatics, there are several with high octane values; all are C6 and above. Their octanes range from 98 to 115MON, BP are about 280F with SpG around .870.

There are NO olefins that have high octane values.
There are several oxygentes with higher octanes but all have high boiling points and high Heats of Evaporation.

Light fuel fractions are very important, (and their losses are intolerable) but not due to octane reduction. They are the igniters that start the combustion process without which proper combustion pressure angles cannot be consistently maintained.
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Re: Induction, carb sizes, fuel types and their effects.

Post by Shrinker »

Methane is an Alkane and its very high octane rated. Ron or Mon.
When gasoline is exposed to air and the light fractions evaporate you get a fuel thats easy to detonate not because of the loss of high octane components but because of inefficient combustion initiation and that leads to incomplete combustion hence residuals on the next engine cycle, there the ones that detonate in this situation.
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Re: Induction, carb sizes, fuel types and their effects.

Post by David Redszus »

Methane is an Alkane and its very high octane rated. Ron or Mon.
Methane has a boiling point of -258F which means it is a gas and not a liquid at normal temperatures and is never found in gasoline.
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Re: Induction, carb sizes, fuel types and their effects.

Post by Shrinker »

Thats true but you could have qualified your statement in that other post better. I was not being critical just clarifying it. Methane fueled engines are not uncommon. Natural gas (methane) fueled Buses are common in Australia.
And methane is produced in exhaust emissions, its one of the gases tested for emission compliance of gasoline engines here. Because its present in exhaust (albeit due to poor combustion conditions) its present in the succeeding cycle due to residency. So it would have an influence.
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