The_7

when the air goes past the bulge in the venturi, how does a vacuum get created. does the vacuum occur at the bulge, or just before? my best guess would be that when the air hits the bulge the velocity increases and its momentum creates the vacuum just before the bulge. am i close?

bill Shurvinton

Go search bernoulli. Plenty of web pages that give nice answers.

Metallic_rock

double post.

Metallic_rock

after the bulge, the expansion of the air going into a larger area from a smaller area causes a decrease in pressure. (also lowers temp, if you were interested.)

The expansion and decreased pressure causes fuel to easily flow into that space and also the expansion allows the fuel to atomize better.

the actual vacuum is caused by the sucking of the engine.

REVHED

Not sure what you meant exactly but remember that vacuum is only present after the throttle plates. (assuming they're not fully open). The air going through the carb is under atmospheric pressure.

The_7

so what would happen if the venturi was straight through with no bulge...would the car run at all?

Sterling

Originally posted by REVHED
Originally posted by Metalic_Rock
the actual vacuum is caused by the sucking of the engine.
--------------------------------------------------------------------------------

Not sure what you meant exactly but remember that vacuum is only present after the throttle plates. (assuming they're not fully open). The air going through the carb is under atmospheric pressure.


Yeah ya did.

Too many people around here get hung up on this "suction" thing.
Merriam-Webster defines "suction" as;
"the act or process of drawing something (as liquid or dust) into a space (as in a vacuum cleaner or a pump) by partially exhausting the air in the space"

-I think a ******* engine qualifies as sucking thru a carburetor! It certainly meets enough criteria for the purposes of illustrating the components of a carburetor to someone who just wants to know how a venturi works!
JEEEE-ZUS CHRIST!

BOING! SMACK

7, the suction would have to be much greater for the same amount of fuel to be sucked into the engine, and it would also not be atomized. If fuel is not atomized, it won't combust well.
Fulies don't have a venturi, just a straight through hole for air because much closer to the chamber, injectors spray a fine atomized mist due to very high fuel pressure.

According to Paul Yaw and another source that I can't recall, at it's most perfect tuned state, there will be at least one throttle position on a carb in which the atomization will actually be better than current injectors.
I don't know if that's true or not.

The_7

cool, so as the size difference between the bulge and the part after gets bigger, the more fuel is sucked in and its atomized better. can the size difference be TOO big? like if you took the nikki carb and made the part after the bulge bigger and then decreased jet size to compensate to the car doesnt run rich would it work better?

Sterling

This ******* FORUM is not letting me post my ******* PICTURE!

mar3

iTrader: (8)

Now, now....take some Prozac and it'll all be OK....

mar3

iTrader: (8)

No

Sterling

Basically, because of atmoshperic pressure, the air is pressurized at around 15 lbs per square inch.
That air wants to fill the void that the rotors make by sweeping the air (mixture) that's in the chamber away.
So the air flows thru the carb as fast as the engine can "sweep" it away. But for the intents of explaining how a veturi works, I usually just refer to this whole thing as suction. It's really just the flow of air.

If air flows through a big tube, and there's a little tiny tube teed to it, as the air flys by the tiny tube, it's going to create a draft that applies vacuum at the end of the tiny tube.

If you increase the speed of air thru the big tube, more vacuum action will happen to the tiny one.

Imagine the big tube as the venturi, and the tiny tube as the fuel nozzel.

Increasing the speed can be done by increasing the pressure (boosting),
or by making the same amount of air flow thru a smaller bore.
Making the bore smaller limits the volume that can flow thru it at hi RPMs. Now your talking about a carburetor that is too small for high end.

The only option left is to get the air at the bottom of the bore out of the way so that atmospheric pressure can do it's job and push the air faster past the fuel nozzle. Necking in the bore creates a spot where the flow rate is high enough to suck fuel from the main circuit. Opening up the bore below that neck creates a void for that air to go.

Altering that venturi angle is a tricky guessing game. It depends on application, engine flow, engine efficiency, fuel mixture, type of fuel, fuel temperature, yaddayaddayadda.
The best you can hope for in a Nikki is to bore out the primaries a bit, but leave the necked in area in the same spot relative to the "booster" venturis. The neck can be made bigger, but it has to be kept at the same hieght.
Widening the neck too much does no good no matter what you do to below the neck because your only going to flow as much as the throttle body will allow (the cast iron bottom part of the carb). Making the neck angle too shallow will reduce the suction on the main circuit, too.

And there's an aswer to some of your questions, I think, just by looking at the little booster venturis.

Holley has been trying for years to develop a better venturi design that incorporates the booster venturi. You can see by the carburetors out there that you can't be too radical in the angle of the venturi neck. If you could get radical and it would help, then there would be no reason for the booster venturi. They would simply have a real thick one-sleeve venturi that's got the circuit going through the thickest part all the way around, with tiny holes drilled on the inside of the venturi to let the fuel get sucked out. It would be all in one. But...if that worked, I think they'd be doing that.
So I think there's a limit to what you can expect a single sleeve to do, and this is why they've stuck with the "venturi on venturi" design.

Keep in mind that you want the system to suck the fuel out of the main circuit like crazy, because the fuel jets regulate the amount that will go thru from the bowls.

The purpose of opening up venturis is to make the carburetor flow easier. The farther from being "maxed out" a carburetor is, the better low to midrange power it's going to make.

If a carb is maxed out (operating at the maximum flow it can handle freely), and tuned for it, it will deliver great midrange to high power.

This is why Weber two barrel carbs are so ideal for racing- their flow rate is greatest at high RPMs.

The_7

damn sterling, you are the carb MASTER! you and rx7carl . i read your last post like 4 times and i can say that i understand it better than i did the first time i read it . so the necking is to give the carb better flow than a single sized bore and to get the fuel to atomize better? i am having trouble imagining your explanation with the big tube and little tube. as the air infront of the little tubes opening moves down the tube, new air flows past it so there is never a lack of air in front of the little tube so i cant imagine how the vacuum is created. and my thanks to you for taking the time to write such detailed posts

dnlloran

Take a bucket of water, put a hose in it, then take an air nozzal and blow across the top of the hose (not into it). watch what happens. same prince. as above, basicaly creating a vacuum at the top of the hose causing a siphon.
D.

The_7

you basically said the same thing sterling said with a different situation. i know that the air flowing past the hole causes the vacuum i just dont understand why. lol sorry im one of those people who wont stop until they know EXACTLY why something happens please dont be mad at me

The_7

bump

The_7

duble bumpy

Sterling

`

The_7

Sterling

Sterling

Here the fuel level is dropping below the halfway point at the window glass in the float bowl. As it's doing this, it's also uncovering holes in the emulsion tube. This does a few things- it bleeds off vacuum signal, and it aerates the fuel with air bubbles, easing atomization.
Since the resulting increase in vacuum signal acting on the main circuit is NOT directly linearly proportional to the increase in velocity coming through the venturi, the air flow would be sucking way too much fuel out at high RPMs.
The emulsion tube is cross drilled with holes that are uncovered as the fuel level gets lower. The more holes uncovered, the more signal gets "bled" away. But the limit to how much can get ever get "bled" away is decided by the orifice size on the top of the tube.

The fuel level drops due to the engines needs-if set properly.
This is why fuel pressure, and running a return line is important for the Nikki to operate properly.

I've read people adamantly proclaiming either that the Nikkis are imfamous for running too rich at high RPMs, or lean out at hi RPMs.
Fuel pressure is rarely discussed in those threads, though.
Go figure.

The_7

i had a little trouble understanding your last post...

what is the emulsion tube?
what do you mean by vacuum SIGNAL?

thanks

Rx7carl

go see how a wing works, like at howstuffworks.com and youll see how the airflow causes pressure drops in the carb. An airplane wing is really just a cross section of a carb venturi.

On the other stuff...........hey Iotus, reprint that pm I sent u a couple weeks back!

Rx7carl

Hah, Nm I found it.

There are jets in the carb. Basically they are calibrated holes (orificies to be technical). Take a simple one barrel carb. As air flows thru the venturi there is a low pressure created, thats where the fuel nozzle is located (little brass tubes in the middle of the venturis of the nikki). The tubes are connected via a passage to the float bowl, so a low pressure will cause fuel to be sucked out of the nozzle. But thats no good cause you need the right balance between fuel/air mixture right? So in the passage (usually located in the float bowl so its accessible) is a fuel jet. It has a hole of a specfied diameter that will flow "x" amount of fuel. Bigger jets will richen the mixture, smaller ones will lean it out. So as vacuum increases, so does fuel flow. Easy right?
But wait! the more vacuum that is created in the venturi can cause a mixture that was perfect at low throttle and mid throttle to be too damn rich at high rpms! Well we cant use smaller jets cause that will cause it to be too lean in the low and midrange....Hmmmmmmmm....what to do.......what to do.......AHA!!!!!!!!! we can put a small hole in the passage (above the fuel level). That will cause some of the vacuum signal to be lost, reducing the fuel signal rise with high vacuums! These little babies are called air bleeds, cause they "bleed" off the signal. When the differential between outside air pressure (which is also float chamber pressure) and venturi low pressure (vacuum) is low, the bleeds dont really do much, but as the differential increases the bleeds keep the fuel flow rate from skyrocketing rich. Also they perform another important function. The air that is sucked into the bleeds has to go somewhere right? Well where does it go Mr. Smartypants???? It goes into the fuel thats on its way into the venturi fool! And as it travels it mixes (or emulsifies) with the fuel, causing the fuel to break up into smaller "droplets". This gives better atomization for a more uniform fuel/air mixture.
Simple eh? The trick is to get it as balanced as possible.

mar3

iTrader: (8)

You really, really need to read up on Physics. Sterling has put it about as simple as he can before you are basically teaching physics and Bernoulli's Principle. I'll answer what I can.

The emulsion tube is that little rod with the holes in it that threads into the carb body from the top. In his pic, the screw head of the tube is army green and the rod's hollow tube is represented by the army green lines going down into the red area. The red represents fuel molecules. Look up the definition, "emulsion", to understand that rod's role in the Nikki. It's a neat little trick that carb has.

Techs and gearheads alike call the vacuum or pressure drop from atmospheric, a vacuum signal. I can only imagine because certain parts of the carb only start working at certain atmospheric gradients and "signal" makes a nice term for those points. Or that certain parts of an engine's subsystems only work by getting a "signal" from the carb to do their thing.