rx7carl, i learned alot from reading your post, thanks .

mar3, thanks for the vac signal definition. even tho im only 17 i do know a decent amount about physics. i understand bernoullis principle, and how fuel is suck through the nozzle with the venturi. what i was confused about was when i asked what would happen if there was no venturi just a straight tube. with the carb/engine example i can see why because the engine takes in more air than the intake system can get through so there is a vacuum. but dnlloran mentioned an example with a hose in a bucket of water. he said that if you blow air past the hose it will create a vacuum. this is what i dont get. it doesnt seem like blowing across the hose opening would create a vacuum. the only thing i can thing of is the small amount of friction of the moving air "grabs the air just inside the hose opening and pulls it out which would then create a vacuum behind it pulling more an d more air and eventually the water. is this close?

sorry if i am annoying you....i do really appreciate you writing all these posts

 

He was trying to illustrate how that would work on a larger scale, but I think he might not have picked the best example.

Getting back to the venturi design; I wanted to add that the the reason it's not simply stepped up to a larger size just past the boosters to create that low pressure system is that the steps wouls allow the incoming mixture to curl.
The whole thing is a compromise in that it can't angles that are too extreme because the flow needs to be a "tumble free" as possible. Any disruption in the flow will cause fuel to drop out of suspension and condence on the manifold walls on the way down.

If you think it takes a little imagination to grip venturi design, you should see the radical things they've tried for intake manifold design! The idea is to keep the mixture from touching the walls in the manifold, keep it swirling but not tumbling, all the way into the engine.
Add to it the fact that as the flow increases, the flow changes its behavior dramatically.

Read "Carburetor Facts" on the Facts page of Mazspeed.com
Tom Turner describes what the emulsion tubes and various jets do. Also check out Paul Yaws site.

 

Yes there will still be a small vacuum created even if there was no venturi, but not near as much of a pressure drop.

 

rx7carl, would it be caused by the reason i guessed?

sterling, thanks for the links, whats the url for paul yaws site?

 

Yawpower.com

 

thanks

ok, i read the carb article at mazspeed.com. it made a lot of things clearer except for one. pretty much the same question had in the first place - it said that as the air flows though the smaller part of the venturi the air speed increases and creates a vacuum. ok, if a given amount of air flows into a smaller space, wont the density increase, causing the pressure to increase?

 

I dont have a good (honest) answer to that. Just that as the air is blown over the "hose" it would draw out the fluid.

 

I thought you knew bernouils principle???

If velocity increases, pressure decreases right? And vise versa. So look at one half of a venturi in cross section. Its now an airplane wing. Airflowing over the bottom goes straight across, but air across the top has further to go right? (cause the curve makes the top of the wing effectively a longer distance). Now the laws of nature dictate that the air when split at the front of the wing, it must re-unite at the back of the wing at the same time. So since the air up top has further to go, at some point it must go faster (Velocity goes ^). So where is the velocity highest? Where the greatest differential is (the crest of the airfoil). Velocity goes up, pressure goes down. That is also called the center of pressure or lift of a wing. (but its not always right at the highest point for other reasons).

 

I'm throwing out numbers just for illustrative purposes.
Alot of times the results from doing the math that subtle changes make can surprise you by being quite counter intuitive.

Think of the air entering a 1 inch diameter tube as an endless column.

The tube represent the top of the venturi all the way to the engine port. (keep that in mind.) But for now, just imagine a straight tube.

The flow rate at the inlet and the outlet is going to be 100 CFM (cubic feet per minute).

If you put an angled step in the tube but kept the bottom half small all the way to the end, the air would be fighting the compression you are talking about, and the flow rate would drop down to 60 or 70 CFM at the inlet and outlet.

But because air is compressable, the venturi design exploits the that to the maximum tolerance that air will deal with it.

So the air compresses slightly as it squeezes through the neck of a venturi, but it only does it because it can squeeze out the other side IMMEDIATELY afterwards.
If the venturi were changed by being made longer, or had a sharper angle to it in order to try to squeeze the air even more for better velocity, the air would say "Nope. Not gonna do it", and the venturi would'nt work well- if it all. Actually, it would only work at a certain air speed or pressure or more.

In a proper venturi, the velocity at the top and bottom are the same. Only the velocity at the apex of the neck is the fastest.

 

where exactly along the carb bore does the vacuum exist?

 

Just below the apex where the air is allowed to expand.
As it expands, it creates a vacuum that draws fuel from the main circuit, but also vacuum that help perpetuate the flow of air through the venturi.

 

YES! thats exactly what i thought. now everything makes sense. thanks sterling and other peeps