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George Ryals wrote:Since there is basically no air flow, there will be no additional pressure drop on the long side. Each side will almost instantly be at the pressure level of the intake tract that it is connected to.

In the 12 mile example? Then that would not be true because a much more substantial volume of air would have to be sucked out of the tube than the minuscule amount sucked out of the short tube.

I do not agree with r90s at all, but you are free to do his simple experiment/test, and then you will know.

Air is not an incompressible medium.

When you have different lengths of sending tube, you are dealing with differing volumes of gas.

A vacuum essentially removes a certain volume of gas from each tube. If the original volumes are different, the percentage of reduction will be different, and the movement of your indicator fluid will be different.

It would not be much of a difference in your case, but you should calibrate anyway.

Cheers,
Jon-Lars
Beverly, WA

r90s wrote:Air is not an incompressible medium.

When you have different lengths of sending tube, you are dealing with differing volumes of gas.

A vacuum essentially removes a certain volume of gas from each tube. If the original volumes are different, the percentage of reduction will be different, and the movement of your indicator fluid will be different.

It would not be much of a difference in your case, but you should calibrate anyway.

I'm certainly no expert but here is my humble opinion anyway. I would agree that "A vacuum essentially removes a certain volume of gas from each tube." I would posit that if one applied the same vacuum to a shorter tube and a longer tube at the same time, the longer tube would have more volume of air removed by that vacuum than the shorter one. But the end result would be the same amount of vacuum in each tube.

Jon, several of us use an el'cheapo homemade balancer like the one here:

http://www.youtube.com/watch?v=nEE508WzNC8

The tube is one piece and the fluid is pulled to one side or the other to show imbalance.

A proper manometer sucks mercury from a resevour via individual tubes. I can clearly see your logic on a real manometer that actually measures vacume, but does the same apply to a linked one where we are just looking at which side is pulling more than the other?

What about the fact that the $4 carb sticks are essentially a differential gauge? My carb sticks have damping valves to keep the columns from bouncing too much...so, I know there is not a constant vacuum that is formed and kept...each carb pulls, then lets go...pull, let go...pull, let go...etc. How does that affect this argu...discussion? When the left side pulls...what is the right side doing? Is it still under full vacuum? Or, does the vacuum need to be re-established on each pulse?

the quinner wrote:What about the fact that the $4 carb sticks are essentially a differential gauge? My carb sticks have damping valves to keep the columns from bouncing too much...so, I know there is not a constant vacuum that is formed and kept...each carb pulls, then lets go...pull, let go...pull, let go...etc. How does that affect this argu...discussion? When the left side pulls...what is the right side doing? Is it still under full vacuum? Or, does the vacuum need to be re-established on each pulse?

In practice, the level of the fluid in the $4 carb balancer does jiggle a bit when in use. I submit that if the tubes were of different lengths, the time it takes for the longer tube to pull down to the same vacuum as the short one would be different, thereby altering the end results.

You don't care how MUCH vacuum they pull, you want them to pull the SAME vacuum.

The $8 deal is a differential manometer, which illustrates the difference in vacuum.

If you don't want the fluid to bounce, you can insert a small orifice in the the line. This acts as a low-pass filter, minimizing the effects of the pulsations associated with the opening and closing of valves. (The faster the engine speed, the more of a continuum this becomes.)

At 1000 RPM, the pulsations occur at about 8 Hz. It isn't difficult to measure ERMS (Eyeball Root Mean Square) to figure out when they're even. If you can get them even with an oil-based fluid, you're doing WAY better than someone using mercury. (1" height difference of of mercury ~15" height difference of oil.) Synch with mercury once, then put on the oil-based differential manometer. you'll be shocked at how far off it appears, and how sensitive the system is when using oil. If you get them close with oil, you've got it nailed.

Boy, did a simple question take some tangents. That's why I enjoy this group.

blitz wrote:
Boy, did a simple question take some tangents. That's why I enjoy this group.

AND we're only on page 2.

blitz wrote:You don't care how MUCH vacuum they pull, you want them to pull the SAME vacuum.

The $8 deal is a differential manometer, which illustrates the difference in vacuum.

If you don't want the fluid to bounce, you can insert a small orifice in the the line. This acts as a low-pass filter, minimizing the effects of the pulsations associated with the opening and closing of valves. (The faster the engine speed, the more of a continuum this becomes.)

At 1000 RPM, the pulsations occur at about 8 Hz. It isn't difficult to measure ERMS (Eyeball Root Mean Square) to figure out when they're even. If you can get them even with an oil-based fluid, you're doing WAY better than someone using mercury. (1" height difference of of mercury ~15" height difference of oil.) Synch with mercury once, then put on the oil-based differential manometer. you'll be shocked at how far off it appears, and how sensitive the system is when using oil. If you get them close with oil, you've got it nailed.

Boy, did a simple question take some tangents. That's why I enjoy this group.

It sure did take some tangents...the question was about tubing length and is it important...which you didn't even mention !. I.e., if the tubes are not the same length, will that affect the vacuum (on one side only)? IF the answer is yes, it does matter...then, if one adjusted the fluid levels to be even, then the carbs would be set INcorrectly.

"You don't care how MUCH vacuum they pull, you want them to pull the SAME vacuum."

Yes, but I think Jon is trying to point out that the two different lengths of tubes will read different levels of vacume, creating an error.

"(1" height difference of of mercury ~15" height difference of oil.) Synch with mercury once, then put on the oil-based differential manometer. you'll be shocked at how far off it appears, and how sensitive the system is when using oil."

Which maybe why different lengthed tubes may not be noticable.