Topic: Homemade manometer / carb sync questions

[narcoticrex]

Hello, I am planning on building a homemade manometer, using the airtubes and splitters, peg board, ATF, and screw valves for pressure equalization.  It will look like the one in this thread:

http://forums.sohc4.net/index.php?topic=23038.0

My question regarding this: shouldnt the length of the tubes be taken into consideration? I mean if you look at picture, 1 and 4 have longer tubes.  Although crossing of 2 and 3 helps compensate, it doesnt fix it...

For equal pressure, wouldnt you need to have markings on the tubing, perhaps every half inch?  These markings would show the difference in length, and therefore vacuum, of each tube.  Is this making sense to anyone else?  In that pic, 1 and 4 appear to be sucking more vacuum, since where the levels are nearly equal, 1 and 4 expend more tube volume to get there. 

Furthermore, can someone clarify adapter size on two models for me: 77 550k and 76 750e
Is it 5mm x 0.8mm thread for both?

[narcoticrex]

Also, for most accurate equalization, wouldnt correct setup be

0 T - T - T - T 0
   |    |    |   |   
   |    |    |   |



where 0 are stoppers, T are T splitters, and - and | are air tubes.  Wouldnt this be better, and the calibration be dead on that the homemade manometer above?

[mark]

The homemade manometers that I have seen (or seen recipes for) have separate fluid supplies for the separate tubes. Has anyone built one tied together as pictured and actually had it work? Not much of a follow-up from the original poster other than to say that he got his carbs " close to sync'ed ".

just curious.

[narcoticrex]

From another board:

http://www.vfrdiscussion.com/forum/index.php?s=e52dc1fc3b7ec05ed82a8f21f1820e17&showtopic=16200&st=0&p=433171&#entry433171

on the other board they mention that gravity equalizes the fluid in all the tubes, so length of the tube (at least in the fashion of my first post) is irrelevant.

[mystic_1]

Here's the problem I see with that setup - as each carb draws vacuum on it's column of fluid, the fluid level will rise.  Additional fluid must then flow into the bottom of the tube, since nature abhors a vacuum.  In the pictured arrangement, the only place for more fluid to come FROM is the other columns, therefore each carb will be affecting the readings for the other three carbs.

What you need is a reservoir at the bottom containing extra fluid.  This reservoir needs a vent so that as fluid is drawn up into the tubes a vacuum is not formed in the reservoir.

mystic_1

[merc2dogs]

connecting the uprights of the T together with a longer loop to keep the crossbars horizontal, then coming off them for the actual tubes while crossing the inner pair of lines would provide equal length tubes, and a larger reservoir for fluid in the loop.
 
 Not sure how it would work with a closed circuit, there's no place to set up any kind of pressure differential to move the fluid, every one I've seen not based on a bourdon tube, (fluid or metal/plastic rods/balls etc) are vented to atmosphere somewhere along the way.

Ken

[mark]

From another board:

http://www.vfrdiscussion.com/forum/index.php?s=e52dc1fc3b7ec05ed82a8f21f1820e17&showtopic=16200&st=0&p=433171&#entry433171

on the other board they mention that gravity equalizes the fluid in all the tubes, so length of the tube (at least in the fashion of my first post) is irrelevant.

aha... I think... There is a reasonable-sounding expanation here.... for those who make it down to post #29....


"This is a differential manometer meaning that it responds to the difference in vacuum between the 4 cylinders. .... Mercury manometers are absolute manometers, meaning that they respond individually to each cylinder's absolute vacuum....".

... makes a little more sense now.

[mystic_1]

Ha ha, well in that case the problem I observed above is not a problem, it's the main operating principle of the thing! lol

Yes, makes much more sense now.

mystic_1

[narcoticrex]

Here's the problem I see with that setup - as each carb draws vacuum on it's column of fluid, the fluid level will rise.  Additional fluid must then flow into the bottom of the tube, since nature abhors a vacuum. 

If it were set up like this, the fluid would never stop drawing, because the vent would act as an air leak, i think.

In the pictured arrangement, the only place for more fluid to come FROM is the other columns, therefore each carb will be affecting the readings for the other three carbs.

Yes, when they are pulling equally, the fluid will be at the same level (or within 1 inch, which I read is a small difference because ATF is so much lighter than mercury)...it is described as a tug o war between the carbs, and they are synced when no one is winning.


Now that I answered my own question, can anyone answer my second?

[mark]

Hondas use the 5mm adapters.

[Tower]

I don't think that setup would properly equalize. For example, to get equalized between 1 and 4 the vacuum of 2 and 3 would come first.   It would need a "loop" to work properly.  i.e. Like this:
 __________
 |  |  |  |  |  |
 |  L  L  L L  |
 |  |  |  |  |  |
Each tube from the carb, hooking into a "T" in the middle of the loop.

This is the way a plumber would set up a multi-head shower, so that each head has the exact same water pressure regardless of where it is in the loop.  The same principle would be needed for a differential vacuum gauge.

[Buber]

Let me ask you a question... - Which one of you actually HAD made such tool, and did the measurement? Because you pretty much sound like good theoretics, but what about some practice?

Well then, I did. To tell the truth, yesterday i synced my carbs with this:

And it works, but it would work better if - I would cross the tubes, as in 1st picture, and if i would add a loop between the outside tubes.

And all else (lenghts, etc) doen'st really matter. This tool is SUPER sensitive, much more than dial ones or mercury one - I know because I tried.

So, my opinion - don't complicate a simple thing. Just plug it it, and go for it. real bugger is to tighten the screws without changing the setup. Especially on 3rd carb, as the access is very poor.

Good luck!

[Tower]

Buber, you could just run the four tubes into a sealed "jar" rather than t-linked crossovers and loops.

btw. rather than "theoretic" how about "engineer" 

[Buber]

True, only this way I'm sure that there are no leaks. Actually I'm thinking heavily of making it that way, but tight seal is a must here. If i will get it - I will let you know!

BTW - carbs synced really nice!