Water Manometer (for balancing throttle bodies)

**geekinavette** · 07-24-09, 06:22 PM

This seemed like an appropriate place to post this...

Ok first of all what is a water manometer and what do we use it for:

Simply, to measure very small amounts of vacuum. A water manometer measures the same thing as a typical vacuum gauge, but in inches (or whatever unit) of water instead of inches of mercury. Because water is much much less dense than mercury, it measures a much much smaller amount of vacuum. Where this comes in handy on the Crossfire setup is measuring the synchronization of the throttle bodies...in other words how much air is being pulled through each throttle plate at idle. Having one throttle plate open even a tiny amount more than the other is very detrimental to overall system performance. Usually this manifests itself with a "hesitation" when taking off from a stop.

A water manometer is one of the easiest tools in the world to build. All it takes is a piece of wood, some clear plastic tubing, some clamps to attach the tubing to the wood and some water (with food coloring helps a lot).

Here is a picture of one I use around the garage:

Just form the clear plastic tubing into a "U" shape, and mark off the wood with horizontal lines at 1" increments. (ignore the blue stuff about halfway up the tube on the left side....that's just paint overspray)

Then, you'll need to get a small amount of colored water into the tubing such that it sits uniformly at the bottom of the "U" and rests at one of the horizontal lines....like so:

(no that's not antifreeze....it is just green food coloring!)

It can be a little tricky to get the water to settle in the bottom of the tube...usually what works well is to submerge one end into a container with the water and apply a small vacuum...kinda like siphoning. It doesn't take much to get the water to move!

When a vacuum is applied to one side of the manometer, the water will move in that direction. A couple of examples....

With 1" of water of vacuum applied:

With 6" of water of vacuum applied:

Now what to do with this....

On the back side of the Rochester 400 model throttle bodies used on the Crossfire engine, there is one "balance" port that has a built-in restriction such that it has only a very small amount of vacuum present. There are two ways to use this port to balance the throttle bodies....
1: make two manometers and simply watch the difference between the two TBs while the engine is idling. With this method one end of the "U" tube is connected to a balance port, the other end is just left open.
2: use one manometer and connect each end of the "U" tube to a throttle body vacuum port.

With method 1 there will always be some amount of vacuum present, and the linkage balance screw is adjusted until they are pulling equal vacuum (somewhere around 6" as I recall).

Method two is a "differential" measurement...the balance screw is adjusted until the vacuum reading is zero (the water is centered in the bottom of the "U"). When the vacuum reading is zero then that means both throttle bodies are pulling the exact same amount of vacuum.

If the throttle bodies have been bored out, then the restriction at the balance port is gone and full vacuum will be present at the balance ports. In this case the differential method is the only option (method 2).

If the balance is off so bad such that one side is pulling more vacuum than the manometer is capable of measuring, then the water will get sucked into the engine. This is why you'll notice in those pictures that the tubing is rather long, and there is very little water in the tube. Don't worry...this is nowhere near enough water to hurt anything. If it gets sucked in...no problem just tweak the balance screw, put some more water in the manometer and start over.

What I have found (several times) is that if the throttle plates' positions at idle are adjusted using a simple piece of notebook paper as a feeler gauge between the throttle plate and the inside diameter of the throttle bore, then they are close enough to use the "differential" method just fine. Of course this "coarse" adjustment must be done with the engine NOT running!

Another nice thing about doing this measurement is that the effect of worn out throttle shaft bores can be directly seen. Give the throttle a little "bump" while idling, if the shaft bores are severely worn then you will see a significant temporary vacuum "imbalance" between the two throttle bodies. It is best to do this with the accelerator pedal as it puts the appropriate force on the throttle plates. If you do it by hand at the TB itself, you may not apply force to the throttle shafts in exactly the same manner as the pedal does, and as a result may not get an accurate indication of how badly the bores are worn.

Bill

**twiget** · 07-24-09, 07:00 PM

Pretty spiffy Bill. I had to use a similar set-up at work not to long ago.

Jason

**geekinavette** · 07-24-09, 07:09 PM

Jason

Yeah it's amazing the difference a couple 'o bucks worth of stuff can make!

**nelson84** · 07-28-09, 12:15 AM

Wow, finally someone explained it clearly with the water in the tube thing.

**elkabong** · 08-06-09, 08:36 PM

Here is my version. Same concept as masta geeks.

$1.55 Carb Sync Tool by Marty Ignazito

**nelson84** · 08-07-09, 03:19 PM

Originally Posted by elkabong

Here is my version. Same concept as masta geeks.

$1.55 Carb Sync Tool by Marty Ignazito

Hows that renegade coming?

**Scottsredvette** · 08-08-09, 07:32 AM

But Bill, how do you keep the harmonics from damaging the phase shifter in the crystal matrix balancer??????

Scott

**geekinavette** · 08-09-09, 09:20 PM

Originally Posted by Scottsredvette

But Bill, how do you keep the harmonics from damaging the phase shifter in the crystal matrix balancer??????

Scott

By using green food coloring instead of red or blue of course...DUH!

**Buccaneer** · 08-10-09, 08:56 AM

Originally Posted by nelson84

Hows that renegade coming?

The Reneage is progressing well, but a tad behind schedule. Here is a link to see the progress, its not entirely up to date and the manifold is in plastic now and getting close to casting. http://www.crossfireinjection.net/renup.html

**Jamesvette** · 12-12-09, 10:45 AM

I have the Hayes manual for the C4s. It mentions the use of an "idle air valve plug set" pn: J-33047. You however do not indicate there use in your instructions. Do I need these plugs, and if do you know where I can find them?

Thanks
James

**geekinavette** · 01-26-10, 10:05 PM

Originally Posted by Jamesvette

I have the Hayes manual for the C4s. It mentions the use of an "idle air valve plug set" pn: J-33047. You however do not indicate there use in your instructions. Do I need these plugs, and if do you know where I can find them?

Thanks
James

Dang totally missed this question...really sorry about that!

Anyway...on the top of the base of the TB you will see a sort of "kidney" shaped hole. This is the idle air port. When balancing the TBs yes this port should be blocked. This "tool" is nothing more than a rubber plug that is the shape of this port. You can easily make one by just trimming a large enough chunk 'o rubber (or other material) such that it prevents any air from getting through this port. Just make sure that the plug itself is large enough so as to not get sucked in.

**Buccaneer** · 01-26-10, 11:42 PM

FYI: You can buy the part that is made for this from Thexton, PN 368 called a GM Idle Air Passage Plug for TBs or do what has been mentioned. You'll need two.

**Jamesvette** · 02-21-10, 06:53 AM

Thanks for the info. I ordered and recieved them this week. The best 2.00 tools I ever purchased.

Thamks again,
James

**Indyvette84** · 04-03-10, 12:13 PM

Here's a silly question:
If I use method 1 and connect one end to the TB and leave the other end open, it quickly sucks my water into the TB. Does that mean I have way too much vacuum to start with?

**geekinavette** · 04-03-10, 01:48 PM

Originally Posted by Indyvette84

Here's a silly question:
If I use method 1 and connect one end to the TB and leave the other end open, it quickly sucks my water into the TB. Does that mean I have way too much vacuum to start with?

Have the TB's been overbored? If so then the restricted balance port is no longer restricted and is pulling full engine vacuum...which is much more than a water manometer can measure.