Water Manometer (for balancing throttle bodies)

[jseremba]

syncronizing the TBIs

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

Bill,

I have a 1984 Crossfire with manual Transmission. What port do I use to connect to the manometer.
So what I am understanding is I connect the manometer to the TBI.
With the car at idle, warmed up and the timing set.
Ar ethe IAC's, TPS and Electronic Spark control connected?
I saw this procedure and lost the link.
Thank you
Joe
jseremba@verizon.net

 

[jimbil208]

I have a question about the water manometer. No one said anything about "while connected to both throttle bodies and you have the water balanced at idle,should you be able to slowly increase the throttle to say about 4000rpm the water stay balanced while doing so?

 

[KANE]

Incidentally... I have Bill's TBs on my Vette!

Bill- this was a great write up on how to do this. I frequently use a link to this page as a reference for how to balance TBs.