Balance tube

[starrider1946]

What is a balance tube? Don't understand exactly what it is and what it does. Thanks

 

[jaxbobg]

Are you referring to carb or exhaust balance tubes?

Years ago (right after cars replaced horses) us kids used to mess with a balance tube between our dual Holley carbs to make idling and throttle response smoother.

Have also heard of exhaust balance tubes. In fact, if you look at the J.C. Whitney web site you'll see some advertised. I think they're also called "crossover tubes" on dual exhaust systems, used to smooth out the exhaust pulses.

Could be that these were also use to smooth out the performance of VW engines.

 

[DeathBySnuSnu]

I have never run twin single barrels like you have.
So i don't have direct experience with it.
But as I understand it to be.....
The reversion shock wave of the intake pulse tends to kill the flow to the adjacent cyl, because of the firing order of the vw. The dual one barrels tend to idle on just two cyls, one on each side.*
The tube from one intake to the other helps the low speed pulse, improves idle, but is not good for hp performance.

But like I said, no direct experience.*

 

[starrider1946]

motor still not right

I may rig one up and see what happens. I can just drill and tap each manifold and run some vacuum line between them. Just see if I can tell any difference.I am not really concerned about high performance. Still looking for tuning advice on the Empi carbs although I know I need webers.Thanks

I have never run twin single barrels like you have.
So i don't have direct experience with it.
But as I understand it to be.....
The reversion shock wave of the intake pulse tends to kill the flow to the adjacent cyl, because of the firing order of the vw. The dual one barrels tend to idle on just two cyls, one on each side.*
The tube from one intake to the other helps the low speed pulse, improves idle, but is not good for hp performance.

But like I said, no direct experience.*

 

[DeathBySnuSnu]

Still no direct experience.*
Just because they are empi does not make them bad.
Them Chinese copied an existing carb. Did not design this from scratch. The problem comes in with sloppy machine work in its manufacture. Go through them very carefully then they might be good. Sometimes you find a machining error that is not correctable and then they are junk.
I have a buddy with a single center mount empi one barrel. Works just fine, don't get excellent gas mileage, probably a bit fat.

 

[Loner]

A balance tube on an aircooled VW can be explained best by looking at the firing order. VW's 4-stroke motors are generally designed to run smoothly through a complete cycle by phasing the four cylinders to push smoothly with one cylinder firing each 1/2 half rotation (180 degrees). That makes for a nice bump-bump-bump-bump of even pushes on the crankshaft. With that happening, vacuum feeding the cylinders from the carburetor plenum is a continuous steady pull of 4 easy pulses. If we totally isolate the two banks of the cylinders, left and right, we not only get half the vacuum pulses pulling fuel mixture from a plenum, but those two pulses are not in sequence but skip a pulse between vacuum pulls (cylinders 1 and 3 on one side, and 2 and 4 on the other with firing order of 1-4-3-2, or right side, left side, right side, left side). So each carb is subjected to two short pulses that can be exemplified as bump-wait-bump-wait and as DbySnu explained the inequities are magnified by resonant effects - but we won't get into those here. Not the premise that smooth engine operation is made of.

That said, a balance tube attaches the two separate plenums so as to mimic a single plenum and let each carburetor get a pulse with each cylinder firing. Not a perfect setup, but better than the totally isolated plenums. A true balance tube in your case would be a larger tube (maybe 1/2-5/8") joining the two plenums directly - exactly what you did on a smaller basis with the rubber tubing attaching the two carbs.

All that said, I can not offer a clue as to whether your setup would noticeably benefit from a balance tube. It would definitely be smoother, but whether it would be noticeably smoother I dont know.

Sometimes a few minutes of hands-on trial and error by a good mechanic is worth months of written advice and recommendations. It may be time to let an experienced professional spend a little while diagnosing and tuning.

 

[DeathBySnuSnu]

Yeah what Loner said.

Just gonna pick one nit.

More of a right side , left, left, right.
So continuing the cycle you effectively get.
One side or bank as bump bump wait wait.
and the other side as a wait wait bump bump.
That's that wierd pulse wave. And also why dune buggy split headers are not actually headers, just a freer flowing exhaust, won't pulse wave tune. To have a balanced split intake or exhaust you would have to pair 1 and 3 together then pair 2 and 4 together.

But yes very much the effect that he described and is how I understand a balance tube is also.

 

[Loner]

Snu's absolutely right - I got my bumps and waits out of sequence. It is bump-bump-wait-wait-bump-bump-etc., etc. Sometimes equating the workings of an engine to simple dance steps is a challenge for me! Cha-cha-cha!