Need for a balance tube on a Stromberg head with Weber downdrafts

[Grumblebuns]

Lumbes problem with his downdraft Weber setup got me thinking about intake manifold balance tubes in general on Stromberg heads. Looking at his Weber setup, I don't see any sort of balance tube. Anyone know why a balance tube setup is not required for the Webers and I assume for the Mikuni HSRs assuming the stock secondary intake manifold is not used.

[Kendo]

For some reason, I can't connect to to https://www.v-performance.com/ at the moment. But I think I remember their application of Mikunis to Lotus TwinCams do retain the balance tube.

[Pfreen]

This is stepping into another controversial subject.
But, I will.
A balance tube only helps idle if the carbs are out of adjustment by equalizing the manifold vacuum in 1-2 and 3-4.  It does nothing at low manifold vacuum.  My car originally came with Hitachi Strombergs bolted right to the head.  Now they have rubber isolated float bowls so no fuel frothing.
It ran fine then circa 1986.

When I ported and polished my cylinder head, I drilled through the head vacuum port all the way through and epoxied in a 1/2" hard copper tube.  That is my balance tube.  I did it because I believed the truism, (it needs a balance tube). Since I smoothed everything out in the port, it does not interrupt the flow very much.

That as 1988 and it has been in there ever since and no leaks.  jb weld is great stuff.

My car runs great.  No issues.

Anyway, in my opinion, a badly done balance tube will do more to hurt performance than it will improve idle. 
I have said it before here, I would try it without a balance tube.  Just for reference, twin dcoe webers (4 chokes) don't have balance tubes.  The manifold vacuum between the front and back carbs can be different as can between 1 and 2 and 3 and 4.

[Kendo]

Would the fact that Webers have a throat per cylinder change the logic of whether a balance tube was needed? Aside from why a balance tube, doesn't the secondary manifold also move the carbs further from the cylinders for some benefit in induction? (I am NO carb expert. Just been reading the Europa forums since listserv days.)

[Kendo]

Wondering and googling about balance tubes, I ran across this discussion: https://sideways-technologies.co.uk/forums/index.php?/topic/7665-su-theory-mixture-spread/

"Mixture spread' is a little known feature of SU operation.  It means that a given position towards the top of the needle will give different mixtures according to the butterfly opening, at the same air flow.

For normally aspirated twin carbs the balance pipe will reduce the pulsing effect at the jets and hence reduce mixture spread."

[Pfreen]

I think that variations of mixture due to throttle position could be mostly canceled out via the carbs needle profile.  The piston position is a function of air flow.

[Lumbes]

The Weber setup on my car had no balance pipe.  When discussing the Mikuni setup with Vintageperformance they sent this as their example of what a previous owner did.  No balance pipe. 

[jbcollier]

Balance tubes are very helpful with constant-depression/variable-venturi carbs (Strombergs, SUs, etc).  They help reduce piston/damper flutter.  On fixed venturi carbs (Weber, Dells, etc), they are not needed.

[Pfreen]

At what rpm do you think the frequency of the piston/damper flutter would become insignificant due to he the piston no longer able to follow the transient airflow?
Also, do you think the damper oil would help to mitigate this affect?

[jbcollier]

Good questions!

Can't answer the first as it is beyond my expertise.

On the second, yes, damper oil will help reduce flutter BUT not necessarily in a good way.  Damper oil serves to slow piston reaction to sudden changes.  The damper valve only allows this in one direction, it slows piston rise, not fall.  An increase in air flow (throttle opening) causes the piston to rise, the damper resists this resulting an increase in venturi vacuum allowing mixture enrichment -- basically a pulse of rich mixture just like an accellerator pump.  If you have a pulsing signal at idle/slow speed, this will also cause pulses of rich mixture making it difficult to jet the carb correctly.

This is why variable venturi carbs have large balance tubes, to allow both carbs to experience a steady draw, especially at low rpm.

[jbcollier]

As an aside, using engine oil as damper oil can be a bad idea on two fronts.  First, it will react with the Stromberg diaphragms causing them to swell.  Second, it may be too thick and cause too much enrichment.  In Jolly Old, you are mostly close to sea level and the air is moist and thick.  Engine oil will probably be fine.  At higher altitudes, and in drier climates, the air is considerably thinner, using engine oil can result in too much enrichment leading to plug fouling issues.  Most of my years pulling wrenches has been at higher altitudes and in a VERY dry climate.  We used Dexron 3 ATF in dampers.  Worked very well and has the added bonus of not reacting with Stromberg diaphragms.

YMMV

[Grumblebuns]

Thanks it all makes sense. One fuzzy area that is not discussed is the diameter of the balance tube. I'm pretty sure Lotus when they designed the high balance tube manifolds for the Euro spec Elans with the Stromberg heads calculated or flow bench tested the optimum size balance tube. That probably all went out the window with the emissions mandated secondary butterfly intake body. All of my high balance tube manifolds were made to the diameter I had for my original master design that someone else created. I don't know how they came up with the "optimum" diameter for the balance tube.

[Kendo]

That link I posted above eventually leads to this presentation: https://supertrarged.wordpress.com/2017/07/10/how-does-an-su-carburettor-work/ which describes mixture spread. They say it's affected by angle of throttle opening, less that 30 degrees eliminates mixture spread. Somewhere in that exploration I thought I read that the effect stops above 3000 RPM, but I can't find it now.