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MG MGB Technical - DCOES- Why no balance tubes?
|No real problem here, only an observation: Why do the Weber DCOE intake set-ups never have a balance tub connecting the two runners?|
My instincts tell me it has something to do with ultimate top end power versus mid range and drivability concerns. Any knowledge out there?
|My local miachinist has commented on the same thing. Normally, he builds very high end 1000+hp engines, but also works on British stuff. Whenever he tunes an engine with a DCOE (or three)he adds a balance tube. Apparently it ALWAYS gains power on the dyno, mainly at low rpms, with little or no effect on the top end.|
|My own backyard theory as to why a balance tube|
is not incorporated into many Weber manifolds for
MGB is because many of these manifolds are
designed to merely mate the Weber carb to the
MGB head. Very few of these manifolds are
subsequently tested on a flow bench, engine dyno,
nor track-tested and tuned on a real running car.
The old yarn that balance tubes are not needed at
the high rpm bands where most Webers will be
applied may be true for other cars - but at speed,
the siamesed intake ports on MGB's become a
howling-mad storm of gas eddies, valve reversion
pulses, mini-vortexes, and cross-port robbing, etc.,
and can use all the help it can get in the way of
smoothing the cacaphony of events that occur
inside the manifold and intake ports when hurtling
down the boulevard at warp speeds.
Get a hold of the factory Special Tuning Manual
and take a very, VERY close a look at the
illustration for Weber Carb Kit C-AJJ3312 (fig 3).
You will see that there is, indeed, a small balance
tube incorporated into the manifold design. This
balance tube is located just inside of the mouth of
each runner. It is internal to the manifold's design
and is not obvious, externally. As such, it is
illustrated in the drawing using thin, dotted (and
easily missed), lines to denote it as an internal port or passage.
The factory works cars were built for all-out battle
at such hallowed places as LeMans and Sebring.
If the fine folks at the Special Tuning Department
saw that it was helpful to incorporate a balance
tube into the C-AEH772 Weber Special Tuning
manifold design…well then there's your evidence
that having a balance tube was found to be helpful
at high speeds with relatively lopey cam lobe
profiles and aggressive driving styles.
Many manifold designers that use the illustration in
the Special Tuning Manual from which to base
their own design often overlook the fact that there
is a balance tube there and so you do not see it in
many aftermarket manifolds.
Having said this, there are manifolds out there that
either have balance tubes, or are easily modified
to have a balance tube fitted to them.
The early (mid-1970's) Cannon manifolds had a
small balance tube similar to the Special Tuning
manifold. Problems with the wood mold used to
produce these manifolds resulted in eliminating
this feature in some later production runs,
however. Aside from this, I'm not a big fan of the
straight runners used on the Cannon design
…but I digress.
I have also seen MGB Weber manifolds fabricated
from steel tubing that also have a balance tube on
them. I cannot remember the name of the
manufacturer at this time, however.
I've found that the best all-around Weber manifold
for the MGB is the one made by Warneford Design
(Australia). I do not know whether if it is still in
production but I've stumbled into them at flea
markets and on eBay. It's got smooth, gently
curved runners. The carb mount flange and all of
the mounting points are nice and hefty (stronger).
The aluminum used is tightly grained. The throttle
linkage setup is neatly designed, works smoothly,
and is easily adjustable. After installation, there
is just enough room to squeeze in an aircleaner.
Better still - on top of each runner is a threaded
boss that makes it rather easy to fabricate a
balance tube between the runners using common
hardware plumbing compression elbow fittings
and a short piece of tubing. No major surgery or
fancy welding needed.
|Webers are Italian, where they don't make 5 port heads.|
|the steel tubular inlet manifolds is manufactured by Maniflow|
Thank you once again for a most informative response. I still have your advice regarding DCOE tunning, posted on my garage wall. (Problem turned out to be a vacum leak from the odd flexi-gasted used on these carbs)
I gather that a couple of hours with the TIG welder and a bit of hose should improve things I want to add a balance tube.
My thanks for an excellent reply.
So far as I can find the Warneford Design manifold has been out of production for some years.
Cheers , Pete
|Certainly the balance tube in fig 3 is there for the PCV valve that's sitting on top of the manifold. It may be there for both purposes - though it looks quite a bit smaller than the SU balance tube. I do wonder why they'd bother with the PCV on a stage 5/6 race engine? In any case, the PCV valve is a better choice for street engine than dumping the fumes into the air filter.|
I know a balance tube helps smooth out the idle (tried it) and I could see how it helps with low speed running - the reversion pulses from the valve closing can be pulled into the other throat rather than out of the carb to soak the air filter.
At high speeds (speculation now) I wonder if the reversion pulses will travel too slow for a balance tube to make a difference, the valves opening up again before they get very far. A vacuum guage on a single runner bounces around at idle, but smooths out as soon as the throttle is opened. I imagine that if a balance tube is added it should be done to minimize turbulence at speed.
|Peter Burgess notes that in theory eliminating the balance tube to reduce turbulence seems like it ought to be good for the high end, and in fact it flows better on the flow bench, but in practice it loses 3 horsepower at the high end and runs like cr@p at the low end.|
Lose no sleep over the turbulence. On a 5-port head you've got much bigger things to worry about, and balance is one of them.
This thread was discussed between 23/04/2005 and 25/04/2005
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