lye_j asks us:
>So I'm appealing to wisdom of the net, especially those of you (Brian,
>Scott?) who undestand the physics of air flow involved to help us out.
>The question really is what does the balance pipe do?
Sorry, I'm neither Brian nor Scott. But that never stopped me. :-)
My books on the subject are at home so I'll have to do this from
memory. I think the problem has to do with the comparative timing
between the pairs of cylinders. With a typical firing order of
1-2-4-3, you get an uneven timing between the first and second
cylinder of the pair on each carb. When #1's intake valve opens,
the airflow in that carb has been relatively still while #4 and #3
were drawing charge. When #2 opens, the airflow is still responding
to the effect of #1. This means the two cylinders will experience
different ram-tuning effects. Since this can also effect the mixture,
the two cylinders might get different mixtures. Finally, the different
airflow characteristics (speed and turbulence) mean different levels of
atomization of the fuel. The balance tube is one way to correct this.
It acts as a sort of airflow buffer to help stabilize the rate through
each carb. It is perhaps misnamed because it connects the main tubes
from each carb, i.e. 1 & 2 to 3 & 4, while the imbalance itself is
between 1 and 2 and between 3 and 4, not between the two carbs. If
you want more detail, I can try to dig this up tonight.
>Are the balance pipes there because of the (primarily British) use
>of constant velocity carburators?
No. It's more the result of having 4 cylinders paired onto 2 carbs:
1 2 3 4
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( ) ( )
If you paired them between 1 & 4 and 2 & 3, this would not be a problem,
but now you get another problem:
1 2 3 4
\ \ \ \ / / / /
\ \ \ \/ / / /
\ \ | | / /
\ \ ( ) / /
\ \ / /
\ \ / /
\ \/ /
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( )
This way, the intake runners for the 1 & 4 pair have a longer length, and
thus a lower-frequency ram length, than the 2 & 3 pair. If you wants two
carbs with four cylinders, you gots to settle for a compr'mise.
Jim Muller
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