Jonathan,
There are some simple clues to judging whether a forum poster knows anything
about
engines or not. If you spot the keyword RPMs (with an S) you know straight
away they dont have a clue! You see it all the time on boy-racer forums for
whom stage one tuning involves installing a Mega Base, putting their baseball
caps on back-to-front and describing everyone as a mud flicker ( .I think thats
what theyre saying).
The links you posted are classic examples.
To understand what the balance pipe does, you need to know how the engine works.
A petrol engine is an air pump at anything less that wide open throttle (WOT),
it is working at below atmospheric pressure. The RPM (no s) and hence power,
is regulated by the throttle butterfly which acts to close off the inlet tract
(thats why its called a throttle) for idle, and successively open for full
power (atmospheric pressure). When you fly in the cruise, you typically use
about
75% power meaning the throttle butterfly is partially closed and the manifold
pressure is below atmospheric.
The Rotax engine is a Boxer configuration meaning two cylinders are on the left
and two on the right and the designers decided that the simplest way of feeding
fuel mixture to them was to use two separate induction manifolds and
carburettors.
Whilst the packaging issues were simplified, this introduced the problem of
ensuring
that both systems were perfectly synchronised so that all cylinders produce
equal power. Any difference and the engine will vibrate.
When you balance the carburettors, you measure the (partial) mean pressure in
each
manifold and adjust them to be the same. If the mean pressures are the same
than there will be NO FLOW through the balance pipe, so there is no need for
a 1.5 drain pipe between the manifolds.
So why do they fit a balance pipe? Carburettors have been developed over the
years
to feed the right amount of fuel throughout the changing rev range and changing
atmospheric pressure but small differences in airflow mean that the two
manifolds are almost never exactly matched throughout the flight profile. The
balance pipe allows the small pressure differential to equalise, and smooth out
any vibration that would otherwise result.
The contributor of the second of your referenced forum postings, Billy-Bob P
Sledgehammer
III, is frantically calculating how much extra flow he could get though
his balance pipe, not realising that he should be aiming to minimise the
airflow through it!
My advice would be to leave it exactly as Rotax supplied it the power unit is
surprisingly powerful for its size and very reliable.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=479355#479355
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