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RE: [T3] Dual port manifolds and the balance pipe


I was hoping someone else would clarify this question.  I'll give it my best
shot as I know it and everybody else can correct me...

The cylinder moves down, the intake valve opens and air is drawn into the
cylinder.  The cylinder moves up, the intake valve closes and the
compression/combustion process occurs.  This is the only part of the process
we care about in this discussion.

Okay, this is all happening rather quickly, particularly as the engine
speeds up (rpm increases).  Now, when air is drawn into the intake system
(be it carb or FI) it has momentum because it has mass (think of air as a
VERRRRY thin fluid, like five million weight synthetic oil :).  It wants to
all travel into the cylinder BUT! when the valve closes, all the air that
was drawn into the intake system and has not reached the cylinder is still
wanting to move (we're now getting into the realm of physics, so follow the
bread crumbs and don't get lost).  With the valve closed, the air kind of
'bounces' off the valve and a shock wave (*think* of it as a really dense
and super thin waft of air) travels in the opposite direction that the air
was moving -- up the intake and back out to the air filter.  This shock wave
can disrupt the air/fuel mixture signal for carb and FI intake systems
alike, but it's more disruptive to carbs.

If the engine is spinning really quickly the shock waves can be rather
strong, in fact, strong enough to push some of the fuel mixture *out* of the
carb and into the engine compartment; this is known as "fuel standoff" and
it's what dirties the bottom of the carb tops.  At low rpm it can disturb
the signal in the mixture and cause a stumbling idle.

What the balance pipe does is give the shock wave more area to move.  With
more area to move it's strength suddenly becomes weaker and its effect
lessened.  The energy of the wave goes through the balance pipe as well as
the intake instead of just the intake.  With less annoying disruptions in
the mixture signal the intake system can meter the correct amount of fuel
and operate like it 'ideally' should.

Naturally, the intake system would like to have a pipe for each valve.  On
single port intakes it's obvious that only one balance pipe be joined to
opposing intake manifolds.  If you have dual ports then you will need two
balance pipes, ideally, however you could join the intakes with a Y or T
pipe, merged into a single balance pipe.

Hey, what about the Type I with it's single carb or the old single-port side
draft Type III?  Well, if the carb is dumping the mixture in a plenum then
that plenum helps to reduce the intensity of the shock wave.  The side draft
T3 carb joins the intake at a 90-degree angle so the shock wave just
continues down the opposite intake.  Some energy will hit the carb but it
will be very weakened (and it's a low hp engine, so why are ya bitchin'? :)

Taking this further, by adjusting the length of the intakes one can increase
torque/hp.  Long intakes improve low-rpm power while short intakes improve
high-rpm power.  This technology is used more and more in the
variable-intake systems of various auto manufacturers (I know Porsche used
it early on).  If you find this interesting then please visit the nearest
library as my feeble brain can't explain it as well as a book written by a
subject-expert ;)

Hmm, all this talk makes me want to whip out my drill and put a pipe on my
intakes to smooth my 40mm Dellortos!  Oh well, later, when I can burn
money...

Questions?  Class dismissed!

   Toby "Mmmm!  Teach you I will!" Erkson
   air_cooled_nut@pobox.com  <-- Please use this address for email
   '72 VW Squareback 1.6L bored and stroked to 2.0L, Berg five-speed
   '95 VW Jetta III GL 2.0L, P-Chipped, Jamex sport suspension
   Portland, Oregon, http://www.icbm.org/

>-----Original Message-----
>This beg's the question:  what the heck do balance pipes do?

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