On 09/13/2010 06:30 PM, rampil wrote:
> Their cross tube looks to be about an inch +/- in diameter. At about
> a foot long, my back of napkin estimate is that there would be quite
> a pressure drop across that tube at WOT,
Pressure drop? Both halves of the engine carry the same manifold
pressure. If there is no inbalance, the air inside the cross tube is
more or less static, although it will cancel out the intake pulses, so
the air will be thrown left and right but never actually leave the tube.
This is a good thing by itself, because it will soften the intake
pulses. This will even out the mixture between the front and rear cylinder.
Let me explain this further: I guess everyone knows that the front
cylinders of a Rotax get a different mixture than the rear cylinders.
Some people think it has something to do with the distance to the carbs.
Wrong. The reason is that the Rotax fires two cylinders at each side,
and then fires the cylinders on the other side. So at each engine halve,
you will find two intake cycles, and then nothing for a while (during
this time the other cylinders on the opposite engine half have their
intake cycles.
This means that the airflow in one carb has to accelerate at the first
intake cycle, and when that intake valve closes, the other cylinder has
its intake cycle. This cylinder benefits from the air already at speed
through the carb. All this has to do with inertia. For the first
cylinder, apart from the fact that is has the task to accelerate the
mixture after it came to a halt during the pause, the acceleration of
the fuel is lower than the acceleration of the air. So the first
cylinder at each engine halve gets a leaner mixture than the one that is
to fire just after it.
So, the idea of a balance tube is to keep the air up to speed, because
both engine halves have the same problem, but at alternating moments.
Balance tubes are common things in cars, both for the intake as well as
for the exhaust. But usually they have a much larger diameter.
But all this has nothing to do with carb imbalances, this is just an
additional advantage. So, let's go back to the claim that this larger
cross tube makes imbalances less obvious:
> Looks doubtful to reduce carb imbalance
> vibration on that basis,
Never took the small balance tube away (for pneumatic carb balancing)?
Without the tube, imbalances become more obvious. Put the tube in, and
some of the imbalance is gone, because the tube allows some of the
mixture from the higher manifold pressure side to flow to the lower
manifold pressure side. Put even more tube in (i.e. a larger diameter)
and more mixture can flow through it, making the difference in MAP
between both halves even less, and thus lessening the imbalance.
> Before we jump on another bandwagon lets see some actual operational
> data, preferably with an accelerometer quantifying vibration.
Fine, if there is such data. If there isn't, we'll have to revert to
basic physics and engine knowledge.
I might install a larger cross tube some day (not for the cross tube
actually, but for the option to fit fuel injectors), and I will let you
know what I find.
Frans
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