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Greetings,

I know that in a perfect world, we wouldn't need the balance tube at all, but I also know that's not the world our engines live in.  The Rotax engineers knew that of course, so there is a balance tube, albeit a fairly small one.  In an ideal installation, with perfect symmetry for the throttle cables and everything else perfectly matched, I'm sure the size is sufficient. 

In the real world, there seems to be a lot of reports of vibration in various RPM ranges.  I've chased this myself, and no amount of syncing seems to work across the full range of RPM.  I've read a number of reports of owners increasing the size of the balance tube, and in almost every case they report a huge improvement in smoothness with no negative effects.  There are companies that sell upgrades for this, and some sizes are as large as 1.5" diameter.  A more typical size seems to be closer to 1".  

In every conversation, there's always someone who claims the original tube is tuned for best power, or for some other reason, and there's the implied doom that comes from changing something the Rotax engineers designed. 

My question is- If there's a downside to increasing the balance tube size, what is it?  I'm hoping for an answer that isn't just speculation, since I've read plenty of that already. 

Thanks,

Rusty

  • Re: Balance tube size

    by » 6 months ago


    Walt Farar has done that.  See his posts on VAF.  His name on VAF is seagull.

    33578_2_Walt Final Lines IMG_6069.jpg (You do not have access to download this file.)

  • Re: Balance tube size

    by » 6 months ago


    Thanks.  I hadn't seen that thread.  https://vansairforce.net/community/showthread.php?t=198160  I send a PM to ask about the size.  I'm guessing it's about 1".  His results are pretty typical of what I've read. 

    Rusty

     


  • Re: Balance tube size

    by » 6 months ago


    Since I am a nerdy engineer I will throw in my 2 cents to further muddy up the water. It can fairly easily be calculated what the airflow rate is thru the standard, 1/2" tube I believe, at at low pressure (vacuum) differential. Even a nominally accurate carb sync should be able to keep that differential at or below about 1/2 psi; at least I am able to do this with my carb sync gauges thru the whole rpm range. At this differential the potential unbalanced airflow rate thru the standard tube is quite large; much larger than the required unbalance at 1/2 psi even at 3000 rpm.

    I won't argue with those who claim to see definite smoothness benefits with the big 1" xover tubes, but my own personal opinion is that it is not because of the increased airflow capacity of the big tube. I believe it is because of the larger intake volume; kind of like tuning your intake and exhaust manifold for increased power. All engines, whether dual or single carbs, have resonance points at certain rpms where roughness is observed, but much of it can be tuned out. I'll bet the same results could be achieved on the Rotax engines by experimenting with added volume chambers added to the intake manifolds; they would not have to be crossovers at all. Take it FWIW, cheers.


  • Re: Balance tube size

    by » 6 months ago


    If your carbs are synced at each annual or if there was an issue then using the larger tube won’t make any difference and certainly one you could feel. When you want something the happen bad enough then it happens regardless of the reality.


    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell


  • Re: Balance tube size

    by » 6 months ago


    It's pretty hard to believe that all the reports I've ready were just from wishful thinkers.  That RV-12 guy bought another set of manifolds because he wasn't convinced it would help. 

    I do agree that this is likely more about volume, than flow.  I read another comment that said not to think of this as flow from one intake to the other, but rather a column of air that sort of shifts a little back and forth.  If you think of the movement of air through the intakes being disrupted by opening and closing intake valves, you can easily imagine the pressure pulses that must exist.  I'm betting the larger volume balance tube helps smooth those out. 

    Of course my original question remains- If there's a downside to increasing the balance tube size, what is it?

    Cheers,

    Rusty

     


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