Just thought I would throw this out there for the PPC fliers who are limited to low RPMs.

Built and added a 1 1/2"D. balance tube to my 912S and it is remarkable how much difference it made in the smoothness at low RPMs. This is not new technology and has been used on intakes and exhaust systems since the forties to smooth out low end vibration. It works.

I saw it advertised recently by an italian racing equipment firm called FLYGAS but they wanted something like $1200.00 for the package. (cost me about $100.00 to build). It does not increase HP, I don't know yet about fuel consumption. Its advantage is to reduce vibration and improve manifold airflow.

Here's what happens. Firing order on the Rotax, and most other boxer engines, is such that when one cylinder is firing, the other cylinder on the same side is on a compression stroke which means that NO air is flowing into that manifold. This inrush of fuel/air mix which has suddenly stopped has nowhere to go except through the balance tube and into the other manifold which is calling for air. The Rotax balance tube is woefully small for this task, even though it is great for idling. You can see this starting and stopping of airflow in the wildly bouncing of the vacuum gauge needles during carb balancing. Because all of this is happening in fractions of seconds, most of the air in the balance tube never actually goes anywhere, but bounces back and forth in the large tube.

All of my operating parameters, EGTs, oil and cyl. head temps remained the same. The only change i noticed was that my prop was pitched to give me 5500rpm at full throttle and now I reach 5500rpm at 95% throttle. I suspect that this means my fuel consumption at a given RPM is somewhat better but I don't know this yet.

I do know that my idling, throttling up, and throttling down are much smoother.

My setup probably wouldn't fit under a cowling and probably wouldn't be worth the expenditure if you are already cruising at the high RPMs that Rotax recommends, but for my requirements, the results were great.
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  • Re: balance tube

    by » 11 years ago

    Hi Bill,

    Quite interesting. What made you pick 1.5" diameter verses let's say .75"? Just curious.
    Let us know how it works out.

    Where are you located?

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

  • Re: balance tube

    by » 11 years ago


    The diameter was just an arbitrary choice. The same approximate dia. as the manifold inlet. Probably 3/4" or 1" would work just as well and would fit better where space is limited. My application did lengthen the manifold by 2" and the tube height is about 4". The FLYGAS tube looks like it is about 1" i.d. Disadvantage to flygas is the price, which include a complete new manifold. They have a picture on their website.

    I'm in NC, about 20 mi. north of Charlotte.


  • Re: balance tube

    by » 11 years ago

    Update on the new balance tube.

    Got my first flight today since installation. Numbers as follows.

    cruise RPM-4250
    oil temp-185F
    oil pressure-51psi

    Fuel consumption is considerably better. 3.3 gal/hr at 4250RPM before-3.1 gal/hr now. Consumption based on a one hr. flight. Not enough time yet to say that consumption is that much better but time will tell.

    One thing for sure. The engine is unbelievably smooth at all RPMs. Very smooth when accelerating or decelerating throughout the throttle range. Although I still idle at 1800RPM, the engine just purrs at 1200RPM. No coughs or misses at all.

    Starting is same as before. No change in hot or cold start routine.


    Outstanding smoothness. Better fuel consumption (so far).

  • Re: balance tube

    by » 11 years ago

    How do you think it would work on a 914

  • Re: balance tube

    by » 11 years ago


    Can't really say for sure since I have no experience with the 914. I would think though, that the theory would still apply for the following reasons.

    1. the firing order is the same
    2. the manifolds and carbs are the same. I.E. two separate and isolated fuel/air feed systems which need to be balanced.

    The only difference I can see is that the 912 is a normally aspirated system fed by a common airbox and the 914 is a turbocharged system feeding a common airbox. Not being an engineer, I'm sure there may be more to it than that but take a look at boxers such as the Porsche and many V8 engines and you will find a large balance tube because street engines spend a lot of time at low speeds and idling. Even single large carb installations still have the airflow problems in the manifolds and make up for it partially by having large plenums or spacer blocks immediately downstream from the carb as well as balance tubes.

    Racing engines in cars and bikes sometimes do not have balance tubes because they are not concerned with idle and low end performance. We rotax owners are concerned however, due to the gearbox issues that can arise with rough idling, particularly with the slipper clutch.

    Even single cylinder racing engines sometimes use a boost bottle between the carb and the manifold. It serves the same purpose as a balance tube in that it gives the air through the carb somewhere to go momentarily when the valves are shut.

    The more I fly mine, the more I like it. Idle is much improved, as is vibration at cruise. Fuel consumption is improved, although I don't yet know to what degree exactly.

    Take a look at the FLYGAS website and you can take a look at their system (which unfortunately sells for over $1200 us.). Mine cost me about 100 bucks total.


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