balance tube
in General Tech Discussion
11 years ago
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.
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.
DSC_0001.jpg (You do not have access to download this file.)
DSC_0002.jpg (You do not have access to download this file.)
