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Polishing intake manifolds

  • Eugene
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5 months 4 weeks ago - 5 months 4 weeks ago #19490 by Eugene
Polishing intake manifolds was created by Eugene
I removed intake manifolds from ULS engine. Planning to replace old balancing tube with 1" line.

When I looked inside of them, found pretty rough surface. Normal for aluminum castings.

In my early motorcycles days, we used to polished intake on 2-stroke engines with pretty good results.

Will it be beneficial to do it on 912 engine?Do we know for sure that ports on intake manifolds lining up perfectly with head ports on Rotax 912? Did anybody ever checked that?




Anybody did this before?

Thank you
Last edit: 5 months 4 weeks ago by Eugene.
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5 months 3 weeks ago #19533 by Roger Lee
Replied by Roger Lee on topic Polishing intake manifolds
This has worked on autos in the past. The Rotax intake manifold is so short and doesn’t flow the same quality of air as an auto I’m not sure it would help, but it shouldn’t hurt. You have to do air flow test before and after to know the impact. That means special test equipment and a bench flow setup.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
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5 months 3 weeks ago #19536 by Eugene
Replied by Eugene on topic Polishing intake manifolds
Installed my new 1" balancing tube today and went for test flight.

Definitely very big improvement on vibration at idle, about 1700 rpm. Almost like sitting in a car.

In flight at 5000-5500 RPM everything is normal. Same power. Same speed.

This should of been done at the factory. I will stop and talk to them at OSH, want to know why they didn't do it.
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5 months 3 weeks ago #19537 by Eugene

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  • Bill Hertzel
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5 months 3 weeks ago #19538 by Bill Hertzel
Replied by Bill Hertzel on topic Polishing intake manifolds
I am confused. ???
A week ago you were asking about the possibility of Modifying your crossover pipe.
Yesterday you post pictures of the manifolds before and after welding.
This afternoon you report a test flight was successful.
6 hours later, the Parts are available for mass marketing on an Italian website.

Either this is the fastest engineered project I have ever heard of, or I am missing something. :unsure:

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5 months 2 weeks ago #19539 by Eugene
Replied by Eugene on topic Polishing intake manifolds
Sorry for confusion.

Manifold kit from FLYGAS is out there for sometime now for about $1200.

I did about same thing for $200 with very good results.

After removing intake manifolds I noticed that they are pretty rough on inside. That was original post with question = if there is benefit of polishing them and port match.

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5 months 2 weeks ago #19540 by Pacheco
Replied by Pacheco on topic Polishing intake manifolds
So am I.

But it is an interesting product.

This topic rises the discussion of the crossover pipe again.

Is there anyone that can explain Why this is not standard if it seems to be the solution for the low rpm vibration?

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5 months 2 weeks ago #19541 by Roger Lee
Replied by Roger Lee on topic Polishing intake manifolds
If the engine is setup correctly, engine mounts aren’t old and carbs synced and the ideal rpm is not set excessively low it shouldn’t vibrate.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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5 months 2 weeks ago - 5 months 2 weeks ago #19543 by Eugene
Replied by Eugene on topic Polishing intake manifolds
Here is result of my homework:

- in 4-stroke engine with 4 cylinders only way to have continuous flow = is 4 - to - 1 situation with 1 carburetor feeding 4 ways. Because in any position there will be always one piston is in suction mode.

In Rotax 912 with 2 carbs we have flow inside of this 2 carbs in constantly "stop and go" mode. Creating giant tsunami back and forth waves inside of intake manifolds. Effect of this "stop and go" we feel only at low RPM as vibration. Makes it worse with unbalanced carburetors.

Large diameter balancing tube is creating one large common supply plenum effect = were is more room for this waves to go. As a result at low rpm 1600-1700 feels almost like sitting in small car.
Last edit: 5 months 2 weeks ago by Eugene.

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5 months 2 weeks ago #19544 by Pacheco
Replied by Pacheco on topic Polishing intake manifolds
Thanks Roger

It seems a perfect solution!!
Is there any issue that we are not seeing that explains why it is not standard????

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5 months 2 weeks ago #19546 by kenryan
Replied by kenryan on topic Polishing intake manifolds
I have no opinion as to whether or not the larger diameter crossover tube represents an improvement. I can, however, think of at least one reason Rotax might have opted for a smaller one. Perhaps Rotax feels that the smaller diameter crossover tube is preferable because a larger one masks effects of poorly balanced carburetors (or other problems). Maybe they *want* the engine to run poorly if the carbs are not properly balanced, because, perhaps, unbalanced carbs would cause other, more serious problems. Just a thought. (It also seems obvious that Rotax would have tested crossover tubes of differing diameters.)

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5 months 2 weeks ago #19547 by Pacheco
Replied by Pacheco on topic Polishing intake manifolds
Thanks kenryan,

This is a discussion that is in this forum for a long time and until now no conclusive statement has been made.
As you wrote there are a lot of maybes that Rotax could explain to us.
Of course the carbs must be well Synchronized but minor deviations commonly seen in mechanical equipment coud be compensated

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5 months 2 weeks ago #19548 by Eugene
Replied by Eugene on topic Polishing intake manifolds
When I was younger, I used to believe that in charge of large corporations or countries are always very, very smart people. People, who simply too smart to make simple mistakes.

Now, that I am getting older, and have seen plenty of mistakes in brilliant German or American engineering, or political decisions of our leaders = I don't think that way anymore!

I will stop and talk to them at Oshkosh, maybe they will explain pros and cons of installing large size balancing pipe. Just like everybody else, I like to think that in the process of developing this engine, they did tests with different tube sizes.

But on other hand, looks like at least one company in Italy did enough research to start making and selling kits to fix this problem.

P.S. another option is, to stop talking and wondering about it, and just install it by yourself. Just like I did. And feel results every time you start your engine.

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5 months 2 weeks ago #19550 by Bill Hertzel
Replied by Bill Hertzel on topic Polishing intake manifolds

Eugene wrote: ... I will stop and talk to them at Oshkosh, maybe they will explain pros and cons of installing large size balancing pipe ...


The problem with talking to them at Oshkosh is that "Rotax" is not there!
The Rotax Tent, and a large and fancy one at that, is supplied and funded by BRP-Rotax but manned by representatives from the various distributors in the US.

You do not get to talk to the Engineers, Designers, or Anyone from Austria.
You get to talk to the Marketing Reps from California, Florida, and Wisconsin.
No one is going to have a heavy Austrian accent.
If you are lucky you might get to talk to one of the repair techs.
Mostly you are going to get the company sales pitch and no one is going to say anything that does not Toe-the-Line with BRP-Rotax Company policy.

If you try to suggest that you can offer them an improvement to their products, they will smile, nod their heads, quietly listen, and politely thank you for your suggestions.
Been there, Done that!

But, by all means, visit the Rotax Tent.
See all the engine displays and new developments and enjoy the show.
Pick up copies of their latest glossy brochures.
But don't expect that they are going to want to contact you after the show to discuss your Mods.
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5 months 2 weeks ago - 5 months 2 weeks ago #19552 by garrett
Replied by garrett on topic Polishing intake manifolds
Well put, Bill. All big business is run by the CFO/accountants.
Last edit: 5 months 2 weeks ago by garrett.

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5 months 2 weeks ago #19553 by Eugene
Replied by Eugene on topic Polishing intake manifolds
Very good summary about how we all feel!!!

We should print this letter and send it to them.

I was living in Germany before I came to this country. So, I will be looking for guys with German accent under Rotax tent. Maybe I get lucky and get to talk to real engineer this time.

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5 months 2 weeks ago #19554 by Roger Lee
Replied by Roger Lee on topic Polishing intake manifolds
Simple answer. It cost millions.it isn’t just a simple add on.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
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5 months 2 weeks ago - 5 months 2 weeks ago #19555 by Eugene
Replied by Eugene on topic Polishing intake manifolds
Something else that I think very interesting.
I did talk to guy in Norway, who makes injection conversion kits for 912 engines.
Their best performing arrangement is what they call 4-in-1. Where one throttle body is feeding 4 cylinders with a specially designed manifold with equal length to every cylinder.($6000)

They also do make kit with 2 throttle bodys to go as Direct carburetor replacement. When I asked him about that system. He said that this type of arrangement doesn't work very good at all. And it's only used in Situation where space is very limited under engine cowling.

Now that we are talking about this, I am making conclusion that manifold 4 into 1 where one unit is feeding 4 cylinders apparently working better for carburetion or injection as well.


webshop.edgeperformance.no/index.php?id_...0&controller=product
Last edit: 5 months 2 weeks ago by Eugene.

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5 months 2 weeks ago - 5 months 2 weeks ago #19556 by Bill Hertzel
Replied by Bill Hertzel on topic Polishing intake manifolds
A relatively easy conversion would be to mount the 912is airbox on a ULS and mount a single carburetor in place of the throttle body.

Unlike the optional USL airbox that is Before the Carbs, the 912is airbox is After the throttle body.

The 912is Airbox has way more design built into it than meets the eye.
The intake manifolds do not stop where they attach to the Airbox.
There is another foot of intake pipe internal to the airbox that extends to the opposite side of the airbox.
The Right cylinders actually get their air from the left side of the airbox, and the Left cylinders get air from the right side.
It is not just a chamber with fittings attached where it was convenient.
It has the look and feel of a tuned intake manifold.
This simple looking airbox was not designed in a week!
Last edit: 5 months 2 weeks ago by Bill Hertzel.
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