Crossover pipe of Rotax 912 Carbys

[Blueadventures]

Has anyone modified their Rotax 912 ULS with a larger size crossover pipe and if so what where you findings?

 

There is a production 1" id manifold (Flygas Engineering) and cross over pipe available, likely $2,000 AUD in price.

 

Seems the main benefit is smoother idle and low rpm's.

 

 

[facthunter]

People can claim anything. I'd want verifiable evidence Before spending that Kind of Money and some way of coping with Expansion/ contraction compared with what it's FIRMLY Mounted to. Nev

[Moneybox]

Looks like a serious overkill. From 8mm to 1"?

If there's an advantage in doing that it might be better value to just retap the existing hole on the manifold and go a little larger. Going from 8mm diameter to 15mm is more than triple the size.

Edited 22 hours ago by Moneybox

[BurnieM]

Can someone post a picture of the existing setup ?

 

[onetrack]

Photo courtesy of John at DOG Aviation ...

 

[Reynard]

There is a string on this topic on Rotalk from 8 years ago.  Here is one skeptical comment from this string :

 

Re: Polishing intake manifolds

by Bill Hertzel » 8 years ago

---

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:

[IBob]

For smooth idling etc, first check the throttle stop and idle jet settings as per the manual. These are static adjustments and about 5mins 'work'.
One of mine was out on a new 912 (can't now recall which one), so worth checking regardless.
After that, synchronise the two throttle actions.
 

[Moneybox]

35 minutes ago, onetrack said:

Photo courtesy of John at DOG Aviation ...

 

I don't have me plane here so I can't check but that thread into the manifold looks to be about 3/8"BSP so with the correct elbow threaded in a larger diameter crossover would be a simple conversion with no modification to the manifold.

[facthunter]

WHY would you need such  a LARGE Balance Pipe? It is normally used where siamesed Inlet Ports exist like on the BMC Motors The Pulsing of the Carburettors is NOT even nor is the Fuel distribution That's fixed by Injection. The Balance pipe is no Magic fix.  Save your Money. Nev

[FlyBoy1960]

On a Rotax 912 (twin Bing carbs), that “balance pipe” is basically a plenum-to-plenum equaliser between the two intake sides. It primarily:

• Equalises manifold pressure between the two sides (helps share load between carb/intake pairs)

• Damps pulsations (intake pulses are strong on a 4-stroke, especially at low RPM)

• Can slightly influence idle/off-idle smoothness and how stable the carbs feel during synchronisation

What changes if you go from ~8 mm ID to 20 mm ID?

Area matters a lot.

• 8 mm ID area ≈ π·4² ≈ 50 mm²

• 20 mm ID area ≈ π·10² ≈ 314 mm²
  So you’d be increasing cross-section by about 6.3×.

That means the balance connection stops being a “small equaliser” and starts behaving much more like a common manifold link between the two sides.

Likely effects you’d notice (if any)

1) Idle and low-RPM smoothness could change (sometimes better, sometimes worse).
A larger tube usually reduces pressure difference between sides more quickly and can dampen pulses more. That can make idle feel smoother if you currently have unevenness between sides.

But… if you go too large, you can also create a situation where:

• one side’s intake pulses strongly influence the other side,

• mixture distribution and response around idle/off-idle can become less “crisp” or occasionally hunt/surge depending on how the rest of the intake is configured.

2) Carb synchronisation behaviour can change.
With a big balance pipe, the two sides are already heavily “averaged together”, so:

• it may become harder to detect a small imbalance using manifold pressure/CarbMate-type methods (because the balance pipe masks differences),

• you might end up with an engine that seems balanced by reading, but still has subtle cylinder-to-cylinder inequality.

3) Midrange and high-RPM power: usually little to no improvement.
At higher RPM and larger throttle openings, the balance pipe generally has minimal benefit because both sides are already flowing heavily and the pressure differences are smaller relative to overall airflow. A larger balance pipe typically won’t give you more power.

4) Risk of unintended side effects goes up.
Going to 20 mm ID is a big jump. Depending on your exact intake layout, you could introduce:

• odd transients (tip-in / tip-out throttle response changes),

• cross-feeding effects if one carb is slightly richer/leaner than the other,

• potentially more noticeable issues if a carb/diaphragm/jetting problem exists (the big pipe can “share” the problem).

The practical/engineering takeaway

• A balance pipe is usually sized to equalise slowly enough to damp differences but not so large that the two sides become one big coupled system.

• Jumping from 8 mm to 20 mm is not a small tuning tweak; it’s a redesign of the coupling strength.

What I’d recommend instead of going straight to 20 mm

If your goal is smoother idle / better balance, you’ll usually get more reliable results by:

• doing a proper carb sync (idle stop + cable sync at ~3500–4000 rpm),

• checking diaphragms, intake sockets, clamps, balance tube condition, and any vacuum ports/caps for leaks,

• confirming float levels and correct Rotax compliance items (many “roughness” complaints are float/diaphragm/leak/sync related rather than balance tube size).

If you really want to experiment, a safer approach is incremental (e.g., 10–12 mm ID) and verify with:

• EGT/CHT consistency (if you have it),

• idle stability,

• plug colour / fuel flow consistency,

• and repeatable run-up checks.

One key caution

Because this is an aircraft engine, changing intake hardware can have certification/maintenance and safety implications depending on your aircraft category (RA-Aus, experimental, certified installation, etc.). Even if it “runs”, it may not be a compliant modification.

[skippydiesel]

This  "modification" has been extensively discussed on Rotax Owners Forum.  Latest revival can be found at https://www.rotax-owner.com/en/general-tech-discussion/6709-polishing-intake-manifolds?start=50

 

My summary of the discussion:

• Supporters rave about perceived smoother running /lower idle speed
• No empirical supporting evidence.
• Most can't see how it can work/make a significant difference to engine performance.
• A few die hard supporters refuse to believe its a con job

Note; that Rotax engines should not idle below about 1400 rpm, as gearbox damage may occur, recommended extended  idle 2000-2500rpm😈

 

[Blueadventures]

53 minutes ago, FlyBoy1960 said:

On a Rotax 912 (twin Bing carbs), that “balance pipe” is basically a plenum-to-plenum equaliser between the two intake sides. It primarily:

• Equalises manifold pressure between the two sides (helps share load between carb/intake pairs)

• Damps pulsations (intake pulses are strong on a 4-stroke, especially at low RPM)

• Can slightly influence idle/off-idle smoothness and how stable the carbs feel during synchronisation

What changes if you go from ~8 mm ID to 20 mm ID?

Area matters a lot.

• 8 mm ID area ≈ π·4² ≈ 50 mm²

• 20 mm ID area ≈ π·10² ≈ 314 mm²
  So you’d be increasing cross-section by about 6.3×.

That means the balance connection stops being a “small equaliser” and starts behaving much more like a common manifold link between the two sides.

Likely effects you’d notice (if any)

1) Idle and low-RPM smoothness could change (sometimes better, sometimes worse).
A larger tube usually reduces pressure difference between sides more quickly and can dampen pulses more. That can make idle feel smoother if you currently have unevenness between sides.

But… if you go too large, you can also create a situation where:

• one side’s intake pulses strongly influence the other side,

• mixture distribution and response around idle/off-idle can become less “crisp” or occasionally hunt/surge depending on how the rest of the intake is configured.

2) Carb synchronisation behaviour can change.
With a big balance pipe, the two sides are already heavily “averaged together”, so:

• it may become harder to detect a small imbalance using manifold pressure/CarbMate-type methods (because the balance pipe masks differences),

• you might end up with an engine that seems balanced by reading, but still has subtle cylinder-to-cylinder inequality.

3) Midrange and high-RPM power: usually little to no improvement.
At higher RPM and larger throttle openings, the balance pipe generally has minimal benefit because both sides are already flowing heavily and the pressure differences are smaller relative to overall airflow. A larger balance pipe typically won’t give you more power.

4) Risk of unintended side effects goes up.
Going to 20 mm ID is a big jump. Depending on your exact intake layout, you could introduce:

• odd transients (tip-in / tip-out throttle response changes),

• cross-feeding effects if one carb is slightly richer/leaner than the other,

• potentially more noticeable issues if a carb/diaphragm/jetting problem exists (the big pipe can “share” the problem).

The practical/engineering takeaway

• A balance pipe is usually sized to equalise slowly enough to damp differences but not so large that the two sides become one big coupled system.

• Jumping from 8 mm to 20 mm is not a small tuning tweak; it’s a redesign of the coupling strength.

What I’d recommend instead of going straight to 20 mm

If your goal is smoother idle / better balance, you’ll usually get more reliable results by:

• doing a proper carb sync (idle stop + cable sync at ~3500–4000 rpm),

• checking diaphragms, intake sockets, clamps, balance tube condition, and any vacuum ports/caps for leaks,

• confirming float levels and correct Rotax compliance items (many “roughness” complaints are float/diaphragm/leak/sync related rather than balance tube size).

If you really want to experiment, a safer approach is incremental (e.g., 10–12 mm ID) and verify with:

• EGT/CHT consistency (if you have it),

• idle stability,

• plug colour / fuel flow consistency,

• and repeatable run-up checks.

One key caution

Because this is an aircraft engine, changing intake hardware can have certification/maintenance and safety implications depending on your aircraft category (RA-Aus, experimental, certified installation, etc.). Even if it “runs”, it may not be a compliant modification.

Regarding 2) Carb synchronisation behaviour can change.  The balance pipe is best disconnected for pneumatic sync.     The benefit as I have been told is the low / idle RPM ranges.  In the past I was planning to fit an increased id pipe taping a larger diameter thread into a manifold set and place nice curved radius elbows (rather than the abrupt 90 degree type on the engine) as that should also smooth air movement.  A few years back someone on this site made a setup whereby he welded threaded flanges to allow a larger diameter balance pipe.  Just chasing any in service findings / results.  I can see it being a benefit one well adjusted  and maintained engines.  

[skippydiesel]

Hi Blueadventure,

 

Can't quite work out, are you are for, fence sitter, against, the Big Tube (BT)t??

 

Say for a moment the BT does what its supporters claim - smoother running and lower rpm. 

 

• Starting with rpm - Rotax advise against extended low rpm operations as this will almost certainly damage the gear box - So no benefit!
• Smoother running - Without data this is just a perceived benefit. Nothing wrong with perception but when you are trying to promote a product/modification, I will always want data ie proof.
• Then you need to consider the engines state of "tune" ie Was the engine that the BT was fitted to,  recently serviced,  carburettors balanced, plugs in good condition , etc

On the last point, if the BT  smooths out a badly running engine, this is a good thing - assuming its not masking an impending problem.

 

I am not an engineer, however my understanding of the Rotax balance tube is that its there to smooth out/make good, any small differences in vacuum between the two induction systems. No significant volume of air flows back & forth in the tube. The proof for this is simple - when doing the pneumatic carby balance, you run the engine with the balance tube disconnected/shut off and the engine runs just fine (smooth) when carby's are balanced.

 

Fitting a BT, will not improve on the action of the Rotax balance tube.

 

I am certain Rotax would have adopted such a simple enchantments if it works as promoted.😈

 

 

[facthunter]

It's too rigid as well and MAY CRACK. Rotax Make them in the Original form. What do They say about it?    There are NOT 2 PLENUMS to connect .There are simply 2 Y branch Manifolds. Controlled roughness is better in Ports than a Mirror Finish (Boundary layer control is better giving better flow) Increasing volume of the GASES between the Throttle valve and the Inlet valve will reduce engine response. The Mixture MIX in the Balance tube is not correct for  accelerating the engine as It is required to Be richer when that's happening. The engine MAY be more likely to Falter and Being a bit lean at that Point, Backfire.  The Sharper the Pulses the faster the Max Velocity and the richer the carb is  ALL other things being equal. Potentially a big NEGATIVE IMHO. Nev

Edited 17 hours ago by facthunter
more content.

[Thruster88]

When the rotax back fires, which turbo says is inevitable, the extra manifold volume full of explosive vapour will not just set fire to the K&N filter(s) it could blow the engine off the aircraft. 🤣

 

On a more serious note, at altitude in very cold air fuel vapour could condense in that large un heated non standard  crossover tube. Not sure if that would be a problem.

 

If it ain't broke don't fix it. Doing nothing is free. Fly or fiddle, that's a personal choice.

Edited 2 hours ago by Thruster88

[turboplanner]

47 minutes ago, Thruster88 said:

When the rotax back fires, which turbo says is inevitable, the extra manifold volume full of explosive vapour will not just set fire to the K&N filter(s) it could blow the engine off the aircraft. 🤣

Anyone can be a comedian, but don't just tell lies to twist the truth. I did not say backfires were inevitable in a Rotax. Any engine can backfire. It's just plain ignorant to paint a picture of a manifold blowing an engine of the aircraft.

 

We're talking about routine issues with engines here.

 

Maybe a crossover pipe helps smooth idling, maybe its some other issue with the combination of equipment and layout.

I worked with 4 carbies a lot and nothing beat careful adjustment there. Some people would use a vacuum balance, but having exactly the same length, optimum jetting, adjusting cables for smoothest idle will give that idle. No matter what you do, the cables will have minor reseating, adjustment in the bends etc, and this maybe is what the balance tube allows for, but the optimum is not to have out of tune settings.

[BurnieM]

Not meaning to be sarcastic (well maybe a little bit) but a single throttle body beats 4 carbies anyday.

 

[IBob]

10 minutes ago, turboplanner said:

Some people would use a vacuum balance, but having exactly the same length, optimum jetting, adjusting cables for smoothest idle will give that idle. No matter what you do, the cables will have minor reseating, adjustment in the bends etc, and this maybe is what the balance tube allows for, but the optimum is not to have out of tune settings.

I recall a comment from a LAME with a lot of Rotax maintenance experience: he said that almost all the carb balancing problems he came across were due to poor installation and/or maintenance of the cables and linkages.
I have only worked on one Rotax (my own) but went to some trouble to set that all up with swept cable bends etc. Some minor cable adjustments were then required in the first 50hrs, thereafter I still checked regularly but no further adjustment was required.
 

[turboplanner]

20 minutes ago, BurnieM said:

Not meaning to be sarcastic (well maybe a little bit) but a single throttle body beats 4 carbies anyday.

 

Yes it does, but some industries start with that, then extract more and more power flowing more carbies. Jetskis are a good example, much the same size hull, but currently selling models over 300 hp.

[facthunter]

Unless you highly restrict the Vacuum gauge, the Needle will be just a Blur. One carburettor per cylinder is Best of all. .  Nev

[Blueadventures]

19 minutes ago, IBob said:

I recall a comment from a LAME with a lot of Rotax maintenance experience: he said that almost all the carb balancing problems he came across were due to poor installation and/or maintenance of the cables and linkages.
I have only worked on one Rotax (my own) but went to some trouble to set that all up with swept cable bends etc. Some minor cable adjustments were then required in the first 50hrs, thereafter I still checked regularly but no further adjustment was required.
 

That’s right, need to check the carby and its fit, then follow the manual for mechanical balance, then ensure throttle travel is same for both; a visual assessment then run up to verify all good.  Two of my experiences are July 2024 asked by neighbour to sort a very rough running Rotax ULS (history was that local lame shop had made adjustments) I adjusted carbs (first recording where settings were) after balance all good. The setting where all over the show, looked in aircraft log no record of the work or who did it. I found out from another source. 
2nd example years before above and on a SavXL did M balance but roughness when rpm increased, a bit of further checking and head scratching found the throttle lever on the carb had been bent for clearance, only a little bit and pretty much impossible to see. Had to fit a straight edge (6” eng rule) to see the slight curve. When straightened all good. All part of learning and building experience.