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 3 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.