All Activity

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.

Maybe find someone who works with Fibreglass and Have Him repair it. ? Just remove the deteriorated material and replace it with glass cloth and resin. . Nev

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

Carry on in Blissful Ignorance. Less than Normal power May easily have been a factor, Drift is NOT always BAD. Don't kill good discussions because a few feathers get ruffled. Any abnormal Running or vibration should be Investigated even IF it appears to Have come good. It's AN aeroplane. Nev

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.

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.

The discussion of engine and carb heat design should have been in a separate topic in the Engines and Props forum. It is not directly related to the Parafield accident.

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:

Photo courtesy of John at DOG Aviation ...

...o, .....

Can someone post a picture of the existing setup ?

......"You're in the wrong saloon and the wrong movie, pardner! You need to be in Alice Springs, and featuring in Priscilla, Queen of the Desert!" Turbine Jones was horrified to receive this response, and immediately fired back, "I can't believe I'm receiving treatment from a saloon in this manner! I know my rights, and I'm going to sue you (utilising Turbine Legal, of course) for refusing to serve me, based on my appearance, my gender, and the product I asked for! If it's good enough for President Trump to sue every time he gets offended, then it's good enough for me to.............

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.

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

Too true! And somehow, I'm put in mind of a conversation overheard at the Port Moresby Aero Club bar, back in the day. Along the lines of: "Oh, aye, we used to fly a tiny, tumble down old Piaggio with great holes in't roof." "Piaggio!? You were lucky to have an aeroplane! We used to haul 26 pax every flight in crates made of coconut thatch and garden furniture. Half the floor was missing and we'd all huddle in one corner for fear of falling." "Crates!? You were lucky to have crates!"

"Tomorrow" turned out to be 8 days later (yesterday). Did an hour. Bit warm under the bubble ( 20C @ 5500') AvTraffic a non starter, due to this computer luddite not realising that a whole heap of "stuff" needed to be selected ON (hope I have now done it correctly). Will do a review after the next flight. For now - Noticed a change in my intercom; Volume needs to be turned to Max, then turned down on headset. I usually do a "click" check, short press of transmit button, on my intercom - doesn't work any more - cant hear the click. Cant explain it, but headset/intercom seems dead - to the point where I did an Any Station Radio Quality Check - came back 5/5. This is the Six Aviation Bluetooth Adapter - not the AvTraffic Should not be related, iPad / WiFi connection with my SE2 dropped out ?? Also lost OzRunways traffic (blue bubble) alerts, for a brief period?? The third event of this nature. I also noticed, after trying, unsuccessfully , to "fix" the AvTraffic issues, in the air, my OzRunways was very slow to come back on ??😈

Yep but one thing over here is heading down the strip with maybe 2000kg just loaded onboard and then doing that over 100 times a day…😁

Ha, ha ... but then, the PNG mob says the same about the NZ mob. Anyway, the bent-wing hill-tribes should support each other. What's a Jodel, after all, but a juvenile Fletcher?

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.

The Piper PA-47 "PiperJet" was a single-engined very light jet (VLJ) that was intended to be developed and built by Piper Aircraft. However, following a change of ownership at Piper, it was decided to redesign the aircraft as the PiperJet Altaire. Despite being technically successful, the Altaire project was canceled in October 2011 due to economic conditions. The aircraft is the first proposed single-engined civilian aircraft with a podded engine located on the tail. The PiperJet was announced in October 2006, as a competitor to the twin-engined Eclipse 500 and Cessna Citation Mustang. The aircraft's fuselage was the same cross section as the propeller-driven Piper PA-46 series, with a 4 feet (1.2 m) increase in length. It was to be capable of carrying up to 7 passengers and cruise at 360 knots (666.7 km/h), at a maximum altitude of 35,000 feet (10,668.0 m). Maximum range was expected to be 1,300 nautical miles (2,407.6 km), with a full-fuel payload of 800 pounds. Piper selected Williams International to supply its FJ44-3AP turbofan engine for the PiperJet. Due to the engine being mounted above the center of gravity, power was highly stabilizing (addition of power would push the nose down), which could have been disconcerting to pilots. Initially, Piper designers incorporated an automatic pitch trim system to coordinate horizontal stabilizer angle of incidence with power setting. This system was later replaced by a vectored thrust nozzle, developed by Williams International, which resulted in reduced weight and simplified manufacturing processes. A design feature of the aircraft was the use of a straight duct air intake design for the vertical stabilizer (tail) mounted engine, similar in engineering design concept to a McDonnell Douglas DC-10, rather than the s-duct arrangement of most trijet aircraft designs such as the Dassault Falcon 900. A selling price of US$2.199 million in 2006 dollars was initially set and as of February 19, 2007, Piper announced that it had received 180 pre-orders. An entry-into-service date of early 2010 was initially anticipated, later changed to 2011-12. In October 2009 the company indicated that it had delayed the delivery of the first customer aircraft to mid-2013 and had informed depositors. The PiperJet did not enter production and in October 2010 Piper announced it would instead develop an aircraft with a larger circular-section fuselage known as the Piper PiperJet Altaire. The 160 customers who had placed orders for the PiperJet retained their delivery positions with the new aircraft and at the same $2.2 million price.

He only needed a tooth count. I know the 700e and 700t use different ratios but he may be able to supply a drive gear and driven gear in 700 e ratio anyway. Otherwise I can flog it off for parts. It's only something to play with.

Just bumping question, are any forum members attending this course? The date should have read 2026 (My bad) 19 to 21 January 2026.

If any doubt you could sit the gear wheel on a piece of thick paper and spray paint around it and post that to them to ensure correct size. Only if there is a doubt about size. Cheers.

Waiting on the agent in America to get back to me. He is going to see if he has the gear. Being a turbo there are quite a few differences to the na version which he has everything for.

What are your plans for it now? Are gears available to recommission it?