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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.

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?

Not saying that your discussion is wrong or not worth pursuing. What I'm saying is that in respect to this particular accident, you are creating unproven assumptions to support your narrative. The students lack of experience may have resulted in him not having the peripheral awareness of the speed if the ground rising to meet him resulting in the bounce.

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.

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!"

An interesting scenario, the possibility of running on all four with a broken throttle cable however I think that would be an unlikely failure on a regularly serviced engine. I think you'd need to do a lot of testing before you went to the expense of making casting patterns. I've built plenty but molds but to cast a manifold is expensive. You first have to form the sand core in a mold. Then the core is fitted into the mold and the molten metal fills the cavity in between. Complex and expensive so you need to sell plenty to cover developments costs.

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.

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…😁

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.

Speculation. I explained How critical the BOUNCE recovery situation is Fully. I've Covered the event Pretty thoroughly and even said I did it On My first solo and why.. It's an Instructors WORST Nightmare when he watches THAT Happen. It's the recovery where it went wrong. Nev

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

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.😈

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😈

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.

...o, .....

......"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.............

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.

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

It would have been a good development to increase the cross over pipe at the time of doing the casting patterns for the intake manifolds as there is good feedback from those that do such about the smoother idle and possibility of operating at 50% power in the instance of one throttle cable / connection breaking and engine going to full power on one carby. Would have been a good change to what Rotax have there in my opinion. (I'm open / planning to increase my R ULS balance pipe i.d. size.)

Crappy's great uncle, Winchester Cook, used to shoot off with Annie too, but he boastfully maintained that his had a different meaning.

Crappy apologizes to his best mate and all others who are involved with the Turbine franchise. Crappy was sucked in, again, by the accuracy of the Turbine family's intergenerational aural storytelling, and the realism of it all, as he could immediately see Turbo (the great man himself, and not his GU), sailing through the air like Nadia in a tutu, then skidding to a halt on the veranda in his R M Williams kangaroo leather high healed brothel creepers.

Turbo was moved to tears at this note from his dear friend Cappy. With the greatest of respect, Turbo points out that it wasn't him, although he is quite capable of it, but a Great Uncle, the son of Wilhemina Turbine . Wil, as she was known used to shoot off with Annie Oakley. His father was Jonathan Jones who carried a big knife like Davey Crockett and was the only survivor of the Alamo, who was saved by climbing up a chimney when the Alamo was overrun by Mexican immigrants.

.... where he sauntered up to the bar, like John Wayne in chaps (arseless leather dacks, and not real blokes), and asked the barman for a light ale from the Moorabbin Boutique Brewery (they distribute internationally, like YellowTail), to which the barman replied .....

My Dear NSEers and our thousands of Avid Readers (ARs). On your behalf I offer our combined thanks to Turbo for the above post, as it is extremely rare (even harder that finding a bloke who landed in France on D Day) to have a first-hand account from somebody who has performed all the procedures for stopping a 19th century Steam Train, and then landing like Nadia Comanechi on the porch of the Lone Star. We are, indeed, a lucky bunch.

...........prominent assets. Turbine Jones, grabbed him by the leg, threw him out of the train's path, and kicked the fireman in the aXXX. The fireman responded by throwing a lump of coal at Turbine Jones, hitting him in the cods. In one fluid action, Turbine Jones swung up into the guards van, kicked the air brakes on, and swung out in an arc, landing lightly on the verandah of the Lone Star Hotel.............

apologies if you have heard this one ............... no joke though Onetracks post above reminds me of the NZ topdressing DC 3's that could take a Bambina or a Mini in the back - also by ramp - for transport of pilot and crew - pilot got off one speeding ticket in the North Island ................. BUT, presumably had to pay the South Island ticket ! (post form NZ web-site below) Was it Gerry Kluck that got two speeding tickets in a day (within the hour I think), one between Blenheim and Woodbourne followed by one from Paraparaumu Airport to the Railway Station. (so engineer could catch train to Palmy)? Magistrate through it out of court as it was thought impossible to get two tickets within three hours in two islands in the same car.

Don’t really see that French stuff as anything to special really. Each day every day over here

And it's NOT really off the topic of the thread Engine related. Safety. BAK,. Nev

Get Near a Merlin with a few revs on and repeat that. ON Load an Open AERO Engine exhaust makes Plenty of Noise. Plenty of People Lost their Hearing ( Including ME. Never Heard of any race engines Getting turned Back wards in any situation and I've had a lot to do with them. It's extremely Unlikely. Turning the Ignition OFF and then Back ON will result ina Lot Of Mixture ending in the exhaust and Muffler will explode and make Lots of Noise and EASILY Produce A Long Flame Out the tail pipe and scare the $#!t out of everybody. BACK Firing is when the Mixture in the intake and supercharger Ignites/ explodes and Subjects things to VERY High Pressure. Superchargers Fly Off the Block Car Vee Block s split. Con rods get shortened etc. High Pressures force Valves off their seat and subject the Engines to Loads they Cannot Take.. They May not Fail straight away But an Overstressed Part is a damaged part.