Crossover pipe of Rotax 912 Carbys

[skippydiesel]

24 minutes ago, Blueadventures said:

Flywheel balance is linked as the pipe effect is to reduce vibration, comments welcomed.  What's needs to reduce is the doubters that keep asking to verify (especially they almost never verify their stuff.)

Here we go again - more claims to pursue (strategy of distraction), rather than addressing the reasonable questions, arising from the first  claim- .

 

"The larger balance tube does give a better idle performance...... "

 

"Please quantify - what is "better" about the idle AND what is the operational advantage of this improvement, over the OM design?"

 

Until you either substantiate your claims, answear some fair questions, I have to assume this is just a load of BS and I advise others to be just as sceptical😈

[skippydiesel]

On 16/01/2026 at 1:35 PM, Moneybox said:

If you want a foolproof balance between manifolds use a Slack Tube Manometer.

 

The only thing I did wrong was that I started using green coolant in the tube. It was too light with the balance well out all the fluid went up one tube. Fortunately it had a shutoff preventing the fluid entering the manifold.

Once I filled the tube with Mercury it worked perfectly. Disconnect the crossover and attach one tube from the Manometer to the LH manifold and the other tube to the RH manifold. When you have balance the Mercury will be balanced.

The next mistake I made was leaving the Mercury in the plastic tube. It's now badly stained. I already had one so I didn't have to buy it but this is a very cheap accurate and long life device.

Hi Moneybox,

I can see how the U tube might work but how do you remove the effcets of manifold pulsing?😈

 

[facthunter]

Have an adjustable shut off valve in the middle. Nev

[turboplanner]

........more complications. 

Standard engines produced to do a job for a long life with basic  maintenance and adjustments usually have one carburettor and a rough cast manifold that allows gas to pass, and complete with sprags, bulges and other imperfections.

Where I come from is taking that engine and modifying it to produce double or more power.

This involves throwing away the shared manifold and replacing it with, sometimes, stand-alone cylinder processing of gases to take advantage of pulsing, as Skippy mentions.

The key process is to have tube lengths to optimise sonic wave directions.

Joining the stand-alone manifolds would interrupt the sonic waves, lowering power.

 I tried to make the point before that the Rotax pedigree comes from light weight/high power engine applications wit some degree of development above the standard engine class.....

Just how far developed it is, I haven't researched, but if it is using fuel packing design at all joining the stand alone environments may result in less power.  Power being more important than the very short time an aircraft engine is idling. Since so far no one has come up with precise figures or explanations, I'd be wary.   So far no one has come up with any real figures.

[Arron25]

2 hours ago, skippydiesel said:

 

Nothing to do with the topic, Just a comment

Ignoring upsets the flow of a 'conversation' BUT as 'Fly Tornado' (former name?) was the most ignored member Is skippy trying to re-emulate the 'record'

[skippydiesel]

12 hours ago, Arron25 said:

Nothing to do with the topic, Just a comment

Ignoring upsets the flow of a 'conversation' BUT as 'Fly Tornado' (former name?) was the most ignored member Is skippy trying to re-emulate the 'record'

Hi Arron25,

 

Aside from the core topic, its quite interesting how some people, can make a claim, with little or no supporting evidence and expect it to be accepted without challenge.

When asked for the evidence, the claimant may ignore the questions, becomes defensive and aggressive. The latter often manifesting as, strident references to their own expertees, combined with personal attacks on the questioner.

This is irrational - the sensible person would either admit that the claim is speculative or provide the evidence and the questioner(s) move on.

Its a sort of bullying tactic, by the claimant, that does nothing posative for the Forum, the topic at hand or their credibility.

""There's nowt so strange as folk". 

😈

[FlyBoy1960]

I just love it !    There are only about 10 people left on this forum, and now you are all fighting amongst yourselves!

 

This resource is slowly self-destructing.

[skippydiesel]

48 minutes ago, FlyBoy1960 said:

I just love it !    There are only about 10 people left on this forum, and now you are all fighting amongst yourselves!

 

This resource is slowly self-destructing.

Hi FlyBoy1960,

 

I would realy enjoy discussing what you have just postulated  but not within this already very shattered thread.

 

I suggest you start another thread on the matter (general discussion?)  - I will respond. 😈

[Blueadventures]

On 17/1/2026 at 3:49 PM, turboplanner said:

Let's just get to the centre of the pipe claim.

When I track down a set of inlet manifolds I will do an increase of crossover pipe size. It will be a positive retro fit. Any leads to a set of reasonable priced inlets appreciated. I do not wish to entertain any justification or why I’m know very keen to do a set. My engine runs sweet as I mech a pneumatic balance it and also dynamic balance prop. Just for info.

[Blueadventures]

I have sourced a set of inlet manifolds and a mate who is a very good ally welder (used to make the inlet manifolds for the EA81’s for the early Terriers) will weld on some 19mm or so elbows when I get some. Getting there. Off an early 80hp engine.

[facthunter]

 Carefully check it's rapid throttle response. That's what I'm concerned about Most.  Nev

[Moneybox]

It may be best to keep your rubber hose sections short. The bigger diameter hose will suck flat easier than the original small hose.

[facthunter]

Put a coil spring in it. Nev

[Thruster88]

36 minutes ago, facthunter said:

Put a coil spring in it. Nev

The spring could fracture allowing a piece to enter the engine. Would not be the first time an unforseen event has occurred from a modification of a system.

[Blueadventures]

1 minute ago, Thruster88 said:

The spring could fracture allowing a piece to enter the engine. Would not be the first time an unforseen event has occurred from a modification of a system.

I plan same set up as Rotax do, alloy tube with manifold pressure takeoff and rubber or silicone joiners.  Yet to decide if I do the 25mm or a 19mm sizing.  Will track down some results with both sizes.

[facthunter]

It's commonly used  (very effectively) in cooling rubber Pipes and other ducting where Low pressure can exist and collapse the section. Pretty unlikely to fracture in those applications. Nev

[FlyBoy1960]

Facthunter, the problem is in a cooling situation the pipes contain positive pressure. In the carburettor location they are in negative pressure, i.e. vacuum, so I don't get what you are saying ?

 

[skippydiesel]

1 hour ago, Blueadventures said:

  Yet to decide if I do the 25mm or a 19mm sizing.  Will track down some results with both sizes.

I await the results with great 😈

[onetrack]

Flyboy1960 - In cooling systems, the reinforcing internal spring is always mounted in the bottom radiator hose. This is because in most systems, the coolant travel is from the highest point of the engine, to the top of the radiator, then down through the tubes, to the bottom of the radiator.

 

The water pump sucks the coolant from the bottom of the radiator, and any restriction in the flow through the radiator tubes creates negative pressure inside the bottom radiator hose, thus leading to its collapse, if unsupported by an internal reinforcing spring.

 

Facthunter is simply pointing out that intake vacuum has the potential to make any intake hose collapse, if it is long enough, and unsupported by an internal spring.

[facthunter]

On the suck side of a Water pump there is usually a coiled  spring to Prevent the rubber hose collapsing. There's NOT ALWAYS Positive pressure and it's limited by the CAP at higher temps. The Pressure CAP raises the Boiling Point. Nev

Edited Friday at 01:28 AM by facthunter

[skippydiesel]

4 hours ago, onetrack said:

Flyboy1960 - In cooling systems, the reinforcing internal spring is always mounted in the bottom radiator hose. This is because in most systems, the coolant travel is from the highest point of the engine, to the top of the radiator, then down through the tubes, to the bottom of the radiator.

 

The water pump sucks the coolant from the bottom of the radiator, and any restriction in the flow through the radiator tubes creates negative pressure inside the bottom radiator hose, thus leading to its collapse, if unsupported by an internal reinforcing spring.

 

Facthunter is simply pointing out that intake vacuum has the potential to make any intake hose collapse, if it is long enough, and unsupported by an internal spring.

Hi Flyboy19660,

 

I beg to differ as to the cause of automotive bottom radiator hose collapse;

 

Older cooling systems (before two/way radiator caps & overflow reservoirs), in which the expansion tank is the top section of the radiator, were prone to bottom hose collapse.

The cause is primarily (not exclusively) due to overfilling the radiator.

When the engine runs/gets up to temperature, the coolant expands & the surplus liquid is forced out of the radiator via the one way cap.

When the engine cools, the coolant returns to its preheated state (shrinks).

The radiator cap closes and a strong vacuum is created in the cooling system.

This vacuum plus any pump action (as you have mentioned) and often the length/diameter of the bottom hose, all acts together to collapse it.

 

The solutions are:

• Only fill & old style radiator to the top of the coolant tubes ie leave a significant space above the tubes for coolant to expand (expansion tank).
• Combine the above with a reinforced bottom hose - internal reusable spring or spring part of hose construction.

Note: In a poorly designed cooling system (UK cars very prone) combined with an unusually hot engine, sufficient coolant can be driven of as steam, causing the above problem, even when the correct amount of coolant has been added.

 

Modern coolant recovery system do not have this problem, due to the two way radiator cap (hot ciilant can escape & return when system cools) & coolant reservoir returning coolant to the system, thereby minimising the chance of a significant vacuum being created.

 

Rotax cooling systems use the modern two way cap & coolant recovery reservoir system😈

[facthunter]

Water doesn't expand anything like as much as you suggest. Air in the system can cause Local hot spots. Real bad  idea.. Stop a motor that has been working hard and it may Boil AFTER it is stopped. High suction can pull air Past the Pump seal into the water passages in the Motor There are Better systems made since the Mid 90s that have the Pressure cap on the reservoir which has a fill level Indicator and often self bleed  capacity. I don't think you can isolate British cars these days. Most are International and built in Many places to similar designs.  Most cars temp indicator sits in the same Place whatever the CAR IS DOING, once it has warmed up.   Nev

Edited 22 hours ago by facthunter
expand

[IBob]

I've specified expansion vessels for a few closed loop boiler systems. It's been a while, but as I recall we used to budget on 4% expansion, from ambient to 100'C.
I can't now recall if that was the actual expansion, or expansion plus safety margin. Water coefficient of expansion is not linear, so you can't just plug one number into the usual calculators.......

[BrendAn]

You blokes do know blue is using rubber /silicone joiners not lengths of hose.  Pretty sure he couldn't give a st#ff what happens in a cooling system 🤣

[facthunter]

It was brought up if there was a risk of the Pipe collapsing, remember. I wouldn't be altering the original set up which is multiples lower in Volume. without a lot of testing including throttle response.  Nev