cobrahead mods

[jetjr]

Seems theres a mod for straightening flow into carb. A vane is put inside the cobrahead adaptor which goes from the air filter to the carb.

 

Apparantly clears up some of the CHT variation issues

 

Anyone know about it?

 

JR

 

 

[Captain]

JR

 

 

 

No but I'd like to know. Is the splitter vertical or horizontal, and is it central? Have you seen a drawing or photo floating around anywhere?

 

 

 

Regards

 

 

[Guest J430]

I have not seen it yet......... but I am sure I will, and I dare say it will be well known about soon.

 

There has long been a problem with the variations in the mixture to different cylinders, and its not just Jabiru's, Conty's and Lyc's have similar issues and hence the value in using tuned injectors from people like GAMI.

 

Back to Jab engines, there is also some interesting things that happen with Bing Carbies in relation to air flow and velocity, I am not fully educated in this however some of you who may know a thing or two about motorcylce carbies etc might understand it better.

 

So the idea is to sort out some airflow issues and make the fuel / air delivery more consistant. How you achieve that by a vane or whatever in front of the carby I do not know, however there must be a relationship between how the air enters the carby and how it exits that has an affect on the plenum chamber and so on.

 

Not sure if that helps or coonfuses....:confused:

 

J:wave:

 

 

[jetboy]

The mod done to quite a few 2200 in non Jabiru airframes is like this:

 

http://www.geocities.com/cffd66/Airplane/Jabinstall/Airbox.html

 

I have tried different systems to achieve proper carb tuning, including cobra head, smooth bore rubber tubing etc.

 

With the cobra, rotating the assembly on the carb clamp made a difference as the inlet tube was offset to one side, making repeatable results difficult. Putting the vanes in the intake at the carb was best. I only had to do this with the original "ultra lean dont worry we know what we're doing" kit but what that did achieve was to show me that my original intake was not optimum.

 

Ralph

 

 

[Ross]

It always seemed a big ask to me for a carburettor to work reliably over a range of throttle settings particularly if there is a disturbance like a bend in the air intake immediately upstream of the carby.

 

In another life I used to work with some instrument fitters who were responsible for looking after and installing flow meters in gas and liquid piplelines.

 

For a flow meter orifice to work reliably the orifices had to be placed sometthing like at least either 5 or ten pipe diameters downstream from any radical disturbance like a change in section diameter or a bend or an elbow so that normal velocity distributions across the pipe diameter would occur at all normal flow rates (throttle settings).

 

I cannot remember the multiple, it was many years ago. Nowadays, there are new electronic style flow meters that do not cause the pressure drop associated with orifice plates.

 

There are Australian Standards for the design of orifice plate installations which might be food for thought in this discussion.

 

A reduction in pipe diameter or straightening vanes (like a cross) can be effective in straightening out stream flow after an offending disturbance if you cannot get enough straight pipe to do the job but preferably not at the price of increased pressure drop. The straightening vanes could also increase the risk of icing.

 

I notice in some other engines there is an emphasis on ensuring that there is a straight air entrance into the carburettor reducing the variation that must occur if the entrance does not come from a straight pipe.

 

A help in this situation might be that aero engines are normally only asked to run over a small range of revs/min.

 

 

[facthunter]

Mixture variations.

 

For return for effort, attention downstream of the carb. is the first area to attend to, and the problem is most likely due to the fact that the mixture is not fully a vapour. There is an advantage to having a single carb. SIMPLICITY!, (and cost), so, they're going to be around for a while. Jabiru seem to be putting a lot of effort into playing with the manifold plumbing and I doubt that considering the lack of space around the engine, any one individual without a lot of resources, would make any useful gains. As a principle the front and the rear set of cylinders, can be paired, as their firing (and inducting) spacing is EVEN. An UPDRAFT carb could be set up better, but this is all academic, it's not going to happen. A change to fuel injection is most likely the only worthwhile step, and that could not be justified on fuelsaving alone. There are other factors ( icing, better mixture management with altitude etc.) which are a PLUS . A downside is cost , reliability?, more speciallised servicing ( a move away from simplicity, which has always been part of the Jabiru package). Nev

 

 

[Yenn]

Ross may have hit the nail on the head with his straight for X diameters. The cobrahead seems to really be reducing the diameter, by turning 1 pipe into 4. I does look rather a messy system with rough finish and rivets in the airflow. I am sure a much smoother result could be achieved with some more work.