Modelling a bunch of bananas - aka downstream manifold

Modelling? Bananas?

There seem to be 2 ways to go about fabricating an exhaust system, particularly the manifold end, where things tend to be more complex: the Honda S800 has a "bunch of bananas" manifold, forming a 4-into-2 system with equal length headers.

The first method is simply to get a pile of tubes, elbow bends, angle grinder disks, welding wire etc and knock it up piece by piece. 

The second is to model up the system using standardised building blocks comprising mandrel bends and sections of standard tubing. Unless you intend to persist with the use of tube benders, you have little choice. Even then, the tube diameter and bend radius are all going to be fairly well predefined. And given that the 32mm tube I have seems to collapse when you so much as fart next to it when trying to bend it more than 30 degrees or so, my attentions have focused on the use of mandrel bends and u-bends, chopped up as required, then butt welded together to form the required final assembly.

And when it comes to modelling manifolds in Fusion 360, there are 2 approaches.

1. Make up a library of bends and tubes, then assemble them so that they replicate the real thing. Then replicate each compt on the bench and assemble / weld them up.
2. Create a 3D sketch that defines the centre lines of the headers, then finally use the "pipe" feature in Fusion to create a pipe that follows the sketch line. Alternatively, you could create your own tube section in a sketch and then use the sweep function to create your own tube / pipe. 
   

NB: Interestingly, the "Pipe" tool in Fusion should really be called a "Tube" tool, as the diameter you specify actually ends up being the OD, not the ID: if you specify a 32mm pipe diameter and a 1.5mm wall thickness, the ID measures at 29mm.

The 3D sketch method is a royal PITA but as log as you don't define the dimensions of the angles and length of the subcomponents, you can edit them in place by dragging the end points of the sketch segments in the 3D environment - assuming you have selected "3D sketch" in he sketch popup dialogue. In contrast, the "build it from blocks" approach is a different kind of royal PITA - each time you want to move the parts around, you have to either use revolute joints etc (for rotation) or edit the subcomponents for each position, noting that you will need to unlink them from the original, to avoid changing all instances at once. So you will end up with a myriad different tubes and elbows and moving each about will be the devil's work.

Here's an example of how to use the 3D sketch method. It's got no commentary but even so, you can see that it's not dead simple:

And LEAD has a similar video in that wonderfully condescending fashion so many Mercan Youtube heroes adapt.

Enough talk - time for some action:

Here's the beginning of an assembly using the "building block" method:

I soon gave up on this, as it was clearly going to become a nightmare very quickly.

Here's the "3D sketch" method, with the centre line sketch being open and edited. The blue lines are not fully constrained, so can often (sometimes?) be edited by dragging the segment end points.

If you set the sketch properties to "Show dimensions" and keep the sketches visible, you can also see the dimensions (look closely):

Changing the Display Settings / Visual Style to "Shaded with visible edges only", highlights where the different segments abut:

And of course, you can render it if you are a CAD tart. The stainless tube I bought is actually polished (it was actually cheaper than the "natural" finish), so it's actually quite close to what we might expect to see, apart from the heat discolouration.

I've concluded that the 2 lower headers may as well join before the final exit (as shown), as this would simplify the assembly and I don't imagine shortening 2 of the headers by a couple of inches will make the slightest difference. That will require some further CAD work of course and the tube is slightly larger diameter (38mm) after the 2 headers merge.