Summer 2005 - Part 1

Building a new exhaust system

Old exhausts
The exhaust system I have been using so far is a pair of pipes taken off the bikes which I bought back in April 2003. I had two pairs of pipes, and I had used two right hand side pipes on the hovercraft - this meant they fitted side by side rather than splaying out like a pair of legs.

I had mounted the pipes vertically from the exhaust flange up over the front of the duct guard to exit near the top of the duct. It had been a quick solution - the pipes had just needed the first bend in the header pipe tightening up a little (by cutting and welding) to make them fit into the space available between the engine and the cockpit side.

The problems with this arrangement were::
1) the mounting was difficult - the front one was tied to the RHS bracing strut and the rear one was tied to the duct guard using jubilee clips. There was no allowance for any flexing or vibration.
2) they blocked the flow of air through the fan
3) they were pretty heavy
4) they raised the centre of gravity unnecessarily.

In the end, the poor mounting arrangment meant that at Gang Warily in May both pipes cracked at the first bend. I welded them up but they cracked again. I decided that I needed to do the job properly and replace them with something better. A lot of TZR-powered hovercraft use custom made exhausts, which run over the RHS deck next to the duct. I was keen to build such an exhaust system which would solve all of the original problems.

Exhaust pipe cut in half
I cut one of the spare pipes in half with a large angle grinder to have a look at the inside. It is made from many individual pieces of steel, pressed and welded together. The basic shell is 1.2mm thick, but in most places an extra sheet has been pressed in, doubling the wall thickness, so the weight is many kg per pipe. The silencer cans are welded on but the wadding inside can be replaced by removing a screw and withdrawing a perforated tube.

Rather than rebuild an exhaust using multiple pieces of sheet, I wanted to try hydroforming - the technique where two flat shaped sheets of steel are welded together around their edges and then inflated under extreme pressure to form a cylinder of whatever shape is required.

As the original pipes had  welded-on silencer cans I would also need to either buy or make replacements cans.


A bit about 2-stroke exhaust pipes

A 2-stroke exhaust pipe is a complicated beast. Unlike a 4-stroke exhaust system teh 2-stroke exhaust is really part of the engine and not just an add-on component.

The 2-stroke exhaust uses several identifiable sections - the header pipe, the front cone, the parallel section, the rear cone and the tail pipe. Each of these components plays an important part, according to the laws of physics - mainly transmission line theory. In particular, the cones produce reflections which send pulses back to the engine to suck out exhaust gases (a negative pressure pulse) and force the fuel air mixture in (a positive pressure pulse). The time taken for the pulses to travel along the exhaust pipe depends on the length of the pipe, and the speed of sound in the hot exhaust gas. Because the pulses travel down the pipe at a fixed speed, the whole system is set up for a particular engine speed - peak power at around 10,000 rpm, which is what tuning is all about.

Testing out the ideas
One of the questions that I needed answering is how much pressure would I need? How easy is it to weld stainless steel? My friend Clive loaned me a high prssure hand pump which could reach 350psi. Would this be enough?

exhaust test piece

As I had a large quantity of 1.5mm thick stainless steel sheet handy, I cut out a pair of identical shapes from this and bent the edges over a little. At one end I welded in a length of pipe with a 1/2"  BSP thread to fit to the flexible reinforcedpipe from the pump, and at the other I welded in an 8mm nut to use as an air bleed off valve.Then I  welded the edges together. This was meant to be just a test piece to try out the technique - which is why it doesn't look much like an exhaust pipe.

I pumped up the pressure a little and bled out the air. Apparently, if you expel all of the air then you are only compressing water. If anything bursts then the pressure will immediately drop, without the whole thing exploding. The first test was too see if it held pressure. At 50psi I noticed that the welded joint around the 1/2" BSP steel pipe was leaking a little so I drained it all down and rewelded the hole.

Then I pumped it back up and continued pumping. At 100psi I thought it was scarey stuff! The pressure soon increased and the bleed valve started leaking slightly around the threads - I hadn't used any PTFE tape (didn't have any). But I kept pumping and the test piece inflated gradually. At 330psi the pump was getting difficult and the test piece looked fully inflated.

Hey this looks easy! It had inflated to a smooth shape - with just a small kink where the edge had a bit of a poor shape The test piece was strong enough that even when emptied of water it could easily take my weight standing on it, but then again it was a little too heavy. If the pump could cope with 1.5mm thickness steel, I thought it should easily cope with thinner stuff. I bought a sheet of 0.9mm grade 304 (that's A2) stainless from Outlooks.

OK. So the test piece showed that it was possible to produce a stainless steel envelope and inflate it. So what I needed now was a design for the actual exhaust dimensions.

dented exhaust
The book provides plenty of guidance and theory but it also makes exhaust design sound complicated. The cone flair angles, cone lengths expansion chamber widths, stinger dimensions are clearly all important but it is always difficult to appreciate how critical each dimension is. Looking at the original Yamaha exhausts, they are clearly a compromise between the theoretical shape and what can be made in practise. In particular, a large dent designed-in to the side of  the pipe allows it to clear part of the bikes engine/frame which sticks out, so it's obviously not too critical in places.

One thing worth noting is that while the dimensions are generally important, the bends in the pipe are not. So as long as the lengths and widths of each stage are correct then you can bend the whole thing to make it follow whatever shape you want. Another point to consider is that hydroforming only allows bends in one plane, so to create a more complicated shape the pipes  need to be cut, twisted and rewelded. This means that you can (in theory) design your entire exhaust pipe out of two flat shapes, allowing for the fact that you are going to twist each section as required. The cross-sectional area is dictated by the theoretical design. But assuming the pipe is circular then the width of the template needs to be half of the circumference of the finished pipe. Then when it is blown up the two sheets form a complete circumference of the required size.

I cut out a paper pattern for the exhaust shape according to the half circumfernce rule above and transferred this onto the stainless sheet. When drawing the bends in the pipe you need to allow for the fact that as the metal  inflates, the bends close up by around 10%, but this is all a bit hit and miss. I wasn't exactly sure that I had the right shape, but I needed to give it a go. I cut out a pair of exhaust shapes using an air powered nibbler (much quieter than a portable jigsaw), welded the edges together, welded on the inlet and outlet ports and pumped it up as before.


This first exhaust shape was a bit of a disappointment after the success of the test piece. The problems were:
The wide section (expansion chamber) inflated first, and this caused a major kink in the metal at the end of the header.
I had allowed 3mm extra width to accomodate the welding - this wasn't enough and the overall diameter came out too small - especially around the header pipe section.
The edge of the pattern had been a bit wavey - this caused ripples in the edge of the inflated pipe.
The bend radius of the pipe was to tight and kinked one side.

A lot of these problems seemed to come down to the fact that I was using stainless steel which is harder and less ductile (stretchy) than mild steel. I modified the design with:
extra width to accomodate welding
smooth straight sides to prevent ripples
greater bend radius to prevent kinking
I tried it again and this time clamped the widest section between two sheets of thick steel to make sure the narrow section inflated first. The result was a lot better, but nowhere near good enough. I came to the conclusion that stainless steel was just too difficult to work, and that I needed to use mild steel. This was a disappointment (and a waste of £60 on the stainless sheet).

mild steel   mild steel

Another set cut out and I tried again with 1.2mm thick mild steel. This was again much better and now almost good enough. But the shape wasn't quite right - I should have started the bend approx 100mm closer to the header. But time was now running out so I fitted the old exhaust back on for the next two events - Whittlesey and Black Ditch Pond.