I was putting this drawl off until I could post pictures of the lovely bit of metal, but I failed to post them again.
I guess that means I will have to fall back on my descriptive powers (sorry maz
). If anyone wants to see pictures you will have to pm me your e-mail.
While my City x was apart having it's motor overhauled by Maz, I could then see into the standard header pipe, and it was not pretty. It looks like all show and no go where the twin pipes converge at the weld. maybe this is behind the stalling gasses of my 9 at low rpms....or not. Certainly this area of the pipe and down to the curve inside the muffler seems to be sensitive to any changes as I found by making a 'venturi' in the entry to my Stealth. Having changed to a 12R muffler, which is both reasonably quiet and yet less restrictive than the standard 9's, I wondered if fabricating another more subtle venturi effect device downstream of the converging header could create a beneficial effect. NB 08 and later models need not apply as the factory appear to have made their own 'flume' or flare into a wider muffler entry. Obviously any reduction will reduce maximum flow at this point and therefore power, but I'm interested in throttle response lower down.
I started by measuring the straight between the headers convergence and the start of curvature in the 12R muffler. This was 63-65mm with an approx diameter of 46mm. I ordered a length of 304 stainless pipe from Metals4U. The nearest od is 48.3 and I went for the thickest wall of 3.7mm to give some meat to work with. These lengths come at 250mm so I could make 3 experiments from a section.
A local engineering firm who specialise in stainless and 1 off specials turned the pipe down to 46mm od. I then started grinding and flapwheeling one end to make a tapered inner. I had read somewhere way back when that an entry of 15 degrees and an exit of 7 degrees was the optimum shape. Whatever, it's good to aim for something. Stainless is hard work and my one larger silicon carbide wheel took a beating. So I could have asked the engineers to turn the job out for me, but I wanted to see how the shape formed and stop when instinct stepped in (enhanced eddy at work
). When the shape was close to looking right I made a longitudonal cut in the section of pipe. This 'springs' the pipe open to leave an expansion gap and ensured a tightish fit inside the standard exhaust.
Turned down to a 46mm od the wall is now 2.55mm and after polishing out with a flap wheel the min internal diameter was 42mm or a tad less in the centre section tapering to knife edge ends where the insert will mate with the standard exhaust. As the steeper entry is at the top near the header the narrowest part of the venturi is also towards the header.
The beauty of this length of section is that it covers the open joint and step where the header slots into the flared joint in the muffler. Hopefully there is a gain here. I tapped the finished section into both the header and the muffler to check the fit and it was good with an almost seamless flow from the knife edge into the existing curves.
For fitting I coated the stainless section with copper grease to ease removal if required. Assembly was straight forward and it all slotted in perfectly. As it happens there is a slight taper in the header straight and muffler, which naturally stop the insert from pushing further in or moving out of position in use.
My trial run of 40 miles showed immediate promise. Once warmed up there was an urgency or ease of running as if the motor needed less throttle to move and the power was smoother over 1500 to 4500 or so. As you would expect there was a subtle loss of urgency as revs climbed, but as the miles went by even that started to change as if the ECU was adjusting itself slightly.
It's too early to gauge all of the pros and cons, so I'll do more miles before deciding whether to leave it in or not. The only downer at the moment is that that lovely gruffly sound I get from the motor when loading the throttle (and stalling gasses?) from 2000 rpm is not there. It just seems to flow seamlessly.