A bit on my muffler:
It rotted out and was in to bad of a shape to patch it up any more. The standard maximum speed in "civilization" is 50km/h here, just a hair under what I was comfortable using fourth gear for. At 50 the car would start behaving sluggish and vibrate a bit so I always had to use third gear in towns and such. My train of thought was that a slightly less restricted exhaust system might just be enough to free up enough torque for 4'th gear to chugg along happily at 50km/h, it would make a noticeable difference in fuel consumption.
I have problems with noise, it tires me really quickly. I wanted a high flow exhaust but still have a quite car. I did some reading and came to the conclusion that a glass/steelpack muffler uses the packing more efficiently if it is in close contact with the perforated core which harbors the main exhaust flow. When further away from the core, less energy is carried by the sound waves and therefore the packing does "less work" on converting the sound energy to heat energy, in other words, less muffling. A short but girthy glass/steelpack muffler should therefore be less effective than a longer but slimmer muffler with the same internal volume. This is a simplification, another aspect is that lower frequency sound waves travel further into the packing material, which makes a girthier muffler better at handling those. My solution was to make my muffler as long and as thick as I could without having fitment problems, starts about 30 cm down pipe from the cat and ends by the rear bumper. Not a perfect solution but it's what I could do with what I had, it gets rid of high pitched noise and what's left is a low rumble.
This was my first muffler build, took some trial and error regarding the shape. If I would do it again I would make the inner core out of sheets with larger holes(5mm) for a better muffling effect, I was afraid that the fiber glass strands would get blown out. After disassembling other straight through mufflers and packing my own I now realize that this probably won't be a problem with the long fibers used and the tight packing. The holed area of the sheets I used was only 33% of the sheet's total, the mufflers I dissected used a larger hole size which should land them at 40% "hole area".
The diameter of the inner core is the same as the original exhaust, which isn't what I intended. I planned to go a little bit smaller to tune the exhaust to my engine and driving style. But... I managed to take my reference measurement from the one and only "larger than original" piece of pipe welded in as a repair. Thought I put in a smaller diameter, landed on stock. 😉
The internal bends are smooth as seen in the picture of the assembled inner core. Just got really tired of fill welding and decided to make the first turn of the outer shell in one go. I also simplified the shape a little to make it easier for myself.
The hair is now a little bit shorter in places since I managed to set myself on fire while laying beneath the car welding mounting points on the muffler. No harm done but the next time I went to the workshop I noticed a couple of Wunderbaums hanging about.
The muffler is mounted and seems to work as intended. It isn't as quiet as stock but I can now drive at my target speed in fourth gear without problems. The old mufflers where both glasspacks but the fibers had detoriated at places and the bigger one wasn't a straight through design, the inner core did a couple of bends in it for a better muffling effect. It was really quite when it was in good condition!
Another note regarding glass/steel pack straight through mufflers. I think they probably work quite good on most hypermiling rigs. They are somewhat alright at muffling at low gas flow speeds but gets increasingly worse as the exhaust gasses speeds up with higher RPM... but that's not the bracket where most of us spend our time.
A stupid project, but fun giving muffler fabrication a go. 🙂