Perhaps it was just the place I got my exhaust done. I have 2 silencers in it now, but the shape of the tailpipe probably gives it the noise.
Lesson 1: A twin round will always be quieter than one large rounded rectangle.
The cat replacement pipe I have is effectively a straight through silencer.
Lesson 2: The Alfa engines like a lot of air, that is why K&N filters make a lot of difference.
The reason they are not fitted as standard, is that it increases the induction roar, which doesn't appeal to some.
Lesson 3: What goes in quickly, must come out quickly otherwise a blockage occurs.
The more obsticles in the way, like silencers, bends and cats, the slower this air flow is. However, a certain amount of back pressure is required to get the exhaust gases out at the fastest rate rather than create a vaccum. In order to acheive this, the exhaust needs to be engineered correctly to what is called a tuned length. The gases swirl down the exhaust, rather than travel straight down the pipe. This creates a vortex, which speeds up the gas flow, rather like the Pipercross Vortex filters, a tornado or a plug hole draining water. In order to acheive this optimum flow, the aerodynamics inside an exhaust need to be considered.
This is where I think custom exhaust fall down by comparison to the likes of the SuperSprint. The Supersprint is engineered for a fine balance between performance and noise.
The reason it is both mild steel and stainless steel is due to the fact that the two metals have different acoustic properties and flexibility. With two idential exhausts, one made with stainless and one with mild steel, the stainless would be louder and more likely to break as it is more brittle, especially when cold. The Mild steel one would be quieter, but prone to condensation in the pipes and rust. They would both give the same performance when new, but different sounds.
I think the Supersprint will have been bench tested to measure the air flow, the optimum back pressure required will have been measured for the engine and engineered into the exhaust etc.
I like my exhaust and the sound it makes, though everyone comments on it noise wise. My car sounds better than the same car with a Supersprint on acceleration for that "power sound", but over time it is not exceptionally discreet and it has had to go back a few times to get it quietened to an acceptable level on motorways. When I specified what I wanted, I said I wanted the same sound volume as standard + K&N, but with a deeper exhaust note. They failed to acehive that, even though the exhaust had a sound guarantee!!!!
Unfortunatly we, as 155 owners, do not have a huge choice. From what I have found out, it is only Supersprint that make a full system for them, though Autodelta also produce a stainless system. Both of these are over £400, which considering the depreciation rate of the 155, is a lot of money. The standard steel one from Alfa is about £300. Any Kwik Fit will slap any Fiat bits that will fit on from what I have seen. Peco now do a rear silencer. I have heard people talk about Sebring and Remus, but I haven't investigated. I have pestered Magnex and Milltek exhaust in the UK and offered my car as a doner, but SuperSprint seem to have got the market sown up at the moment.
Richard S & Jaques: It is that classic "If I was doing it again" routine!! With custom exhausts, it depends on how skilled the guy who is building it is. Some treat it a journy from A (Downpipe) to B (Rear bumper) and take no pride in their work. Some don't appreciate the aerodynamics or acoustics involved and are just welders. I am a big believer on Tuned Lengths as it works for induction as well. The GT Ferraris you see on Eurosport had tuned length air intakes, and experienced a drop in power when they were told to adjust their length and position. This was just the input air!!!
I would say that 3 silencers are essential and take the cat out, as this is £500 if this goes wrong!!! Also consider your tailpipe size and length as this adds a lot of the sound.
I hope this helps. Sorry for the long email. Again this is just my opinion and I am certainly not rubbishing anyones exhausts. The place that did my exhaust now at least know how to make a quiter one and will give us discounts.