The stock exhaust system of Moto Morini 3-1/2 and 500 street bikes are widely considered to be quite good, and it is known that the designers treated the entire induction, combustion, and exhaust systems as an integrated system. Nevertheless, as with almost every motorcycle ever built, people want to change things and make it "better". In this section I've collected photos of various aftermarket and home-made systems, without making any attempt to judge which ones are good, or even as good as the stock system.
The main advantage of 2-into-1 systems seems to be less weight and better "style". Usually louder, too. Some are claimed to boost power or torque, usually in the mid-range, at the expense of the top-end.
The cross-over pipe in the stock system is thought to have the primary function of decreasing noise. It does not have the same potential advantage for scavenging the exhaust port on a V-twin as it does on a parallel twin.
Click the thumbnails to get the full image.
Plans for a factory-designed racing exhaust for Morini 3-1/2. Thanks to Peter Booth for supplying the plans and to Graham Pearce for rendering them in his CAD program.
A commercial 2-into-1 system
A racer with separate pipes exiting high on the right.
A fairly typical 2-into-1 system.
Calvin Grandy's homemade high-level 2-into-1.
Racer with a very circuitous 2-into-1 system.
Very cool 2-into-2 system.
Another high-level 2-into-1 system.
A home-made 2-into-2 exiting lower right. Looks like it would ground very easily.