I think unless you have a lot of resources at hand, you´ll have to settle for basic rules-of-thumb, and common sence.
When talking about 2-1 exhaust systems, there are a couple of different versions to consider.
In certain engine configurations "2-1 exhausts" can be used to enhance engine performance in some respects, in others you have to take precautions to avoid detrimental effects.
Basically, I believe engines with evenly spaced combustion events tend to belong to the first cathegory. By implication, this would render Ducati engines "type 2" engines; i.e., there´s not much scope to use a 2-1 exhaust to enhance engine performance.
Since I like 2-1 systems from aestethically ( I just like the look) and practical (no muffler obscuring access to the drive chain) reasons, I´ve spent some time thinking about and making 2-1 exhausts.
Basically, what I came up with was that the best compromise if you don´t have the opportunity to test things out on a dyno is to use the principle of independent headers and just join them in a common collector / silencer assy.
Since I´ve been exclusively working on 2V engines, and using whatever formulas generally available as well as empirical testing, I have arrived at systems using headers 700 mm to 800 mm long (from the exhaust valve face), exiting in a collector section of considerably greater section area (150% to 200%) in order to terminate the header section, and to promote a negative pressure wave reflection. Mainly, I beleive by adhering to these principles, you do not optimize anything, but you do avoid major disasters.
The graph below (hope it comes out readable) shows the same M900 MY 2002 with everything stock except Silmotor slip-ons (red) and homemade 2-1 as per above + open airbox lid and dynoe´d PCIII.
This happened before I had learned that owning a Duc means spending money, so I did not dyno the bike with the 2-1 system only. Would have been interesting, and worth the money.
In my opinion, if you´re just looking for something that looks the way you want, and are not into heavy performance tuning, use whatever exhaust configuration you think looks right, observing the following rules-of-thumb:
1) Use standard header diameter (or very close, like 38 mm inside diameter);
2) Use header lengths around 700mm to 800 mm;
3) Use as little back pressure as possible in the basic system, since no 4-stroke engine "likes" exhaust back pressure
4) Provide Db killers or whatever you like to get your bike thru whatever noice tests you´re likely to encounter.
5) No rule, really.... more back pressure will likely improve lower-RPM torque, and hurt higher-RPM power. Might actually be more rewading to ride ....
