Harley uses a crossover on some engines to increase the muffler volume for each cylinder. When a restrictive (quiet) muffler is used it will increase power and it is also known to quiet the exhaust note without a muffler. This seems to comport with Tim's comments, including the cork. A single would need a larger diameter silencer because it does not get to use the the siamesed pipe that a twin can take advantage of.
Exhaust and intake tract tuning is a black art because it takes so long and is so expensive. The math gets you in the ball park, but from there it is all dyno tuning. On a stock street bike you just do not have the flexibilty of changing the size and length of both the intake and exhaust before you start running into problems. The acoustic wave tuning is the same on both sides of the cylinder. The intake benefits from a positive wave arriving at the intake valve just before the valve closes. There are so many variables it is not worth making a complete mathematical model. It is better to go to the dyno and adjust the cork.
David
Exhaust and intake tract tuning is a black art because it takes so long and is so expensive. The math gets you in the ball park, but from there it is all dyno tuning. On a stock street bike you just do not have the flexibilty of changing the size and length of both the intake and exhaust before you start running into problems. The acoustic wave tuning is the same on both sides of the cylinder. The intake benefits from a positive wave arriving at the intake valve just before the valve closes. There are so many variables it is not worth making a complete mathematical model. It is better to go to the dyno and adjust the cork.
David