- exhaust is pressure waves (aka as sound)...waves pulsing through pipes cause low and high pressure zones at varying places at varying times...this is what your neighbour complains about
- pressure waves reflect and deflect where pipes change diameter or direction or terminate, creating positive and negative interferrance patterns...a combination of diameter changes, pipe ends, baffles and chambers (of exactly the right volume) at exactly the right distance from the exhaust valve help tune the system to improve one or more engine characteristic
- an ideal exhaust system would cooperate perfectly with desired exhaust valve flow, creating negative pressure or flow most of the time the valve is open and positive or back pressure stopping flow at just the right moment, esp. at high revs, preventing fresh air-fuel mix escaping
- 'H' pipes, equalizers, balance or crossover pipes create pressure pulses out of phase with the cylinder, can replace short or long systems and connect the two exhaust pipes, balancing exhaust pulse pressure, reducing sound while theoretically increasing power...should be installed as close as possible to the header collector, and should be 50-100% main pipe diameter
- for V motors they increase low-end torque by eliminating 'puffing', periods of time when there were no exhaust ports open on one side of the engine and gases just sit, not moving...when an exhaust port does open, the gases have to be re-motivated towards the tailpipe, creating more high pressure at the manifold, robbing some torque/power...crossovers supposedly prevent the pressure points and smooths the exhaust gas exit
- when removing, unless you optimise the complete system, you may lose some low and mid range torque...in extreme cases carb rejetting may be necessary
- in practice, with XS650s, the limiting factor tends to be port shape...optimising this and installing freer-flowing exhaust have positive effects...i personally like the sound of separate pipes...they also make final carb sync polishing easier