Re: How to know when broken in? Fuel ratios?
If you take off the exhaust you can see the rings in through the port.
When it's broken in they will be scuffed and rough looking. If they still look new, or only partially scuffed up, you got some more breakin in to do. Yes, the engine will become a more efficient pump as those rings seal up all the way around, and it will make more power.
An expansion chamber pipe can be tuned to produce a burst of power in a certain RPM range, with the header being lengthened or shortened to choose that range. It has to do with the intake and exhaust flow through a piston ported engine, resonance and a bunch of pressure wave/technical jargon I would leave to those that understand it better. It's like a boost bottle for the exhaust, but unlike a boost bottle it actually works.
Buzz zzzzz zzz