I don't get the back pressure thing me and everyone i know runs little to no back pressure.
A 2-stroke engine has no valves a like a 4-stroke.
It tries to do intake and exhaust at the same time and your intake starts going out the exhaust.
Just follow the green fresh air/fuel/oil mixture.
The stock mufflers provide some good back pressure, a tuned expansion chamber just more, like a reverse turbo charger.
All that gas/oil mix you have on your bike/driveway/etc. is power you paid for, and literally left on the road with no gain.
The SBP X-chamber is difficult to install, but very flexible in it's configurations.
It is tuned to go one 'as is' for a 48cc engine as they are legal most places and need some help making more power.
For a 66cc I just cut that ~4" straight piece of copper in half for the perfect tune.
Throw in an NGK 7544 (CR7HIX) Iridium spark plug in to ignite all that extra fuel and you will feel all that extra power right away ;-}
Hope that helps ya ;-}