Adjustable back pressure means there is some sort of movable restrictor in the stingers.
More BP = More power, but holds in more heat. There is a very small window on what "works" as far as back pressure.
I have messed with 1/8" thick spacers behind the cylinder flanges, sifferent thickness exhaust donuts, etc... to change the signal and back pressure of a pipe. Small changes can make a big difference. Ask Hooper, they are a very good builder and can give the best insight.
