The water adjusting screws on the headpipe control where the pipe makes it's peak hp.
"Traditionally, if you wanted low RPM torque and high RPM horsepower, it required
several pipes. A few of our competitors cast rings into their pipes to achieve pipe tuning by Acut
and try.@ In 1992 Factory Pipe introduced the first truly tunable pipe using our variable water
injection system. This system allows you to modify where and how much water injects into the
exhaust by the turn of a set screw. Where our competition had you change the length of the pipe,
the Factory Pipe allows you to vary the exhaust gas temperature which in turn changes the sonic
wave speed within the pipe. Changing the sonic wave speed within the pipe has the same
tuning affect as changing the length of the pipe."