Here's some info:
A SHORTER PIPE LENGTH WILL RAISE THE R.P.M. or more exactly it will raise the RPM point where maximum power is produced.
A LONGER PIPE LENGTH WILL LOWER THE R.P.M. or more exactly it will lower the RPM point where maximum power is produced
Adjustment
1. Fix the manifold and pipe in position at the length required for your engine, making sure that there are no steps or leaks in the manifold to engine joint.
2. Run the boat on the normal propeller and set the needle valve for the best straight-line performance.
3. Turn the boat in 90-degree turns. If the motor loads or slows considerably then the pipe length is too short or the propeller is too big. If the motor slows a little or not at all then shorten the manifold slightly or try the next size up propeller
4. Shorten or lengthen the manifold until the best balance in performance is obtained between turns and straight-line speed.
This information can be found here:
http://www.nimrifmodels.co.uk/setup.html
Dan.