"Tuned length" refers to the total combined length of the exhaust header and tuned pipe from the face of the piston to the end point of the convergent cone. This is the factor that is generally calculated first in pipe design. The tuned length is a function of certain fixed engine parameters and more significantly, a target RPM for maximum performance. Of course, a designer can only choose one target RPM. One RPM equals one best-tuned length. For a maximum benefit over a greater range of RPM you must change the tuned length.