Header pipe length:

The stock header pipe is about 30". Multiple tests, made by several groups, confirm this length as being very nearly the best for all-round performance. Shorter (less than 27") and longer (over 32") header pipes significantly reduce peak power, throttle response and over-all performance. An exception to this "rule" are a couple of the high performance 2-into-1 systems which work very well with longer (and un-even) header pipe lengths. Stock Harley header pipes are near-perfect in diameter and length.



Source: http://www.mikuni.com/tg_poor_mid_range.html