If theres an interest in the engineering and math behind this, I actually have a book from 1962 that details the tuning of intake and exhauset tubes based on RPM's and displacement, and it discusses 2 and 4 stroke, small dirt bike to full size car, na and blown engines. I built a few engines with this information in the early 90's.
There will be a "sweet spot" rpm range where the engine will indeed feel much more responsive and powerful, and if you get too far above or below this range, it will indeed fall flat on its face, just as Andrew learned. The other side is that in the tuned range, the engine will have (iirc) in the neighborhood of 25% more torque.
I am extending my bikes wheelbase by 20" right now, and when that is done, will be installing a 50cc 2 stroke on it, with a goal of 25mph max, 180mpg, and able to power up anyhill in town. To do that, I'll need all the help I can get, so I will for sure be using the tuning info in my little book.
In case anyone wants to find their own copy at a used book store, the title is "Scientific Design of Exhaust and Intake Systems" and the credited author is Philip H. Smith. It was published by Robert Bently, Inc.
There is really a lot to this. I remember spending the better part of a months worth of evenings reading and studying this book! And here I sit, 15 years later with barely a memory of the details! Ugh time is a killer!
Maybe I'll build some tuned manifolds and see what I can sell them for!
So, If there is any interest, I'll be happy to pull out the relevant formulas and definitions for calculating intake and exhaust pipe lengths and post them here.
I looked at the calculator link that was posted, and the problem I see there is that thers no variable for displacement, which is a pretty important variable. Also, there isnt a mention of tube diameter. I do remember that the tube length changed for a givem rpm if the tube diameter changed, so that calculator is a good start, but for as small as our engines are, it wouldn't be very accurate, IMHO.
