i think the TT pipes for triumphs are 38 inches by 1-1/2? can't remember. i'll have to measure. but i'm not convinced that the length of the pipe or the diameter are what's really happening.
my 1-5/8-inch by 32-inch long pipes have a nominal volume of 66.4 cubic inches, each.
a 1.5-inch by 38-inch pipe has a nominal volume of 67.2 cubic inches.
i suspect that a long skinny pipe will tune the same as a short fat one, all other things being equal. the speed of sound varies with density, and a fat pipe will have a less dense pulse moving more slowly than a skinny pipe. so the faster pulse in a long skinny pipe will return to the exhaust valve at the same moment as a slower pulse in a short fat pipe. ie they will tune to the same rpm.
more or less.