The Austin-Healey 3000 with a 3-liter inline six had split manifolds and exhaust pipes that entered a siamesed-pipe muffler at a distance that became a tuned-length header when the muffler was removed. In the days of SCCA and other club sponsored races, this was no accident. On a series of dyno runs we did with the Ford I mentioned previously the best low end torque performance was recorded with manifold and header pipe volume that matched, or was a multiple of the volume of one cylinder. This allows the entire cylinder volume to be contained in the pipe to provide the maximum scavenge effect from the volume and speed of the exhaust gases. At higher speeds, best horsepower was recorded with relatively short exhaust runners. Like every other modification I can think of, there's a trade-off between HP and torque. Increasing one reduces the other. "There ain't no free lunch!"
Jerry
