Though the H-pipe does not flow any significant amount of exhaust, it presence is nonetheless critical to efficient operation in dual-exhaust systems. These cross-overs perform a very important function in balancing exhaust flow and performance.
Hpipes are installed in a dual-exhaust system as close to the exhaust headers (manifolds) as is physically possible.
Dual-exhaust systems without cross-over pipes can build up enough back pressure to inhibit flow and reduce horsepower.
Hpipes do not actually carry much exhaust gas. These cross-overs are designed to give pressure pulses inside the system a place to go to keep them from inhibiting flow.
A close relative of the H-pipe is the X-pipe, which gives the exhaust a place to cross over smoothly and helps to increase flow.
Crossover pipes modify sound waves in the same way as they do other pressure waves. H-pipes tend to have a louder and more muscular sound, and X-pipes tend to emphasize mid-range notes and make an engine sound more exotic.
