Some cars have dual exhaust systems.
An H exhaust pipe gets its name from its shape. It has right and left-hand pipes attached to a crossbar. The H pipe makes up a part of a dual exhaust system. As a part of this system, the H pipe helps to keep your car engine safe.
Structure
According to Walker Support's "Exhaust 101" article, the H pipe usually attaches to either a header pipe or a catalytic converter. Header pipes collect exhaust gasses from each cylinder in your car and direct them to the H pipe. Catalytic converters are emission control devices that reduce engine pollutants. The H pipe serves as one of the intermediate pipes in a dual exhaust system, helping to filter emissions and gasses away from the combustion chambers of your car.
Purpose
According to MOPAR Muscle Magazine's article on Dodge Challenger exhausts, the H pipe essentially tricks your car's dual exhaust system into thinking it is bigger. The pipe's two-armed structure sends excess pressure from gasses from one arm of the pipe to the other. It does this when there is no exhaust pulse on the second side, which builds up more pressure in the pipe than normal. In an H pipe, engine gasses follow the path of least resistance (straight through each pipe's arm) instead of going 90 degrees through each pipe and into the other half of the dual exhaust system. The H pipe's two-armed system also mimics your engine's cylinder banks and helps balance them, according to Street Directory's website. This is what makes the H pipe an effective part of a dual exhaust system.
The X Pipe
Newer cars with dual exhaust systems often have X pipes instead of H pipes. This is because the X pipe's design is simpler than that of the H pipe. The two arms can join at a single location, reducing noise and synchronizing the exhaust pulse of both engine cylinder banks instead of alternating between them.
According to MOPAR Magazine, H pipes are effective in dual exhaust systems, but X pipes improve throttle response time and smooth the exhaust flow. This also changes the noise from the deep growl the H pipe produces to a higher, smoother sound.
