H-pipes are aftermarket exhaust components designed to improve exhaust flow in performance applications. They replace the restrictive factory catalytic converter and typically feature two inlets and two outlets that merge into a single pipe. This design helps reduce exhaust pressure and allows the engine to breathe more efficiently, potentially resulting in increased horsepower and torque. When choosing H-pipes, consider material (stainless steel for durability) and diameter (to match your existing exhaust system) for optimal performance.