Why is there a crossover in the exhaust system or H pipes?

Question:

Please explain why a good dual exhaust system should have an interconnection between the two exhaust pipes; perhaps referred to as an “H” pipe. What purpose does that serve? What’s the disadvantage of just two separate pipes?

Answer:

After each cylinder fires, a hot charge of compressed air is forced out through the exhaust valve and down the exhaust pipe by its own pressure and the upcoming piston. Behind that is a relative vacuum that helps scavenge exhaust gasses further as the new fuel/air charge is being pulled in.

At low rpm the exhaust gasses have plenty of time to exit the engine and pass down the exhaust pipe without interfering with one another, but at high rpm, pressure in the system builds up and stifles performance and fuel economy. This is called backpressure, which is actually a misnomer. The real problem is resistance in the system caused by the muffler and catalytic converter plus the diameter of the pipes, and bends and kinks in the system. .

The standard V8-powered American classic works well at normal driving rpm with just one exhaust pipe and a two-barrel carburetor, but for people who want more performance, a four-barrel and dual exhaust pipes allow more horsepower at higher revs. Lowering exhaust backpressure helps engine efficiency—to a point. If the exhaust ran straight out of the exhaust port in the head with no exhaust pipe there would be virtually no scavenging effect because there would be no pulse of hot air followed by a vacuum.

So to answer your question: “Why a crossover pipe?’ It is there to increase efficiency and smoothness. As the alternating pulses go past the crossover the pressures equalize and the exhaust becomes steadier. Also, backpressure is lowered, with the result that the engine runs more efficiently, the exhaust note becomes quieter, and there is less vibration.