Both H and X pipes, are used to equalize the exhaust pulsation between the cylinder banks. This usually tones down the lumpiness at idle to some degree. The equalization of pressure between the banks helps with cylinder scavenging on the port and can help contribute to better cylinder filling. Generally speaking an H pipe provides better low end torque, while X provides better mid range and top end. H pipe gives classic muscle sound while X pipe is a little raspier tuned sound. I run a 3" TTI X pipe on my W2 head 69 Swingers 408.
