All V-8 engines with 90-degree cranks have an imbalance in the firing order between the right and left cylinder banks. The firing order imbalance is a necessary sacrifice so that a dynamic balance of the rotating and reciprocating assembly (crank, rods and pistons) can be maintained. Each time the firing order is complete (two full revolutions of the crank), two cylinders within each bank fire and exhaust within 90 degrees of each other. Because of this, this pair of cylinders will be exhausting into the header collector (on one bank) almost simultaneously. This overlap creates a back pressure. Meanwhile, the header on the opposite bank has no activity (or pressure) in it at all. Conventional full-length four-tube headers help separate the exhaust in the cylinders until it reaches the collector. After the header primary tubes dump into the collectors, the two cylinders close to each other in the firing sequence again are fighting for space in both the collector and exhaust pipe. This results in reflected pressure waves traveling backwards though the exhaust system. What you get is more backpressure, diminished power and fuel economy, and the V-8's characteristic growl......

....Some people have found small low and mid-range torque improvements using an H-pipe. But at high RPM, the gases cannot bleed across the H-pipe fast enough to help power very much. The double cross, X-crossover, is an improvement beyond the H-pipe, but does not resynchronize pulsing and pressure spikes completely.