The crossover pipes just help equalize the exhaust. Not so much back pressure, more of a exhaust back up or traffic jam. If one muffler is full, the exhaust can get out though the the other side by way of a crossover. Since the exhaust is really just a bunch of pulses or burst of hot exhaust gases, the X-pipe type crossovers can actually pull exhaust from one side to the other due to high and low pressures of a exhaust pulses.
Normal driving won't have much effect on the crossovers other then noise, they tend to take out some of the harshness. Nascar really started the crossover use, the high rpm engines really like them.
Joe
