X pipes have come from racing. The design improves power and Torque (slightly) by increasing efficiency. The cross-over allows the existing escaping exhaust gases to work in tandem in order to scavenge the next cylinder as the exhaust valve opens. Leaving the cylinder more empty, ready for the next compression stroke.

Straight pipes are best ONLY on huge HP motors. They are unrestrictive and allow for a cleaner flow of gases. The X allows the motor to work more efficiently at all RPM on a motor with more realistic levels of HP.