Originally Posted by AIR RAM
What they had lost was velocity. Velocity through the system is needed to keep the exhaust gasses flowing through the system... with high velocity you create vacuum... other wise know as the "scavenging effect". If you go too large on the ID of the system for the amount of air the engine can pump, you lose the velocity.... NOT BACK PRESSURE.
Agree 100%. 2.5" to a 3" exhaust is the culprit. Exhaust velocity was lost, resulting in lost low end torque.