When i wanted to weld on a straight pipesomeone told me that my 89 fleetwood needed the "back pressure" from my catalist converter. Is this true? if so then what is back pressure?

 

old wives' tales .. i cant believe that any mechanic is still walking around that believes that old myth.

the secret to more POWER is breathing better, inhale / exhale as best as possible.
you get plenty of flow trying to exhaust several liters of gas per second on some 2" pipe,
so less restrictions and bigger pipe is always better.