i got a 05 f150 5.4 with straight pipes off each cat 45 out the back, pretty loud but was wondering if i replaced my two cats for two high flows would it make it that much louder? Also, would two high flows cats plus say slp loudmouth 1s be louder than my current setup?

 

do u have any crossover pipe? (H or X pipe?) yes, high flow cats with make it louder, and free up some flow. slp loudmouths are just about AS loud as straight pipes, but a different tone. they are raspy, and wont snap crackle and pop as much as ur straight pipes. adding the lm1s and an H pipe will gain you back some low end tq that u lost with ur straights and no crossover, as well.