I am wanting the best flowing one. Also which would be louder w/o cats through headers? lt1 car btw

 

Both are good flowing catbacks and should provide decent gains. I'm guessing the SLP will be louder and raspier without a cat in place. On my old setup, I had no cats but had the Maganflow catback and when I had cats re-installed, there was no difference at all in tone.

 

I have a 93 LT1 6speed shorty header no cat, the magnaflow is definitly quiter i had SLP's before. Even though you can see straight through the Magnaflow muffler its definilty quiter.