Is the factory a 2-1/2" size pipe. If yes, is it O.D. or I.D.? Thanks. Going to put Magnaflows on and they are I.D. Hoping the muffs will slide right over the factory pipes.

replacing the exhaust with anything other than factory exhaust could result in the check engine light coming on an staying on......
I know others have done it, not sure how they over came the computer reading an issue when the new exhaust was added.
Not sure on the answer to your inner or outer question though

I replaced my entire exhaust with headers all the way to the tips and no light.


On the diameter question, I'm sure what size the GXP splits to after the Y but it'* probably smaller than 2" (the pipe before the Y is only 2 1/4" or 2 1/2")

I cut my mufflers off and have been running straight pipes for almost 6 months now. No lights, and absolutely no drop in performance.

I think if you have a light come on something else was going wrong with the car already and the timing of the light coming on just happened to coincide with the exhaust change.

I put on Flowmaster V50'* 5 years ago, no probs