I have a few exhaust questions that I need answered as soon as possible.

I have a 2001 3.9L V6. I cut off the muffler, left the cat on, and am going to straight pipe.

What's the stock exhaust diameter?
I tried to measure and it seems like it's 2.5" off the cat, but then close to 3" from behind the muffler. However, when I measured, it's not exactly 3".

What exhaust diameter would give me the best sound/performance without sacrificing power?

And one more thing. I cut the muffler off after the cat, and when I started it up that day, the sound was very loud, "powerful", and kind of deep. Well, I ran it like that for a few days, and as the days went on, the quiter the sound got. Do you know the reason for this?

Thanks!

 

it should be 2.5inches, and any muffler will work for you just depends how loud you want it. FM, magnaflow, glasspack etc.

hearing loss

 

the fm10 wont be too loud but it sounds awesome. same with the flowmaster 40 series

 

I did the same to mine I just have a foot of pipe after the cat thats it and really you might think it got quiter but really thats just you getting used to it. The pipe diameter should be 2.5 the whole way back thats what mine is.