[man show]

I think generally a "bellmouth" downpipe begins as a 4" (measured lengthwise) oval opening at the turbo and then tapers down to 3" (or 2.5" or whatever size) where it connects to the cat back. I don't think there is such a thing as a 3" bellmouth because such a design would be a pretty significant restriction right at the turbo and very similar to the stock plate design. I'm only saying this because I measured my Cobb bellmouth dp before I put it on and that's what I saw.

Then again, I guess it's all about perspective. I could say I have a 4" bellmouth downpipe (since the bellmouth part of it is actually about 4" wide where it connects to the turbo) if I want it to sound big, or I could say I have a 3" downpipe since that's the general size of the pipe. I could also say it tapers from 4" down to 2.5" since that's what it actually does. No matter how you describe it, the bellmouth design is pretty good.