Johnny, 3.25 inches for a Titan on up to 36 lbs of thrust. 3.5 inches diameter up to 40 lbs of thrust. Those are rule of thumb.

I have a p-200 pipe in my motorhome, I'm picking it up today so if you call me tomorrow morning I can give you a dimension on that.

I don't know what airplane your are upgrading, but chances are you could call Tam and tell him what engine you are putting in it and he could probably tell you exactly what you need and sell it to you to boot.

Unless you guys are fabricating your own, if so, you could go by what I wrote above.

If you taper the back of the pipe 5% or so off the front diameter you'll get some extra blow for free.