I ordered the 3" unit as the 5" would not fit. The inlet size was the one that matched the outlet of of the exhaust. I used the hose clamp to initially hold it on, but after a half-hour of stump griinding it fell off. VERY noticible increase in noise when that happened. I reattached it with a couple of screws until I can get a chance to weld it.
You install discs in this muffler according to HP. It uses 1 disc for each 3 HP. For my 22 HP PT, 7 discs would've probably been fine, but they come in packs of 3 so I used 9.
No perceptible change in power, VERY obvious change in noise. Sounds about like a normal riding mower now.
Phil