I have some direct experience here...

About a month ago I put on a 17733 Dynomax Super Turbo, with 2.5 inch pipe from the Y back. I found it to be subjectively too loud, especially at around 2500 rpm (highway speed). I think the loudness was similar to a PV, which I heard on some VXs at Moab last spring, although the Dynomax tone is somewhat different.

So, I switched to a 17748. This is essentially the same design as the 33, but 6" longer body for about 43% more internal volume. (FWIW the 48 is called a "hemi" muffler, and was intended for big blocks such as on motorhomes and the like).

The 48 is perfect for me, a mellow throb at idle, a nice growl upon accel, and no drone on the highway. I like it a lot. <img src="/forums/images/graemlins/notooth.gif" alt="" />

You would want at least a 2.25" pipe, as that is the minimum internal diameter of the muffler. I'm not sure if 2.5" gains you anything over 2.25". The guy who did the work said the inlet pipe from the Y to the muffler was 2.25", but the tailpipe was 2".