Hey guys, how about kicking this around a bit...specially you Fred, with your experience in the diesel performance area.......
Just for the sake of this discussion lets diregard the very cool sound you can get with upgrade exhaust systems ok...:D

The larger exhaust systems are used to reduce or eliminate all the kinks and bottle necks in the piping/exhuast flow (as it expands and passes) so that the exhaust moves quickly and efficiently out...
Now my thinking (and my understanding) is this...as long as the volume of exhaust being produced or, expected to be produced, is not an "overload" in terms of volume, the 'overly large' exhaust is not needed.
All you need is the ability to move the exhaust out and without restrictions or bottle necks...yes...???

Now, throw into this the idea of exhaust velocity.....

Take the inlet to a turbo exhaust housing here...particularly when you port it's inlet....you don't want to over-do this process because you can and, actually do, reduce the exhaust gas velocity as it enters the 'hot end'. Yes, you derive your turbo spoolup from the thermal energy but you also benefit from the velocity of those hot gases right
We often do this porting or at least gasket match the inlet to our exhaust housings to reduce/eliminate the "backup" or delaying of the exhaust passing thru the housing....

Ok, so, if you port your hot end just right and, you have an exhaust sytem that provides just a hint of backpressure (to hold the hot gases just enough to transer their heat/velocity) you stand to benefit from this "tuned and/or complimentary" combination....???

BTW, if HDM48 is around maybe he can jump in here to address the issue of 'loss of velocity' as it pertains to diesel/turbo charged engines...HEY DAVE.. !!!!!!.....

Right now, many of the guys running twin turbo systems and lots of fuel are using a 5" exhaust system on their trucks....for many reasons....one is the outlet of the final turbo happens to be 5".....kinda a practical reason for the larger diameter pipe I'd say...right..:D
Keep in mind that these trucks (including the 1st gen guys) are usually very heavily fueled and, they run higher boost pressures as well...


So, the question is this (and this goes along with the question asked about exhuast diameter(s).....
"...Is there a "good" or "bad" size of piping to use on our trucks based on fueling/performance setups etc. ?????...."

bob.....:D:D