I have a flow thru muffler and then i have one of them pipes with the twisted deal down the center, so i can always run that too. Only thing is with that i know for sure that it was a restriction when i had compounds on my 6.4
The Moroso/FTE auger style muffler/resonators are a restriction no doubt...
Do you want to kill/cancel the turbine whistle sound as well as toning down the overall volume or do you want turbine noise? Or do you not care either way?
are you running full 4"? full 5"? 4" into stock 3.5"?
My recommendations will change for each scenario, but they will follow a similar trend in that I'm going to recommend a smaller muffler/resonator up front and a bigger one back near the tailpipe.
Highest flow with decent sound suppression and almost complete abatement of turbine sound I would do something like a Dynomax 17537 as soon as possible after the downpipe. It's a 4" in/out 6" diameter with a 22" long body.
Then in the traditional muffler position, do a 4" packed style non-louvered muffler. Biggest one you can find.
SLIGHT reduction in flow with better sound abatement, but less turbine abatement would be the Donaldson M085171. But the packed Dynomax up front will eat up most if not all of the turbine sound.
Or you could sub the M085171 for the water heater M090544 for even more silencing with minimal flow impact. It's a long son of a gun though. almost 4ft long body length with a 9" diameter vs. a little over 30" long body w/ 8.5" dia on the M085171.
If you want all the turbine, the M085171 out back and possibly a small non-packed resonator up front. There are outfits that make 12" or so long perforated tube muffler/resonators with no packing.
Push comes to shove, you can take something like a 5.9 CR cat and hollow it out and integrate it. A hollow expansion chamber acts as a resonator and as long as it has a nice taper on the rear, it should flow fine. It's not quite as effective as a perforated tube style, but it is effective.
HTH
