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I've got an 06 5.9 that has 5" straight pipe from the turbo back (no cat - no muf).
Here is what he sounded like running on the dyno a few months back DYNO RUN It is loud there is no question about that. There is a bit of a drone at freeway speeds. But it only bothers me when we have a trailer on the truck. Other than that it just sounds good
There was a slight improvement in HP / TQ over stock
I pulled:
HP Max - 276.4 HP Avg - 236.9
Torque Max - 534.7 Torque Avg - 482