Someone correct me if i'm wrong, but my understanding is that the engines are not "detuned" per say, but not rated as highly due to EPA.
my understanding was vehicles over a certain GVWR had to pass a drive by sound test with the EPA. the larger vehicles, 1 tons and i believe the Megacabs also fall in this category, have to be below a set DB rating.
To meet this DB rating, the engines must be ran at a lower RPM than peak power to not exceed the DB allowed. This means the engines are rated at the max power they can make within the DB allowed.
Read where chevy just increased the rated numbers on its 3500 6.0l vortex because of increased sound insulation meaning they could run the engine to max power and still meet the EPA DB levels.
my guess is that the numbers listed for the 6.4 where it appears to be "detuned" actually mean that's the max power the engine will make and not exceed the DB. The engines truly make the same power as the other 6.4 engines, they just don't list that as the rated numbers.
basically, my understanding is that it they added some more sound deadening material to the engine bay, they would rate the larger 6.4's (1 ton and mega cabs) at the same higher hp/tq numbers.
open for spears if this isn't correct.....
