I thought the 5.3 V8 was the only engine that had Displacement On Demand. Does the 3.9 really have it?
I think only the ones in the Monte Carlo and Impala have that feature, or at least was in the works. They do have the variable intake and rudimentary (compared to Japanese engines) VVT. And yes, the previous motors are HIGHLY bottled up. My Monte Carlo had the 3.4 and when I put dual Flowmasters on it, not only did it sound so much better, it got about 35 mpg on the highway. Gotta love that!