Problem here lies in the muffler. To really understand the sound you need to toss the muffler out. When you start dumping more fuel and air into the cylinder, it gets louder.
There are changes in the sound itself, but I am not sure if that is from a pitch shift, which i dont think it is, or rather from being able to hear more of the variety of tones the engine is producing.
If HITECH moves to the pitch change, that would be grossly inaccurate for the planes modeled in AH. The only plane without a variable pitch propellor that I know of in this plane set is the hurri 1, and ours may be a later model with the fancy prop.
I think the best answer would be to introduce a new sound on top of the engine sound which would produce the loud "BRAAAAAAAP" sound kids imitate, and mustangs make when you stomp on the gas.
Hell, when I think about, just imagine yourself when you were 6 years old trying to make car sounds, its not the sound getting louder as the imagined car speeds up (increased rpms) its the sound moving up in pitch.
Now think of that car with its wheels held tight and you flooring it, it will get loud, but not higher pitch.
talk about flogging a dead horse.