I wonder about this. If you turn on AWD then you will hear the engine scream under heavy load easily, but if you turn it off, it wont scream as easily. My guess is that when you turn off AWD you will just lose that extra power instead of it redirecting to the rear wheels. Is this right?

Also, what happens irl? My guess is that the power goes all to the back but I dont know for sure.

I ask because sometimes you might want to disable the AWD in exchange of giving the rear wheels the extra power when you have the front wheels barely touching the ground, but if the power doesnt get redistributed then its useless doing this.

The way the engine works in this game is the "engine tension" system. The sound relates directly to the load the engine is under, not the RPM or throttle position. Not entirely realistic, but that's how it's coded. When not in AWD mode, the engine may be spinning the drive wheels, and thus, are not under full load.