I removed the engine cover (black plastic with silver stripes) on my 6.0L V12 to do some maintenance/tuning. During road testing, really liked the slightly louder growl of the engine when you step on it. Any reason not to leave the cover off?

 

You might be imagining the difference in sound.
Either that or it was something else you did, like leaving the air box cover loose or misaligned.

 

No reason not to leave it off. You are not imagining the noisier sound. Diesel engines have covers for noise suppression reasons.

 

One good reason might be the chance you might misplace it. The 6L cover is apparently hard to replace.

 

Reason to leave it off -- the Valley of Death gets hot and I imagine that cover only makes things worse.

 

You can leave it off, but keep the cover. They are no longer made and if you ever want to sell the car it's a plus to have the cover and it's attaching screws - they are also no longer available.

 

I originally removed the cover to check the TPS, had "hesitant" acceleration to the 1500 RPM range, at 2000 the engine and trans seemed to smooth out and take off much better.

Turns out the TPS was set to .34 volts, and not to the .60 volts required by the six wire red TPS in the 6.0L V12. Made the adjustment, and that seemed to help a lot. Encouraged, I tackled the throttle linkage, and that further smoothed out both my idle and my acceleration.

The engine is much quieter overall, especially at idle. So I'm pretty sure the growl I hear at higher RPMs is due to the cover being removed.

I have the coverstored away with its four springed screws in a safe place. Need to give it a good cleaning and repaint the silver stripes in case I want to replace it for show.