Funny, you make the comment about the the echo and turning heads a block away. My wife used to say the same thing when I would return home after a romp in my '67 E-Type. Need to find my sound clip.
These are her muffs......
Found it....

 

I faintly remember my 66 E and back when I owned it there wasn't any digital cam/recorders nor was there youtube. Thanks for the memories.

 

What a car. Love the sound too. One day, I have a nice spot in the garage waiting for one of these.

 

Would it be worthwhile to do the x-pipe modification on a n/a 4.2 engine or is it best only on supercharged engines?

 

An X-pipe would benefit a N/a or Supercharged engine design in the upper RPM's.
There has been a fair amount of testing that justifies this. HotRod magazine has conducted a few very well documented studies on this.
When the x-pipe is located properly and a proper design, there will be benefits.

Link:
Dodge Challenger Exhaust Install - Mopar Muscle Magazine

Regard,
D. Fricke

 

Well I fitted the Adamesh x pipe kit. Not much louder except when giving it the beans. Bit more volum, but not hugely noticeable to me, but then I had the Quicksilver Supersport on the Vantage and that has spoiled me, even if the neighbours are grateful now.

 

I had the magna-flow x-pipe fitted to my xk 4.2 and completely disliked the sound it was very raspy to the point of not wanting to drive it anymore . After looking through eBay I found a 4.2 xkr quad system which I jumped on straight away . I couldn't be more happy with the sound and have fallen back in love with the car . If you want the extra sound try and locate the xkr rear box it's money well spent