I believe the main difference in the idle characteristics of a twincam vrs shovels/evo's is the older engines have a longer stroke with a smaller bore. The twincam has the shorter stroke and larger bore. I'm talking carburated bikes only.

 

has to do with the domed piston and hemi head on pre EVO engines....they snarl.

 

Heads, port design and cams all make a difference in the sound. Pontiac and Chevrolet have the same firing orders and some engines are very close bore/stroke wise but sound nothing alike. And I still want a shovel as my second bike!!!!!