From what I hear & read the term came up back in the day cause of the drag track. Chevy had a small block "The Mouse", when Dodge came up with the 426 Big Block Hemi they called it the "Elephant", size of the block & valve covers. This term was used cause the elephant was scared of the mouse. when Chevy came up with the 396, 427 & the 454, they were considered a large mouse so the term "Rat" came into play.
How true is this ? Did the "RAT" letters stand for something?