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It is said that when the Chevy SB V8 came out 1955 model year & was taken to the drag strip, they called it a mouse motor. When it won, they called it Mighty Mouse. The Mighty is understood. The BBC was deserving of rat, i.e. a big mouse.
Even much later had an interesting reaction. After shutting down everyone in Galveston (they did not know me there ) w/ my cousin riding, she said she had to see the engine. She was accustomed to fast cars, drove fast, had a boyfriend w/ a modified GTO, etc. She was shocked at the small size of Mighty Mouse. Later ran a Mighty Rat in the other Hot Rod.
The Complete CHEVROLET Book by Petersen Publishing Co. 1970 p.5
"And, believe it or not, 20 years of Ford dominance in the American youth market melted away almost overnight. The kids started buying V-8 Chevys en masse as early as the spring of 1955."
"And the rest is history. Within weeks the new V-8 Chevys were moping up their stock classes on the drag strips- and running away from the Fords and Olds 88's at the stoplights."
My understanding of the RAT nickname.....because the valves on the big block heads are not in line, they stick out a various angles, they were called porcupine heads. Because the porcupine is part of the rodent family..... Since the small blocks are smaller than the big blocks, and mice are smaller than rats.....