Encased under glass and lit up like a museum piece, Audi’s ten-cylinder totem to human ingenuity looks as if it should be accompanied by a soft-spoken docent to narrate its very existence: At 610 horsepower, this naturally aspirated 5.2-liter V-10 is the most powerful engine ever put into production by Audi. It revs 20 percent quicker than its predecessor, spinning from idle to its 8500-rpm redline in 0.66 second.