F2 started to decline with the arrival of the 2.5 L Formula One in 1954 . A new format of Formula Two was introduced for 1957 with 1.5 L engines . F2 went into decline with the arrival of the 2.5 L F1 in 1954 ( with small-capacity sports car racing becoming particularly popular ) , but a new Formula Two was introduced for 1957 , for 1.5 L cars .