The new 737 Classic series featured CFM56 turbofan engines , which had a better fuel economy and far less noise . However , it also posed an engineering challenge given the low ground clearance of the 737 . The 737 Classic series featured CFM56 turbofan engines , which yielded significant gains in fuel economy and a reduction in noise over the JT8D engines used on the -100 and -200 , but also posed an engineering challenge given the low ground clearance of the 737 .