The 737 Classics had CFM56 turbofan engines. These engines were a very big improvement. They burned less fuel and made less noise. However, when the 737 is on the ground, the bottom of the plane is quite close to the ground. This meant that if the engines were not changed, they would be too close to the ground. Boeing and CFMI solved this by putting the engine in front of the wing, instead of below it. They also moved some parts of the engine to the side of the engine, instead of the bottom. These changes mean that the front of the 737 engines are not circles.