Boeing has confirmed a roughly 2.5cm (1in) growth in the diameter of the CFM International Leap-1B turbofan for the 737 Max.
The airframer and engine maker will continue to make "little tweaks" to CFM's new powerplant as they seek to optimise fuel efficiency within the ground clearance limits imposed by the 737 wing.
The latest change increases diameter of the Snecma-designed fan of the Leap-1B from 173.7cm to about 175cm, but the overall width of the nacelle will remain the same, Boeing says.
CFM made a similar change last year for another version of the Leap engine selected to power the Airbus A320neo. The diameter of the Leap-1A grew about 5cm to 198cm, which was still nearly 8cm less than the 205.7cm diameter of the competing PW1100 geared turbofan.
For the 737 Max, Boeing anticipated fan diameter growth for the Leap engine, and may still have room to grow further. The Leap-1B is 15.2cm wider than the 160cm-diameter fan of the CFM56 installed on the 737 Next Generation. To accommodate the larger turbofan, Boeing extended the nose gear of the 737 Max by 20.3cm, or 5.1cm larger than the new size of the Leap-1B.
Boeing has targeted a 13% specific fuel burn reduction for the 737 Max compared to the previous model, though Airbus argues that the real number should be smaller.
However, Boeing notes that the lighter airframe of the 737 Max can be more efficient with a small engine fan diameter than the A320neo.
Boeing and CFM plan to complete the firm configuration of the Leap-1B in 2013.