CFM International is busy developing the Boeing 737 MAX’s version of its Leap turbofan and is zeroing in on specifications. Critically, the Leap-1B will have a fan diameter somewhere between the current CFM56-7’s 61 inches and the Leap-1A’s (for the Airbus A320neo) 78 inches.

As Jean-Paul Ebanga, CEO of the Snecma/General Electric joint venture, told AIN, the fan will be scaled down from that of the -1A. As a result, the bypass ratio will be smaller, which is usually not favorable to low specific fuel consumption (SFC, measured at the engine level). But this should be partially offset. “A smaller-diameter engine will cause less drag and will be lighter,” Ebanga said, pointing to the fuel burn benefits that this will bring.

The high-pressure turbine will have two stages, again to answer the airframer’s needs in fuel burn. This will reflect the architecture of the eCore2 core engine GE is testing. “We are refining the core’s configuration; the -1A and the -1B will have the latest,” Ebanga said.

The first test for the full Leap-1B engine is scheduled for 2014. Trials will take place both in Villaroche, France, and Peebles, Ohio. Certification is then pegged for 2016. The engine will start commercial service the year after.

CFM is pledging the aircraft’s fuel burn will be reduced by 15 percent thanks to the new engines. In addition, nitrous oxide (NOx) emissions should be cut by 15 percent. The noise footprint reduction target is 75 percent. Meanwhile, thrust levels will be “similar” to those found on the CFM56-7, Ebanga said.

How will CFM’s design engineers fit a bigger engine under the 737’s wing? There are at least two ways, Ebanga said: first, by improving engine-wing integration, and second, by adding a hydraulically extensible landing gear strut.

GE and Snecma will share the work on the Leap-1B. GE is responsible for the core engine and Snecma is in charge of the low-pressure spool. Engine control and other equipment is shared, too. For example, GE is in charge of the full authority digital engine control, but the unit will be made by a Safran-BAE Systems joint venture.

The Leap-1B will be the only engine available for the 737 MAX.