Boeing and CFM International have defined the fan size of the Leap-1B engine for the 737 MAX at 68.4 inches, a slight increase in diameter which will increase performance without negatively impacting weight and drag.

Revealing the upward revision of the fan diameter first set late in 2011, Boeing product development vice president Mike Bair says the new size is in the “sweet spot” of a three to four inch range of potential fan diameters. “At one end of the ‘bucket’ it gets lighter, because a bigger diameter will change the weight,” says Bair who adds “it’s a trade-off because bigger fans are quieter.”

Although Boeing aims to keep change to a minimum with the transition to the MAX to keep development costs down and leverage commonality with the current 737 production system, some systems will inevitably change, says Bair. The MAX configuration will include a digitally controlled pneumatic system in place of the current analog bleed system. This will provide “more precise control of the air bleed from the engines for de-icing and cabin pressurization.” The new model will also incorporate 787-like on board servers for improved maintenance control.

Bair’s comments, made at the International Society of Air Transport Aircraft Trading (Istat) meeting in Phoenix, came as the war of words over the relative merits of the A320NEO and 737MAX continues to intensify.

Both manufacturers argue that each new product has distinct fuel burn and operating cost advantages over the other - with wildly differing performance estimates of each others products. Airbus claims the NEO will deliver roughly double the improvement over the current A320 that the re-engined 737 MAX will deliver relative to the present 737 production standard. Boeing, by contrast, says the 737 MAX will have a 17% fuel burn advantage over the current A320, and a roughly 5% edge over the re-engined A320NEO with either the Pratt & Whitney or CFM International option.

Airbus’s senior vice president of leasing markets, Andrew Shankland, says the A320NEO is expected to produce overall performance benefits of 15% compared to the current A320 models. The engine is expected to contribute 15.3%, and the Sharklet winglets an additional 2.4%. The extra 1.8 tonnes in maximum weight added through the larger engines, strengthened structure and winglets will, however, reduce the overall improvement by 2.7%, rounding it down to around 15%.

Shankland also presented Airbus’s analysis of the 737 MAX and says the European manufacturer’s estimate of the performance of the re-engined 737 indicates a relatively modest 8% improvement over the baseline 737. The bulk of this, around 6%, will be derived from the improved core performance of the CFM Leap-1B engine, with a further 4% from the 24% increase in fan area that comes with a growth in diameter from 61 inches to 68.4 inches. A further 0.5% improvement will come from optimized wing-engine drag shaping, and an additional 0.5% in airframe drag reduction. However, Airbus predicts a 2.4 tonne weight penalty for the changes which it says will reduce the fuel burn improvement by around 3%.

“It’s remarkable how different physics are in Europe versus U.S.,” retorts Bair who reiterates points made at last year’s Paris air show about the lower weight of the 737 model versus the A320.