So I was doing some research on the Boeing 747-8/4 engines. I discovered that the PW4056/62 (747-400) has a much lower bypass ratio and weighs a lot less, and it is much smaller than the GEnx-2B67 (747-8). So it really seems the PW4084 is the more efficient engine. If it is not true than what improvements have been made on the GEnx-2B67 to make it the more efficient engine? GEnx-2B67 engines on 747-8IGEnx-2B67 engines

PW4084 engines on 747-400PW4056/62 engines

  • $\begingroup$ Related: What prevent jet engines design with bigger by-pass ratio? $\endgroup$
    – fooot
    Sep 4, 2015 at 23:52
  • $\begingroup$ Efficiency, of any machine for any purpose, means the ratio of useful work done to energy used (which for engine means fuel burnt). Weight or size of the engine has nothing to do with it. $\endgroup$
    – Jan Hudec
    Oct 3, 2019 at 17:51

2 Answers 2


In most of the engine performance parameters, the GE GEnx-2B67 is a better and more efficient engine compared to the Pratt & Whitney 4084. There are multiple reasons for this.

  • Fuel Consumption: This is the parameter of the engine airlines are most interested in, as it affects their bottom lines. The Thrust Specific fuel consumption gives the fuel consumed by the engine for unit thrust produced. In this case, the GEnx has over 20% improvement over the PW 4000 series (0.27 versus 0.348).
  • Bypass Ratio: In general, a higher bypass ratio means a more efficient engine. The GEnx has a much better bypass ratio compared to the PW 4084 (8.6:1 versus 5.3:1).
  • Engine Noise: The GEnx engines are specifically designed for reduced noise levels, incorporating the Chevrons.
  • Thrust to Weight Ratio: This is a much better parameter compared to the weight alone for engine performance as the weight increases due to larger bypass fans, for example. In this case, both the engines have almost the same values (5.5 and 6).
  • Thrust: Higher thrust means improved aircraft performance. The PW 4062 produces a maximum of 62,000lb thrust while the GEnx produces over 67,000 lbs of thrust.
  • ETOPS: Another advantage of GEnx engine is that it is certified for Extended-range Twin-engine Operational Performance Standards (ETOPS), (in Boeing 747-8) which indicates that the reliability of engine is high. The PW 4062 engines used in 747-400 does not have this certification (Engine is only a part, albeit important of ETOPS certification though)

The engine used in the 747-400 is the PW 4056 and 4062. PW 4084 is used exclusively in Boeing 777.


Yes. According to GE, the GEnx-2B consumes 15% less fuel than the CF6, which was contemporary to the PW4084 on the 747-400.

The bypass ratio of a jet engine makes it more efficient, not less. A higher bypass ratio provides more thrust from the fan air, which means the core section can use less air and therefore less fuel to provide less thrust. This increase in efficiency is the reason turbojets evolved into turbofans and have been increasing in bypass ratio as technology improves.

One consequence of a higher bypass ratio means the engine does get larger and heavier. However, the increase in efficiency is more than enough to offset these costs.

Besides increasing the bypass ratio, there have been many internal improvements in the GEnx line of engines, discussed in the GE link above. These changes include composite fan blades and fan case, which can be lighter than the metal alternatives. Throughout the engine there are also aerodynamic improvements to increase the efficiency of air flow through the engine, as well as more new materials and manufacturing technology.


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