I see some questions on the site that talk about the relative positives and negatives of a high bypass turbofan (HB) vs a low bypass turbofan (LB). But I don't see anything explaining what actually makes a turbofan engine HB or LB? I assume it has to do with the ratio of air that is bypassing the compressor stages and just going through the fans. But what ratio makes an engine HB v. LB? And why was that ratio chosen?

Please note: I am not looking for usage cases, I am looking for differences in design specifications.

  • $\begingroup$ May be related to the development of higher bypass ratios $\endgroup$
    – fooot
    Apr 6, 2016 at 15:10
  • $\begingroup$ Something on the order of between 4 to 5 to one is the limit between high and low bypass. I just did some Google search on several engines and that is what I came up with. $\endgroup$ Apr 6, 2016 at 15:11

3 Answers 3


The first turbofan, the RR Conway, had just a bypass ratio of 0.25. It was designed for installation into the wing root of airplanes like the HP Victor bomber or the Vickers V-1000 transport project, so RR strived to keep the diameter small. Of course, back then nobody distinguished low- and high-bypass engines. The P&W JT-3D, which powered most of the second generation of jet airliners (Boeing 707 and DC-8), had a bypass ratio of less than 1.5.

But when the new generation of engines (GE CF-6, JT-9D) with bypass ratios of around 5 were developed for the first generation of wide body jets, marketing needed a term to make clear that this was a new generation of engines, and the high-bypass ratio engine category was born. Generally, the threshold is around 5, but note that some CF-6 versions have bypass ratios as low as 4.24 and still fall into the high bypass ratio category. Therefore, Wikipedia sets the limit at a bypass ratio of 4.

There is no technical difference between a low- and a high-bypass engine, and the limit is arbitrary. The jump in bypass ratio from 1.5 to 5 brought significant improvements in specific fuel consumption and noise reduction, but part of the technical progress was also from better materials and aerodynamics, which allowed higher compression ratios and turbine entry temperatures.

Now, almost 50 years later, we see the first engines with bypass ratios above 10 entering service. Designs are being readied with bypass ratios of 12, so marketing will need a new word to describe those creations. With "ultra high bypass" already taken by the unducted fan crowd, the search is on.

  • $\begingroup$ So there's no structural difference you can point at (other than the arbitrary 4:1 ratio), that might easily distinguish between the two? It's really just a loose marketing term with no meaning rooted in actual engineering? $\endgroup$
    – Jay Carr
    Apr 6, 2016 at 16:17
  • $\begingroup$ @JayCarr: No, even most low-bypass ratio engines use two spools, and a few with bypass ratios above 4 have only a single spool (Lycoming ALF502) $\endgroup$ Apr 6, 2016 at 16:19
  • $\begingroup$ @JayCarr: The new generation in the mid- to end-sixties had a substantially lower specific fuel consumption due to the higher bypass ratio, so it is not all marketing. But, yes, the term is an arbitrary definition, and some fuel saving was also due to better materials, better internal design and so on which also benefited the low-bypass engine crowd (just compare the P&W F100 with the J79 or even the J57). Remember that the J57 became the JT-3D when the fan was added ... $\endgroup$ Apr 6, 2016 at 16:25
  • 3
    $\begingroup$ They can't jump to Ultra High Bypass! They skipped Very High Bypass! $\endgroup$
    – TomMcW
    Apr 6, 2016 at 17:12
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    $\begingroup$ @ROIMaison: Yes, indeed Wikipedia has a medium bypass category, but this is not widely used - such fine grained differences are of limited usefulness. Why not refine further into low medium and high medium? At some point it becomes pointless. $\endgroup$ Apr 7, 2016 at 10:00

There is no particular ratio at which the engine becomes a 'high' bypass, though the it is generally taken to be around ~5:1. The reasons are more historic than technical.

The first large scale development of the high-bypass turbofans was spurred by the USAF's CX-HLS competition, which lead to the C-5 Galaxy. The requirements of the competition (range and fuel efficiency) lead to the development of a new generation of engine by GE and PW, which developed the TF39 and JTF14E respectively. GE won the contract with the TF39 engine, which had a previously unheard of 8:1 bypass ratio.

PW figured out that it better develop a high bypass engine itself or lose the market and developed the JT9D, which had a 5:1 bypass ratio. This engine was the first widely used engine with high bypass ratio (the previous engine had bypass ratios in the range of ~1 or so) and necessitated a special mention, which came to be called high-bypass engines, once it became widely used, especially in the Boeing 747 (Ironically, it was the two losers of the CX-HLS competition PW and Boeing, which won the immediate race in civil aircraft).

Joe Sutter acknowledged the effect of the CX competition in the development of 747:

I should add that fostering large high-bypass engines was all that the USAF C-5 competition contributed to the Boeing 747, as my new airplane would be called.

Not to be left behind, GE developed the CF6 series with bypass ratios of 6:1 and RR developed the RB211-22 which had a bypass ratio of ~5:1. Due to these, the ratio of around 5:1 (or >4:1) is usually taken as the dividing line between the 'high' and 'low' bypass ratio turbofans.


"Bypass" in this context is how much air transits the actual fan stage and is used to directly generate thrust, vs. how much air transits the fan stage and moves into the compressor stage to be burned in the engine.

Low-bypass turbofans don't use a lot of air for thrust; most of the air that transits the fan stage is routed into the engine, compressed, combusted, and exhausted like a standard turbojet. High-bypass turbofans route more air outside the combustion core, so the spinning fan generates most of the thrust (rather than the actual engine). In this case, the turbojet core of the engine is used (mostly) to generate engine rotation, and the fans generate most of the thrust.

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    $\begingroup$ The question does not ask for the physical differences between HP and LP turbofans. It asks for the mathematical ratio that puts and engine into the HP or LP category. $\endgroup$ Apr 6, 2016 at 21:53

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