Editing Turbofan (section)

===Boosted two-spool===
Higher overall pressure ratios can be achieved by either raising the HP compressor pressure ratio or adding compressor (non-bypass) stages to the LP spool, between the fan and the HP compressor, to boost the latter. All of the large American turbofans (e.g. [[General Electric CF6]], [[GE90]], [[General Electric GE9X|GE9X]] and [[GEnx]] plus [[Pratt & Whitney JT9D]] and [[Pratt & Whitney PW4000|PW4000]]) use booster stages. The Rolls-Royce BR715 is another example. The high bypass ratios used in modern civil turbofans tend to reduce the relative diameter of the booster stages, reducing their mean tip speed. Consequently, more booster stages are required to develop the necessary pressure rise.