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Message: [QUOTE="Charles, post: 37294, member: 103"] The 42 is simply way too small to act as a fiirst stage charger on one of these engines. The GTP38 would have been fine with a good gate and at [i]least[/i] a GT45. People just don't understand that a compressor must be much, much bigger to provide the same flow at a fraction of the pressure like what a first stage must do. Instead of running a PR of 4.5 to 5.0 or so like it would as a single, it must move that same, or more air at a PR of ~2.0 to 2.5 or so. Here.... This is a GT4202 compressor map: [IMG]http://inlinethumb45.webshots.com/44652/2787041310082519711S600x600Q85.jpg[/IMG] The Green dot is running ~47lbs of boost while moving just over 90lbs/min. The Red dot is running ~20lbs of boost and moving the same flow. At 47lbs of boost that compressor will move that air with an efficiency of ~73%. At 20lbs of boost it is below the lowest listed value of 65% and technically beyond "choke" for that wheel. In contrast.... look at that same point... just over 90lbs/min and a PR between 2 and 2.5 on this GT45 compressor map... [IMG]http://inlinethumb30.webshots.com/33053/2066327120082519711S600x600Q85.jpg[/IMG] Bingo... We just figured out why the 42 isn't large enough for a first stage. [/QUOTE]

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