Question regarding CFM56-7B engine ratings

[ShaunSAP]

I'm trying to wrap my head around the various thrust ratings on the 737NG series. i read this post: 737-800-fde-improvements, post #24 by _Jx, which states the following thrust ratings:

7B27 = max SSL rating 27,300lbs @ 100.9% N1
7B26 = max SSL rating 26,300lbs @ 98.7% N1
7B24 = max SSL rating 24,200lbs @ 96.25% N1
7B22 = max SSL rating 22,700lbs @ 91.8% N1
7B20 = max SSL rating 20,600lbs @ 90.2% N1
7B18 = max SSL rating 18,500lbs @ 87.3% N1

He then states; "Contrary to popular belief, most thrust variants of an engine model are usually reduced throttle positions on the same engine."

I understand this, but what I don't understand is; for a 7B27 engine, 18,500lbs would be achieved at 87.3% N1. Let's say for example I'm flying a 737-600 equipped with 7B18 variant engines. Would its EICAS be locked at 87.3% maximum or be displayed on a caret like the image below, or would the software be adjusted so the 18,500lbs is now achieved at around 100% N1 like add-on aircraft seem to do?

 

[Tim Collins]

By 'reduced throttle position', effectively what is meant is that fuel flow is restricted. There is actually a great deal more to it than that, but a modern turbofan engine is controlled by a 'FADEC' (Full Authority Digital Engine Controller). It does exactly the same as the ECU in your car, and controls a number of variables to produce the intended result. From the flight deck, rated thrust for any variant of the engine will be produced at a given maximum N1 RPM. That will be shown essentially at 100% on the EICAS (there may be slight differences on different models of engine). I'm not qualified on any variety of 737 (I'm an Airbus man), so I'm not intimately familiar with any of the detailed engine readings, but essentially your second hypothesis is correct: 100% N1 is max power whichever variety of engine you are using.

 

[Heretic]

N1 indicates the fraction of the maximum rated engine RPM, which is constant across all engine ratings (see CFM-56B7 type certificate).
Derating is nothing more than telling the engine when to cap the fuel flow. Hence, the only difference between a 7B18 and a 7B27 is that the higher rating will yield higher N1 while moving the throttle lever on the lower rating ain't gonna produce extra fuel flow and thrust. So all you're gonna see for a 7B18 is 87.3% N1. If you take the exact same engine and program a higher rating, it will merily continue its life as a 7B27. AFAIK, 737NGs also have a "soft" derating, where you can limit a 7B24 to a 7B22 or 7B20 in the FMS as part of entering takeoff data.

However, the EECU might adjust throttle lever position sensing, so that full forward will always yield the max rated RPM, consequently offering finer thrust adjustment granularity on a 7B18 and lower granularity on a 7B27, effectively normalizting power input.
Same for an Airbus, where the throttle lever notches tell the computer what kind of power level you want, with the computer doing all the actual control work.