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Looking at the stats breakdown on Wikipedia here for the Merlin 61 series reciprocating engines used in Spitfire planes, I noticed that maximum horsepower is given at approximately 3000 RPM. But AirPages has what looks like a manual here that lists the maximum RPM as 3000.

Are they simply stating that the same thing: that torque starts to fall off at 3000 RPM? Or are they saying that the engine is just physically incapably of spinning faster than 3000 RPM (seems unlikely)?

In the case of the first one, what is the (approximate) redline for one of these engines?

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  • $\begingroup$ Power is torque x RPM and goes up linearly with engine speed. And the limit speed of the Merlin was indeed 3000 RPM, which was quite fast for its time. $\endgroup$ – Peter Kämpf Jul 24 '16 at 12:32
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Power output of a reciprocating engine goes up with speed, and yes, maximum power is reached at maximum RPM. Considering the size and age of the Merlin 61 (27 liters displacement) it had a rather high maximum speed. Some contemporary engines ran even slower:

Note that the considerably smaller GA engines still only allow a maximum speed of 2700 RPM. The smaller the engine becomes, the higher the maximum RPM will be. Model engines for RC racing easily top 50.000 RPM.

Age is also a factor. Modern engines run much faster, and the early rotary engines of WW I would run 1350 RPM, max. The radial engine of the Boeing Stearman which I once flew was limited at 1700 RPM, requiring me to throttle back when accelerating for a loop-the-loop so I would not overspeed the engine in the dive.

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Maximum RPM and Red Line RPM are the same thing.

They can be exceeded but not without the chance of engine damage.

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  • $\begingroup$ What is confusing to me is that the red line RPM would be the same as the RPM at which the maximum horsepower is produced. Compared to a car engine, for example, where the maximum horsepower may be produced at something like 3000 RPM, but the red line RPM would be something like 6000 RPM $\endgroup$ – Habitablaba May 27 '16 at 18:34
  • $\begingroup$ I suppose that is because aircraft engines have to be more reliable than other types of engines. $\endgroup$ – Mike Sowsun May 27 '16 at 18:39
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    $\begingroup$ @Habitablaba; You are confusing torque with power. Torque might peak at 3000 RPM, but power goes up until you hit max. RPM. $\endgroup$ – Peter Kämpf Jul 24 '16 at 12:34

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