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I checked my car's specs sheet this morning and in it I noticed:

Torque NM@rpm: 145@4600

What does NM@rpm mean?

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Newton meters of torque at 4600 RPM.

NM is the metric way of measuring torque similar to Foot Pounds being the SAE way.

The engine torque output changes with the RPM because the engine breaths differently over the RPM range.

  • But is there a specific reason why they chose 4600 RPM for documentation? How do they choose which RPM to document? – Jomar Sevillejo Feb 19 '16 at 0:08
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    @JomarSevillejo Theoretically the manufacturer listed the peak torque and the RPM it happens at. Without seeing the torque vs RPM curve it's only a guess. – vini_i Feb 19 '16 at 0:30
  • @JomarSevillejo when an RPM is quoted like that it is usually because that is the RPM at which maximum torque is produced – Zaid Feb 19 '16 at 12:04
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This is a typical torque diagram (this appears to be for an LS1 engine), this is in lb-ft instead of Nm but the idea is the same. The peak torque seem to be around 350lb-ft @ 4200 RPM. The peak power ~340hp @ 5500 RPM.

The hp (power) is a function of the torque and the revs per minute and a constant (which comes from pi, and minutes to second etc). power[hp] = torque[lb-ft] x RPM ÷ 5252 So the power at peak torque would be 350*4200/5252 = 280hp which seem to correspond to the diagram.

Realising that the power of the car is a curve and not just a single figure (like 250hp) will explain why different cars with the same power figures can have very different characteristics. For example an engine with higher low rev torque (and power) than the one in the diagram would be faster even though the peak power was the same.

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