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I think that it is affected (not determined) by the number of cylinders, the mass of the flywheel and the stroke.
• A 12 cylinder engine will be firing more than a 2 cylinder at a given rpm
• Ever hear a single cylinder John Deere tractor idle? Fires about twice a second
• Strokers have better low end torque
That said, air-cooled VWs like revs, because the volume of cooling air is related to rpm.
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