Created with Sketch. Created with Sketch.

RPM Calculator

An alternator needs 1800-2000 RPM at idle to make power and will make maximum output at 6000 rpm but will fail at 16,000 RPM

Measure Alternator pulley size (for example 2.5")

Measure Crankshaft Pulley Size (for example 5")

The Ratio is 2:1 (5 divided by 2.5 = 2)

To find MAX RPM: take your Max engine Speed (for example 5800 RPM) and multiply it by the calculated ratio (5800 x 2) = 11,600 shaft RPM

To find IDLE RPM: Multiply Idle RPM x Ratio (for example 1000 x 2) = 2000 shaft RPM @ Idle