RPM Calculator

RPM Calculator

An alternator needs 1800-2000 RPM at idle (shaft speed not engine) to make power and an alternator will make maximum output at 6000 shaft rpm but will fail at 16,000 RPM or higher

Measure Alternator pulley size (for example 2.5") ______

Measure Crankshaft Pulley Size (for example 5") ______

Determine the Ratio (in this example it is 2:1, or 5 divided by 2.5 = 2) ______

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

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

Note: lower output alternators require less shaft speed at idle to perform.


We recommend the following:

95A minimum idle 1600 shaft

130A minimum idle 1800 shaft

200A minimum idle 2000 shaft (2200 is ideal)