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How to Determine Ratio and Rotor Speed

alternator ford focus

The alternator rotor RPM is not necessarily the same as engine RPM. To calculate the actual alternator RPM, determine the ratio between the two pulley diameters.

Ratio = Crankshaft Pulley Diameter/ Alternator Pulley Diameter

Now that we know the ratio, we can now determine the rotor speed:

Rotor RPM = Pulley Ratio x Engine Speed (example; 2.1 x 870 = 1827 Rotor RPM)

Tech Tip: Generally, the alternator should be 1:1 with the motor for circle track racing. For drag racing, the alternator should be overdriven by a ratio of 1.75:1 or more. This will allow charging on the return slip and in staging. For street use, we recommend 3:1.

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