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.