Calculate Alternator "Excite" RPM
Craig Borden  


If you are adapting an alternator to your Z car that wasn't really meant to be bolted up, you need to check and make sure that the alternator will see enough RPM at idle to excite. Logically, the alternator RPM is not the same as the engine RPM.
Instead of counting, use the method below.
Find the pulley ratio:
Ratio = Crankshaft Pulley Diameter/ Alternator Pulley Diameter

Now with the pulley ratio, we can determine rotor speed:
Rotor RPM = Pulley Ratio x Engine Speed Example: 2.1 x 870 = 1,827 Rotor RPM