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