Next, measure the overall diameter of the crank pulley.


The alternator has an OD of about 2.43" and the crank has an OD of about 4.43" so we simply divide 4.43" by 2.43" and we have a 1.8:1 pulley ratio.


This means that for every revolution of the engine crank the alternator makes 1.8 revolutions.


By knowing this we can determine that at 1000 engine RPM our alternator is spinning at 1800 RPM. Once you know this you can compare it to the curve for your alternator to see where you need to run your engine at to develop the current you want..