How To Calculate Pulley Ratio And Rotor RPM For Your Alternator

Setting up a properly functioning charging system is a crucial step for ensuring a reliable ride. To help readers that may be looking for some tips we’ve cited a couple from our friends at [Powermaster]. Below we’ll look at how to address power pulleys and pegging down the correct ratio based on application and output capacity.

When replacing an alternator the safest and most practical thing to do is to match the original in terms of amperage. When building a custom car from the ground up it is critical to take into account the power curve of various setups.

“An alternator’s output is dependent on speed, but this can be deceiving because this output is not linear,” says Powermaster. They continue, “Because of the preceding, pulley ratios are very important, especially when using high amperage alternators. The pulley that are supplied with the alternator are matched to the winding and power curve.” Because of this, “it is important that any dress up pulley sets do not deviate from this ratio.”

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. The formula for determining pulley ratio is:

Ratio = Crankshaft Pulley Diameter / Alternator Pulley Diameter

You can then determine the rotor speed using your known ratio:

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

The following ratio breakdowns courtesy of Powermaster can offer a ballpark estimate for where you want your ratio to be based on the type of application your car will most likely be suited to, “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.”

For more information and tech help on your car’s charging system be sure to check out the Powermaster homepage.

About the author

Andrew Almazan

Andrew Almazan is a graduate of CSULB with a degree in English and a passion for traditional kustoms and hot rods. His first exposure to out of this world vehicles was through the prevalent low rider, baja bug, and kustom culture of LA county.
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