For most people, the only alternator rating they are familiar with is the amperage rating. Alternators are spoken of as a 65 amp or 100 amp alternator. When replacing the alternator on the family car, this is probably the only information that is necessary. After all, all one needs is an alternator that matches the original.

When building a custom car from the ground up however, a deeper understanding of the power curve of an alternator is required. Usually a custom pulley or so-called "power pulley" set is used with a performance alternator. A mismatched pulley ratio and alternator will spell trouble, especially at idle speeds where alternator performance is critical. To avoid this, it is important to understand the alternator's capability at slow speeds.

An alternator's output is dependent on speed, but this can be deceiving because this output is not linear. Instead, it follows a curve. Each alternator has a unique curve, and at idle small changes in the alternator's speed can make a big difference in its output capacity.

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. It is important that any dress up pulley sets do not deviate from this ratio. Typically, a street driven car should have a pulley ratio of at least 3:1. If the vehicle has an automatic transmission with a low idle and the vehicle spends a lot of time cruising, then a higher pulley ratio - perhaps 3.5:1 - should be used. Alternators can take high speeds up to 20,000 RPMs for short periods, so overdriving the unit is not a problem.

The output of high amp alternators can drop off substantially under 2400 rotor RPMs. Therefore, Powermaster does not recommend power pulleys with high amp alternators.

Powermaster uses a state-of-the-art computer alternator dyno to measure the performance of each alternator we manufacture. Output curves, engine idle speeds, and alternator pulley ratios are carefully considered to assure good drivability at idle and slow cruising speeds.

How to Determine Ratio and Rotor Speed

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.