Most people are familiar with the alternator rating as the amperage rating.
Alternators are referred to as a 65 amp or 100 amp alternator.
When replacing the alternator on the family car, this information is likely all that is needed.
However, when building a custom car from the ground up, a deeper understanding of the power curve of an alternator is necessary.
A custom pulley or so-called 'power pulley' set is typically used with a performance alternator.
A mismatched pulley ratio and alternator can cause issues, especially at idle speeds where alternator performance is crucial.
Understanding the alternator's capability at slow speeds is important to avoid this.
An alternator's output depends on speed, but this can be misleading because the 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 significantly affect its output capacity.
Due to this, pulley ratios are very important, especially when using high amperage alternators.
The pulleys supplied with the alternator are matched to the winding and power curve.
It is essential 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 spends a lot of time cruising, a higher pulley ratio—perhaps 3.5:1—should be used.
Alternators can handle high speeds up to 20,000 RPMs for short periods, so overdriving the unit is not an issue.
The output of high amp alternators can drop off significantly under 2400 rotor RPMs.
Therefore, Powermaster does not recommend power pulleys with high amp alternators.
Powermaster employs a state-of-the-art computer alternator dyno to measure the performance of each alternator manufactured.
Output curves, engine idle speeds, and alternator pulley ratios are carefully considered to ensure good drivability at idle and slow cruising speeds.
