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 a family car, this is likely the only information needed.
However, when building a custom car from the ground up, a deeper understanding of the power curve of an alternator is necessary.
Usually, a performance alternator is used with a custom pulley or so-called 'power pulley' set.
A mismatched pulley ratio and alternator can cause issues, especially at idle speeds where alternator performance is critical.
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 small changes in the alternator's speed at idle can significantly affect its output capacity.
Therefore, pulley ratios are crucial, especially when using high amperage alternators.
The pulleys supplied with the alternator are matched to the winding and power curve.
Any dress-up pulley sets should 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 of 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 utilizes 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.
