The only alternator rating most people are familiar with is the amperage rating.
Alternators are referred to as a 65 amp or 100 amp alternator.
When replacing the alternator on the family car, this is likely the only information necessary.
After all, an alternator that matches the original is 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 required.
Usually, a performance alternator is used with a custom pulley or so-called 'power pulley' set.
A mismatched pulley ratio and alternator will spell trouble, especially at idle speeds where alternator performance is critical.
To avoid this, understanding the alternator's capability at slow speeds is important.
An alternator's output is dependent on speed, but this can be misleading because this 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 make a significant difference in its output capacity.
Because of 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 crucial 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, then 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.
