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 information is probably the only detail that is necessary.
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.
It is important to understand the alternator's capability at slow speeds to avoid this.
An alternator's output depends 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 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.
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, 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 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 manufactured.
Output curves, engine idle speeds, and alternator pulley ratios are carefully considered to assure good drivability at idle and slow cruising speeds.
