There are four things to know when considering whether to use a high-amp alternator.

First, do you really need a high-output alternator?

Most factory alternators are rated at 65 to 100 amps and are capable of handling the factory power demands from things like headlights, gauges, fuel pumps, air conditioning, etc.

Typically, these alternators also come with a 10- to 15-percent reserve to handle additional accessories.

That said, many readers won’t have a stock vehicle so a factory alternator may not cut it.

So how do you really know if you need to upgrade your alternator?

There are a few telltale signs such as dim headlights, poor stereo system performance, or an alternator that wears out quickly.

You can also check your electrical load using an ammeter or estimate your vehicle’s electrical load by checking the accessory fuses.

The amp ratings, although slightly higher than the highest draw of each component, will give you a good estimate of your needs.

If you decide to use a higher-amp alternator, the next consideration is: What amperage do you need?

Required alternator amperage depends on the current draw, along with any future accessories you plan to add. Each accessory will have drastically different draw. An electric fuel pump can draw anywhere from 3 to 8 amps while a high-power audio amplifier can pull as much as 70 amps.

Keep in mind that you can never have too much amperage.

Amperage is basically the amount of electrical current your alternator can supply. High-output alternators will not harm your components or charging system, no matter how high you go with the amps.

The third consideration is: What accessories do you need?

Although a performance alternator doesn’t require many modifications, it is recommended that you replace both the ground straps and charge wire.

Keep in mind that the factory cables weren’t designed to handle the juice of a higher-output alternator, and can restrict the flow of electricity. In the case of the charge wire, you really can’t go too large.

See Powermaster’s chart with recommended cable length and gauge for your vehicle’s amp draw.

What is Pulley Ratio?

Pulley ratio is a comparison between the crankshaft pulley diameter and alternator pulley diameter.

This ratio is derived by dividing the crank pulley diameter by the alternator pulley. For example, a 6-inch crank pulley with 2-inch alternator pulley will yield a 3-to-1 pulley ratio.

Powermaster recommends using this common rule of thumb:

  • Street use = 3-to-1 ratio or slightly higher
  • Drag racing = 1.75-to-1 ratio
  • Circle track = 1-to-1 ratio

Again, the ideal ratio depends on your application (street, drag racing, circle track racing), but you need to understand the effects of altering pulley ratio.

From amperage to pulley ratio, this video covers the most important things to consider when choosing a high-amp alternator. Check it out.