How to Choose a High-Amp Alternator

Considering making the jump to a high-amp alternator?

There are four things you need to consider.

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, A/C, etc. These alternators also typically come with a 10 to 15 percent reserve to handle additional accessories. That being said, most of our viewers don’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 simply 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

Once you’ve determined the need for a higher-amp alternator, it’s time to move on to the next consideration:  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. Powermaster supplies information on amperage draws here:

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As you add up your accessories’ draw, keep in mind you can never have too much amperage. That’s because 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: What accessories do you need?
Although a performance alternator really doesn’t require much in the way of modifications, we do recommend 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. Here is a chart with recommended cable length and gauge for your amp draw.

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Finally…What is pulley ratio and why should you care?
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 follows 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, we’ve covered the key consideration for choosing a high-amp alternator. If you’ve got more questions, our experts are ready to help!

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