PWM-47796_HH_mlSo you’re considering swapping out your stock alternator.

There are plenty of good reasons to make the leap to a high-output alternator, but you’ll need to do a little bit of homework first. Luckily, we’ve got smart friends to help us with our studies, so you can ace the topic. In conjunction with the alternator experts at Powermaster and MSD, we’ve compiled the five things you need to know before upgrading to a high-amp, or high-output alternator.

This starts with the most basic of questions:

Do You Really Need a High-Output Alternator?

If you’ve got a basic, stock vehicle, chances are you don’t need a high-output alternator. Most factory alternators are rated at 65 to 100 amps and are capable of handling your vehicle’s basic necessities, such as headlights, gauges, fuel pumps, A/C, etc. These alternators also typically come with a 10 to 15 percent reserve to handle additional accessories.

However, many of our readers don’t have a stock vehicle. For example, you may have a custom-built street rod with a unique combination of accessories. Or you may have a high-end stereo system or a race vehicle with an array of on-board electronics. As the electrical load of all these accessories add up, you may find yourself in need of a higher-amperage alternator.

But how do you know?

There are a few ways to figure out whether you need to upgrade your alternator. A few telltale signs are dim headlights, poor stereo system performance, or an alternator that simply wears out quickly. You can also check your electrical load using an ammeter. Simply connect the ammeter in series with the battery’s ground terminal (with the engine turned off), switch each electrical component on and off, and note their amperage draws. Add up the total electrical draw and compare with your alternator’s rated output. The output should be 50 percent greater than the draw.

One final way to estimate your vehicle’s electrical load is to check the accessory fuses. The amp ratings, although slightly higher than the highest draw of each component, will give you a good estimate of your vehicle’s electrical load.

What Amperage Do You Need?

That depends on the current draw, along with any future accessories you plan to add. For that reason, we’ve supplied a list of some common accessories and their amp draw:

Electrical Load of Common Vehicle Accessories

AccessoryAmp Draw
Air Conditioner20-21
Audio Power Amplifiers10-70
Back-Up Lamps3-4
Cigarette Lighter10-12
CD/Tuner with Amp7-14
CD/Player/Tuner without Amp2.5-5
Dome Light1-2
Electric Cooling Fans6-15
Head Lamp Dimmer2
Head Lamp (Low Beam)8-10
Head Lamp (High Beam)13-15
Heater Defroster6-15
Ignition (Racing)8-36
Instrument Panel0.7-1.5
Lamp, Gauges1.5-3.5
Lamps, License Plate1.5-2
Lamps, Parking1.5-2
Lamps, Side Marker1.3-3
Lamps, Tail5-7
Nitrous Oxide Solenoid5-8
Power Windows Defroster1-30
Power Seats25-50
Power Windows20-30
Power Antenna6-10
Pumps, Electric Fuel3-8
Starter Solenoid10-12
Voltage Regulators (1 Wire)0.3-0.5

How Much is Too Much?

You can never have too much amperage when it comes to alternators; therefore, you never have to worry about choosing an alternator with too high of a rated output. Here’s why:

Amperage is basically the amount of electrical current your alternator can supply. And it basically operates off of supply and demand. That is, your alternator will only supply the amount of amperage a particular component demands—and no more. So high-output alternators will not harm your components or charging system, no matter how high you go with the amps.

prf-30709_xlWhat Gauge Wire Do You Need?

A performance alternator really doesn’t require much in the way of modifications. However, Powermaster and other alternator manufacturers do recommend you replace both the ground straps and charge wire. Keep in mind 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. However, here is a chart that matches cable gauge size to total amperage:

Recommended Cable Gauge and Length for Amp Draw

Cable LengthUp to 4 ft.4-7 ft.7-10 ft.10-13 ft.13-16 ft.16-19 ft.19-22 ft.22-28 ft.
0-20 Amps14 AWG12 AWG12 AWG10 AWG10 AWG8 AWG8 AWG8 AWG
20-35 Amps12 AWG10 AWG8 AWG8 AWG8 AWG6 AWG6 AWG4 AWG
35-50 Amps10 AWG8 AWG8 AWG6 AWG6 AWG4 AWG4 AWG4 AWG
50-65 Amps8 AWG8 AWG6 AWG4 AWG4 AWG4 AWG4 AWG2 AWG
65-85 Amps6 AWG6 AWG4 AWG4 AWG4 AWG2 AWG2 AWG0 AWG
85-105 Amps6 AWG6 AWG4 AWG2 AWG2 AWG2 AWG2 AWG0 AWG
105-125 Amps4 AWG4 AWG4 AWG2 AWG2 AWG2 AWG2 AWG0 AWG
125-150 Amps2 AWG2 AWG2 AWG2 AWG2 AWG0 AWG0 AWG0 AWG

What is Pulley Ratio (and Why Should You Care)?

In short, 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 3:1 pulley ratio.

The ratio has a direct effect on how fast the alternator spins.

In order to understand the importance of pulley ratio, you first need to understand the “power curve” involved with alternator output. Although the alternator’s output is dependent upon engine speed, it follows a unique curve. At idle, small changes in the alternator’s speed can make a big difference, so the pulley ratio becomes very important.

Powermaster supplies its alternators with pulleys matched to the alternator’s power curve. The company follows this common rule of thumb:

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

So why should you care?

Because differing ratios can affect performance, you should take care to maintain the same pulley ratio if you decide to use dress-up pulley sets. A mismatched pulley ratio and alternator can lead to big problems, especially at idle where alternator performance is critical. That’s because these high-amp units typically lose output under 2,400 rotor rpm. Rotor rpm are a factor of pulley ratio multiplied by engine speed. So, if you have a pulley ratio of 2:1 multiplied by an engine speed of 870, you’ll get a rotor rpm of 1,827.

At 1,827 rpm, you’ll see a significant drop in alternator output.

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.

With all this in mind, you’re ready to choose the right alternator for your application.

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Author: David Fuller

David Fuller is OnAllCylinders' managing editor. During his 20-year career in the auto industry, he has covered a variety of races, shows, and industry events and has authored articles for multiple magazines. He has also partnered with mainstream and trade publications on a wide range of editorial projects. In 2012, he helped establish OnAllCylinders, where he enjoys covering all facets of hot rodding and racing.