Choosing the Right Alternator for Your Musclecar or Hot Rod

Your musclecar’s electrical system was state of the art forty plus years ago but time and technology have taken it’s toll. As such, it’s probably not up to the task of handling today’s modern electrical components. When these cars were originally built, a 60-amp alternator was the largest you could get, if you happened to check the correct box on the order sheet. Today, modern cars often have their smallest alternator sized at 90-amps, and in many cases the minimum is over 100. Attempting to run modern components on an old alternator is a recipe for charging system issues, or worse yet, damaged or failed electrical components.

Upgrading your current alternator is one of the easiest bolt-on enhancements you can make to your musclecar, hot rod or even your daily driver. For any enthusiast who is handy with tools this is typically a simple upgrade on most cars.

Not to leave you in suspense, we got a hold of Powermaster to get the inside scoop on choosing the correct alternator for specific applications. We cover everything from determining the amp load, choosing an alternator, various finishes and what you should expect to upgrade along with the new alternator.

Determining Amp Load

We got together with Powermaster Performance to get the details on how to determine what alternator is best for your vehicle. While you could simply default to the largest alternator available to fit your application, this may not be the most astute choice. Powermaster’s Brady Basner points out, “Amperage is like horsepower, you can’t have too much,” however he also cautions that the bigger the output the larger the alternator. This could cause an issue with a mounting solution, it also often increases the cost, which might put the upgrade out of your budget. “The largest possible alternator will work in any situation, but size and mounting options often make that choice impractical,” said Basner.

Various connector types are available depending on your car's wiring harness.

Wiring and Its Effect On Amps and Voltage

Amperage is like horsepower, you can’t have too much.

For good measure, it’s always wise to upgrade the wiring, especially if the current setup is still running the original wiring. Also, if the wiring has become a tangled mess over it’s lifetime, undergoing numerous repairs, splices and backyard fixes, it may also be time to upgrade.

That said, upgrading the alternator will not affect the car’s wiring. Powermaster quickly points out to consider electrical current much like water. Using this idea, amps are the volume of water, volts are like water pressure, and the wiring acts as the hose or line that carries it all. Having a larger than needed amperage, much like having a reservoir of water, will mean the electrical components can run without a drought. Furthermore, too much voltage is like too much pressure in an old garden hose. Much like the old hose which could bubble or burst under too much pressure, meaning – the factory wiring could fail since it was never intended to carry that much load.

Summing it up, higher voltage can damage wiring and electrical components. More available amps, which is what you get from an alternator upgrade, simply give you more available power when the demand arises. Since upgrading your alternator changes the amperage and not the voltage (on 12-volt systems), installing a higher amperage alternator is not going to cause your musclecar to burn to the ground or your wiring to melt. but having too small of a gauge wire could be the cause of an under powering issue.

How Many Amps

Test Run

All Powermaster alternators include this Proof of Performance card attached. Before an alternator leaves the company’s Chicago facility, it is tested and inspected to ensure that it’s performing properly. Save this card for reference, it could help you diagnose electrical problems if the need arises in the future.

Figuring out the necessary amperage for a vehicle is probably more simple than you think. No matter what duty the vehicle is serving, be it a resto-mod, daily driver or show car, the process for determining amperage is the same. We suggest compiling a list of everything on the car that uses electrical power.

An example of components would include:

  • Lights
  • Power windows
  • Power locks
  • Gauges
  • HVAC fan
  • AC compressor
  • HVAC blower
  • Windshield wipers
  • Stereo components
  • Radiator electric fan
  • Other electric fans
  • Electric water pumps
  • Computer/ECM
  • Nitrous system components

It’s important to note here that a race car or street/strip car may have some or all of these components, but may be less likely to use all or many of them in combination. A street driven car, or daily driver however needs to account for all of these items to determine maximum load.

You should be able to refer to the manufacturer of any aftermarket components you’ve installed for their amp draw. However, if that information is not available to you then Powermaster can help you make a safe determination based on their over thirty years of experience in building quality starters and alternators.

Choosing The Right Alternator

When the maximum load is figured out, then add twenty percent to that as a safety buffer. For example; if you’ve calculated the maximum amp load is 80-amps, with a 20% buffer your desired output would be 96-amps. With those numbers in mind, the closest match would be a 100-amp alternator. Running a slightly larger than needed alternator will improve the life of the component as it is not running at 100% all the time. It can also improve belt life, or belt issues, which we will address in a moment.

Case Size and Brackets

The case size of an alternator from Powermaster is directly correlated to the output of the alternator. For example, an 80-amp alternator will have a much smaller case size than a 200-amp unit for the same application.

What this means is that your second step in choosing the proper alternator for your vehicle will be to determine bracket compatibility.

In many cases the OEM bracket may be a suitable mounting point for your alternator but there are however a number of reasons why the original brackets may not work. The mounting points may differ in such a way that the bracket can not be utilized. Interference due to case size could be the other reason. It could also have an interference fitment problem with other components such as the engine block, power steering pump, or air conditioning compressor.

Powermaster offers a variety of case sizes and mounting options.

If you determine that brackets are needed, Powermaster can provide these as well. They have a number of chrome bracket kits for a variety of Chevy applications including small- and big-blocks to suit your particular application. If you can’t find what you need it may have to be sourced elsewhere, or custom-built to suit your particular needs.


As Basner states, “This is where the fun starts,” when picking an alternator. Powermaster offers a variety of finish options for their products that should help fit any style of build.

Finish options include:

  • Chrome
  • Polished
  • Black
  • Satin
  • Natural/OEM

Top Left: OEM Finish, Right: Black, Bottom Left: Satin, Right: Chrome.

Powermaster also offers several fan and pulley options as well and Powermaster can help you select the correct pulley and fan combinations if you are looking into an aftermarket product to match the rest of your accessories or build. This can ensure that these components properly match the alternator’s operating characteristics such as having a pulley that will allow the alternator to spin at the proper operating RPM.

The One-Wire Solution

Powermaster's alternators are highly detailed and take every aspect of installation into consideration.

A popular upgrade for decades has been a change to a one-wire alternator setup. To simplify hook up and installation, traditionally, a one-wire setup required the engine to rev after startup, which signaled the alternator to turn on and charge, otherwise at idle you’d simply run your battery down.

That is not the case anymore. As Basner tells us, “A high quality one-wire alternator with a regulator and rotor will help give excellent turn on speed and excellent idle output. So whether you’re running an old school hot rod, or just want a simple installation and less wiring to deal with for your musclecar, a one wire alternator is a viable option.”

What Else Needs Upgrading

Aside from upgrading the alternator, and possibly the bracket system there are a few other things that you need to keep in mind when taking on a project such as this.

Charging Wire

A proper charging wire is necessary to carry the current from the alternator to the battery. This is an area where your original wire designed for a low amperage system is definitely not going to work. These charging wires are relatively inexpensive, and should be of a heavy, 4- or 6-gauge wire with a fine strand design.

Amperage (or current) travels over the surface area of a wire. So the greater the surface area the more efficiently it can get to where it’s going. Again thinking of electricity, and power flow as similar to water can be a helpful visualization. Pushing or moving water through a small tube or funnel takes longer than through a larger one. Powermaster can supply you with the proper wire for your system as well as the necessary attachment hardware.

Powermaster offers the proper charging wire for your application if yours is the original size or not up to the task.

Belt System

This could be the biggest area where Powermaster encounters issues when customers upgrade their alternators. It takes more torque to turn a higher output alternator than an original lower output model. “Belt tension on a 63-amp stock GM alternator can be much looser than a 140-amp upgrade,” says Basner.

These photos from Powermaster show examples of the most common belt issues encountered when upgrading your alternator.

Installing a new belt when you upgrade your alternator is an affordable and simple solution and probably something you’ll need to do for maintenance anyway. It’s also important to check your belt tensioner or tensioning system. Misalignment is another area that will affect alternator output. These are other areas that may need attention in the form of upgrade or replacement.

A simple test that Powermaster recommends to check for proper belt tension is to place a socket on the alternator pulley nut, and attempt to rotate the alternator clockwise with the belt at what should be proper tension. Under this test you should be able to rotate the entire accessory drive system and crankshaft (Note: high-compression engines will be more difficult to rotate). If the alternator pulley slips or rotates separately without moving the belt, then there is not enough tension.

Multiple pulley options are available to accommodate your belt system.

What’s It All Mean

Upgrading the alternator on your car is a wise decision. With so many modern conveniences, something as simple as your cell phone charger or GPS unit could be enough to throw a dated, and low output charging system into chaos. This is an easy project on most vehicles and will give you added piece of mind on the road as well as good looks under the hood for years to come.

Article Sources

About the author

Don Creason

Don Creason is an automotive journalist with passions that lie from everything classic, all the way to modern muscle. Experienced tech writer, and all around car aficionado, Don's love for both cars and writing makes him the perfect addition to the Power Automedia team of experts.
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