Street Muscle’s Guide To Choosing The Right Alternator

Choosing an alternator for your car sounds like a straightforward, simple decision, doesn’t it? When it comes to an all-stock daily driver, it’s as simple as tracking down the right OEM unit to fit the specific year, make, and model. However, when you’re talking about street machines (and we’re talking about street machines), you need to do your homework to ensure the alternator you choose can keep up with your car’s electrical demands from low to high RPM.

Remember, the alternator is responsible for keeping the battery topped off so it can meet the demands of every electrical component with its required current and voltage. In the case of our muscle machines, we can easily tack on a few more high-current-drawing devices, such as a pair of electric fans, a performance electric fuel pump, transmission cooling fan, high powered ignitions, and more, that can overwork the alternator. If the alternator can’t keep up, there will be issues with pretty much the entire electrical system of your car. 

Modern muscle machines require a lot of current to feed a growing number of electronic controls and devices. Powermaster understands how to keep things running at full power while keeping the charge.

Think of it this way: The alternator gets the beer to the battery and the battery passes out the drinks to the party of electronic devices. If the party gets too busy, the alternator won’t be able to keep up with the demand and the battery will drain its reserves until the party ends – poorly. 

What To Expect When Charging

When you’re selecting an alternator, there are several things to consider, starting with the current demands of your vehicle. Take the time to consider your electrical components and jot down their current needs – you’ll be surprised how fast things add up. For instance, a standard electric fuel pump will use at least 10 amps with higher output pumps requiring more (use 10-20 to be safe). A single fan can pull 30-40 amps, air conditioning up to 20 amps, air compressors for suspensions, cooling pumps, stereo amps, etc…it all adds up quickly. 

We asked Todd Ryden, one of Powermaster Performance’s experts, to expound on this concept in further detail. Todd says, “remember, an alternator’s output is not linear! For example, a 140-amp alternator may only produce that amount at over 2,500 rpm (engine), but only about 80 amps while cruising at 1,800 rpm at 60 mph. If the alternator is putting out 60 amps at idle, yet you’re using 100 amps with your fuel pump, A/C, and electric fans running while waiting to pull into the cars and coffee, well, you can figure out the result.” 

Adding up the total current from the components on your car will give you a better idea of what size alternator you’ll need. With today’s low idles and cruising RPM, it’s important to consider the current demands at low RPM, not just the advertised peak numbers.

An interesting thing to note when shopping for alternators is that most units are advertised or promote their highest output, which also happens to come at higher engine RPM. As much as we like to look at high RPM engine numbers, we all need to face the fact that the majority of the time, your engine is at a very low RPM and even idle speed. Sitting at lights, rolling through a cramped show, and even cruising down two-lane country roads can keep your engine RPM below 2,000, thanks to modern overdrives. 

Hairpin Replacement & Special Considerations

One company that understands the importance of high output at low rpm is Powermaster. They show the output of their entire alternator line at idle, cruising, and high rpm so you know exactly what you’re getting. This info is extremely helpful in getting the right alternator to fit your application with balanced output through the entire RPM range.

Powermaster’s new HPR-series alternators are designed to deliver high amounts of current from idle to redline engine RPM. The trick is to use a stock style pulley so a new belt isn’t required, and that’s what Powermaster has pulled off.

The Powermaster team understands that even with higher current demands, modern street machines are also idling at lower rpm and thanks to overdrives, they even cruise at lower engine speeds. Their engineering crew set about answering the needs for today’s high current/low RPM operation and developed an all-new product line – the HPR Alternator series. These units are designed to produce high current at low RPM, yet, still have plenty of punch as RPM increases. 

HPR stands for Hairpin Replacement. “Hairpin” refers to the modern winding techniques of the stator and rotor in late-model applications. Many hairpin designs use a unique square copper wiring, allowing more material to be used on the rotor and stator, thus improving output. You may also see references to “six-phase” models as well. A standard alternator is typically a three-phase design while most hairpin models are referred to as six-phase due to the advanced component design of the stators that house two three-phase rotors which create more current. Of course, this oversimplifies the process. But with improved efficiency comes improved output (with less heat to combat as well). 

Powermaster’s unique winding design of the stator and the rotor creates a much more efficient assembly capable of producing large amounts of current at low RPM, it does so while running cool and not adding to parasitic losses. Their HPR units operate on the similar six-phase technology. To convert this high Alternating Current (AC) into the Direct Current (DC) required for a car, they also incorporate a heavy-duty rectifier capable of handling the extra current while dissipating the heat created. 

In the end, Powermaster’s HPR-series alternators are extremely efficient with a very balanced output from idle to high RPM.

They offer two versions, a smaller unit designed to bolt in OEM and aftermarket brackets for common alternators used from the ‘60s through the ‘80s, and a larger model for late-model engine swaps including Coyotes, LS/LT engines, and even HEMIs. The smaller HPRs produce over 145 amps at idle while the larger units deliver a stout 195 amps at idle speeds (with 175 and 245 amps at higher rpm). Perfect for today’s modern muscle machines – with low idle and cruising engine speeds.

An advanced winding design combined with the highest quality materials creates very high current capabilities with efficiency. A heavy duty rectifier is also used to handle the increased current and effectively remove heat. 

Charge Cables

Once you’ve selected an alternator, you’ll need to ensure that your wiring is up to the task of handling the improved charging capabilities. The best place to start is with the charge wire that connects from the alternator to the battery positive terminal. Chances are that the cable leading from the alternator’s charge terminal to the battery is either old and brittle, or not the right gauge to handle the added current. 

We asked Todd what he would tell people who call asking about common charge issues related to their cables, and he has this to say. “Do yourself a favor and just plan to replace the charge cable with a high-quality, multi-strand copper cable. When choosing a cable, you’ll need to consider the length of the cable, as a trunk-mounted battery will need a much heavier gauge cable compared to an application with the battery under the hood. In the case of a charge wire, bigger is better!” 

A high-output alternator needs a high-strength charge cable. Powermaster now offers a line of multi-strand, pure copper cables in a number of lengths and gauges. The multi-strands provide more paths for current to flow. While you’re at it, be sure to connect a ground wire to the alternator to complete the circuit.

According to Todd Ryden of Powermaster Performance, “One of the leading issues on vehicles with a lot of electronics has to do with grounds. Powermaster has added a ground terminal to all of their HPR alternators (and other high-output models) to ensure a proper ground is connected. And when it comes to the gauge of the wire, they recommend using the same size as the charge wire to complete the circuit.” 

To complete the charging circuit, Powermaster equips their high-output alternators with a ground terminal.

The higher the output and longer the length of cable needed, means an increased gauge size. Charge wires and ground wires are not a place to skimp. Do it right, do it once.

Pulley Ratio

Pulley ratio is the comparison of the crank pulley diameter with that of the alternator pulley. The ratio is the result of dividing the diameter of the crank pulley by the alternator pulley. As an example, a 6” crank pulley used with a 2” alternator pulley comes out to a 3:1 ratio.

Depending on your application, looking at pulley diameters compared to the crank pulley is important. Powermaster offers a number of different pulley diameters for street and race applications.

For street applications, a 3:1 ratio is a good target. This means that at a 700-RPM idle, the alternator is running at 2,100 rpm. Conversely, at 6,000 engine rpm on the same setup, the alternator is spinning at 18,000 rpm! Running at that high of sustained rpm can take its toll on an alternator which is why you may notice larger diameter pulleys on race applications. A road course application will realize sustained high RPM, so it is best to move to a larger pulley, such as a 2:1 ratio, to reduce the RPM of the alternator. Powermaster recommends not running their alternators over 18,000 rpm. 

On Powermaster’s late-model HPR alternators, they use a stock diameter pulley so you don’t have to worry about over-spinning the unit, nor having to purchase a new belt. They even offer models with the stock decoupler or clutch-style pulley to ensure an OEM fit and operation. 

Article Sources

About the author

Vinny Costa

Fast cars, motorcycles, and loud music are what get Vinny’s blood pumping. Catch him behind the wheel of his ’68 Firebird. Chances are, Black Sabbath will be playing in the background.
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