How to Choose the Right Alternator

Powermaster’s New Line of HPR Alternators

By Todd Ryden – Photography by the Author

Choosing and installing an alternator sounds easy enough, right? In the old days you simply bolted in the new (or rebuilt!) unit, connected the wires, and tightened the belt to where it had an inch or two of deflection and done. Alas, those days are long gone, along with a factory “high-output,” 50-amp alternator.

- Advertisement -

Today, you likely need at least twice as much output to power the growing list of electronic controls on a modern-built classic truck. We’re talking about electric fans, fuel pumps, HVAC systems, EFI, and more modern amenities. With increased output requirements also comes more pressure on your wiring connections, cables, and grounds, not to mention belt and pulley choices.

It’s important to note that the alternator is producing the electricity your vehicle needs while it is running, not the battery. The battery’s main function is to act as a reserve storage of electricity that is primarily used to get the engine started. Once running, the alternator is tasked with maintaining the battery at its full capacity as well as powering everything on the vehicle that requires voltage. If you’re using more than the alternator is capable of producing, well, you’ll be on the side of the road eventually, or as a best case, sitting in a parking lot waiting for a jump.

Read More: How To Install A Universal Wiring Harness Kit

- Advertisement -

Choosing an alternator sounds easy enough, but before clicking the “Buy Now” button, it’s best to take note of just how much current your classic truck needs to keep you cruising on the road for thousands of miles. This is easily accomplished by making a list of the electronic components on your classic truck; you’ll be surprised how quickly they add up.

Current Requirements

Most components have their current use specifications on the manufacturer’s website or the instructions, but to give you an idea, a standard electric fuel pump will use at least 10 amps and more for higher-output versions (use 10-20 to be safe). Typical headlights and tail lamps use 5-6 amps (up to 10 amps for a Halogen and HIDs), a single electric fan can pull 30-40 amps, air conditioning up to 20 amps, air compressors for suspensions, power windows, and so on all add up quickly.

02 Before buying an alternator, make a list of all the electronics on your car and add up the amount of current each circuit requires. With that info, select an alternator that exceeds your demands.
Before buying an alternator, make a list of all the electronics on your car and add up the amount of current each circuit requires. With that info, select an alternator that exceeds your demands.

Yes, every component may not be running all at once, but it’s better to be prepared, and you should always select an alternator that exceeds the max current requirements. Once you have a feel for the output requirements your classic truck needs, don’t just pick the highest-rated alternator—you need to consider the rpm range of the alternator’s output.

- Advertisement -

Most alternators are rated at their highest output possible, which generally occurs at its top rpm.  For example, a 140-amp may only produce that amount at 5,000 rpm (engine), but only about 70 amps when your engine is cruising at 1,800 rpm at 60 mph thanks to that overdrive transmission. An alternator’s output is not linear! If the alternator is putting out 60 amps at idle yet you’re using 100 amps with your AC and the electric fan is running while waiting to pull into the cars and coffee show, well, you can figure out the result.

Powermaster’s new line of Hairpin Replacement (HPR) alternators are designed with low-rpm output in mind. They offer two series: a smaller unit designed to bolt in place of common alternators from the ’60s-’80s and a larger model for late-model engine swaps, including Coyotes, LS/LT engines, and even Hemis. The smaller frame HPRs produce over 145 amps while the larger units deliver a stout 195 amps at idle speeds (with 175 and 245 amps at higher rpm).

05 Powermater’s new line of HPR alternators are designed to deliver a much more balanced output from idle to top end.
Powermater’s new line of HPR alternators are designed to deliver a much more balanced output from idle to top end. The small HPRs are one-wire models that bolt in place of common 12si, CS130, and 10si units and deliver 145 amps at idle with 175 at higher rpm.

It is always important to ask the alternator manufacturer the output at idle, mid, and high rpm. This is very important and quite different between a street cruiser or a high-rpm road race truck. In most cases, there will be many choices to fit your current requirements, but if you’re running something unique, you can consider the pulley diameters to tailor the rpm that the alternator runs.

Pulley Ratio

As mentioned, the output of an alternator is dependent on the speed at which it is driven, just like tire diameter and rear end gearing. 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-inch crank pulley used with a 2-inch alternator pulley comes out to a 3:1 ratio.

06 Different diameters of pulleys will affect the charging output of the alternator in relation to engine rpm (crankshaft speed).
Different diameters of pulleys will affect the charging output of the alternator in relation to engine rpm (crankshaft speed). A smaller pulley will overdrive (turn it faster) while a larger pulley will underdrive (slow it down) in relation to engine rpm. A rule of thumb is about a 3:1 ratio for street trucks.

Read More: How To Install Classic Instruments’ New Bel Era III Gauge Panel

For street applications, a 3:1 ratio is a good target, which means that at an 800-rpm idle the alternator is running at 2,400 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 see larger alternator pulleys on race applications. Short bursts of high rpm are OK but sustained screaming rpm will take a toll on the internal components of the alternator.

Charge Cables

With your current needs covered, there’s nothing left but to bolt in the new unit and connect the old charge cable, right? Not so fast! If you’re stepping up to a modern high-output alternator, do yourself a favor and update the charge cable and add a ground cable.

The charge wire on your truck is likely old and brittle, or was replaced with a box-store replacement cable designed to transfer 30-60 amps max. It’s like expecting your 100,000-mile TH350 transmission to handle your new 500hp LS engine. And when a charge cable can’t handle all of the new current it will get hot and hotter until it fails—which could go horribly wrong.

03 Nearly as important as the alternator itself, the charge wire needs to be scrutinized in length and gauge
Nearly as important as the alternator itself, the charge wire needs to be scrutinized in length and gauge. As current requirements and the distance from the battery grow, the gauge of wire also needs to be considered. Always use a high-quality, multi-strand cable for battery and ground connections.

04 Always use a high-quality, multi-strand cable for battery and ground connectionsWhen upgrading the charge wire, you again need to consider the current requirements as well as the length of the wire. The thickness (gauge) is dependent upon these variables but the good thing is that you can’t really choose too big of a charge wire on the alternator. Always use a multi-strand, copper cable and you might as well go big to ensure years of flawless current flow.

08 On their standard models, they recommend running a ground wire from the housing to ensure a proper ground path
On their newer HPR alternators, Powermaster provides a ground stud right on the housing that should be connected to the block or battery negative terminal. On their standard models, they recommend running a ground wire from the housing to ensure a proper ground path.

09 On their newer HPR alternators, Powermaster provides a ground stud right on the housing that should be connected to the block or battery negative terminalWhile you’re considering the charge wire, it will behoove you to add a ground wire from the alternator to the engine block or negative battery terminal. With the volume of electronics, having your vehicle grounded properly is key to the overall success of everything electronic on your hot rod. This is a good practice with any high-output alternator, but even more important due to brackets being painted or powdercoated, which creates a challenge for electrons to find a ground.

Adding Tension

A final consideration in selecting an alternator has to do with the pulley. Are you planning to run a V-belt or have you upgraded to a serpentine belt system? If you’re going with an aftermarket drive assembly, the pulley will be included, but for custom installations you need to be sure to choose the proper pulley, which also leads to considering the pulley diameter in relation to the crank pulley.

For standard V-belt applications, it’s time to ditch the deflection way of setting belt tension. A slipping belt causes heat and excessive heat is not something alternators like. Today’s high-output alternators need that belt to be good and tight! Bearing technology in the water pump and in the alternator produces much stronger units so there’s no need to worry about bearing damage from an over-tightened belt.

07 Powermaster recommends that V-belts be tightened until the alternator cannot be turned with a 3_8-inch ratchet connected to the pulley bolt
Powermaster recommends that V-belts be tightened until the alternator cannot be turned with a 3/8-inch ratchet connected to the pulley bolt. The bearings can support the pressure and a slipping pulley causes heat and turmoil to the inner workings of the alternator.

 As you can see, there’s a bit more to selecting the right alternator for your hot rod application. None of this is difficult and it largely revolves around making sure you install and connect your new alternator with the best practices to ensure trouble-free installation. Also, once you know the type of unit you need, you can be sure the manufacturers offer several designs and coatings to make it look as good as it performs. Happy charging!

10 Billet Specialties offers a Tru Trac kit is supplied with a high-output, one-wire Powermaster alternator
If you make the move to a crate motor there are many options available for complete accessory drive systems, including complete kits such as this example from Billet Specialties. Their Tru Trac kit is supplied with a high-output, one-wire Powermaster alternator as well as the other accessories, brackets, and serpentine belt.
11 If you prefer the look of a vintage generator, Powermaster offers their PowerGEN, which has the cool, classic looks but the charging reliability of an alternator
If you prefer the look of a vintage generator, Powermaster offers their PowerGEN, which has the cool, classic looks but the charging reliability of an alternator. No more dimming lights at stoplights or questionable at lower speeds.

12 Powermaster offers a vintage looking alternator

13 On some hard-core race applications, we’ve seen alternators mounted backward, which also reverses their rotation.
On some hard-core race applications, we’ve seen alternators mounted backward, which also reverses their rotation. The alternator will still create current, but the cooling fans will not be effective. Also, the pulley bolt can easily back off unless a strong thread-locking compound is used. Powermaster offers or can build reverse rotation alternators if needed.

Read More: Engine Wiring for LS Engine Swaps

 One Wire vs. Factory Connector

When you’re deciding on an alternator, one thing that may come up is to go with a one-wire alternator or stick with a factory plug-in style. From the factory, most alternators have a battery output terminal as well as a plug-in harness that could have two or up to four terminals. When the ignition key is turned on, the alternator is powered up through a 12V signal and ready to produce current as soon as the engine is turning. Other wires may be for a charge light on the dash, sensing the voltage for an external regulator, or even to power an electronic choke.

A one-wire alternator is exactly what it sounds like; hook up one charge wire from the alternator terminal to the battery positive post and done. These alternators use a unique internal regulator that basically “wakes up” once it senses rpm. Years ago, there were skeptics about one-wire alternators because they needed to see a certain rpm to start charging but today’s one-wire alternators start charging at very low engine rpm. For ease of wiring and cleaner installation, you can’t beat a one-wire alternator.

Source
Powermaster
(630) 849-7754
powermasterperformance.com

- Advertisement -

Related Articles

Search Our Site

More Classic Truck Performance

Masterful Metalwork With Scott’s Hotrods N Customs

Behind The Scenes Look at Brian Schutte’s 1951 Chevy...

Dino’s “Black Widow” 1973 Chevy C10 Stepside

Gittin’ Down With Dino Battilana’s LS swapped 1973 Square...

Retrofitting 1973-80 AMD Inner Fenders On 1981-87 Square Body Chevy

Gaining some clearance With Harrison’s Rod & Custom By Jason...

The Perfect Hot Rod Recipe? Lenhart’s 1947 Ford Truck

John Lenhart’s Stunning Orange Pearl Paint and Pro Fabrication...

How To Rebuild Control Arms on a 1967-72 Chevy Truck Or Suburban

Budget Front Suspension Overhaul For 67-72 C10 & Suburban By...

Kenny’s One Of A Kind Show Truck

Kenny Kimbrough’s “Killer” 1949 GMC Truck By Rob Fortier &...
More Classic Truck Performance

1949 Chevy Truck Runs Low 12s At The Dragstrip!

From Drag Strip to Long Hauling, this 1949 Chevy...

1956 Chevy 3100, a Tribute to a Supportive Wife

Ricardo Lopez Pays Tribute to Many Years of His...

PARTS DEPARTMENT – Latest CTP New Products include Pertronix and PPG

Why Seal-4-Good? The Seal-4-Good product line is a staple in...

Massaro’s Award Winning Atomic Silver 1956 Ford F-100

No Compromise 1956 Ford F-100 Is Worth The Wait By...

How To Install a TR3550 Tremec Transmission Into a Dentside Ford

Improving Shift Feel With A TR3550 transmission in a...

Shop Tour: Old Anvil Speed Shop

Complete Custom Fabrication, Intricate CNC Work, Restoration, Chassis Design,...