Rotor rpm are a factor of pulley ratio multiplied by engine speed. 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. Street use = 3:1 ratio or slightly higherīecause differing ratios can affect performance, you should take care to maintain the same pulley ratio if you decide to use dress-up pulley sets.The company follows this common rule of thumb: Powermaster supplies its alternators with pulleys matched to the alternator’s power curve. At idle, small changes in the alternator’s speed can make a big difference, so the pulley ratio becomes very important. ![]() Although the alternator’s output is dependent upon engine speed, it follows a unique curve. In order to understand the importance of pulley ratio, you first need to understand the “power curve” involved with alternator output. ![]() The ratio has a direct effect on how fast the alternator spins. For example, a 6-inch crank pulley with 2-inch alternator pulley will yield a 3:1 pulley ratio. This ratio is derived by dividing the crank pulley diameter by the alternator pulley. ![]() In short, pulley ratio is a comparison between the crankshaft pulley diameter and alternator pulley diameter. What is Pulley Ratio (and Why Should You Care)? Recommended Cable Gauge and Length for Amp Draw Cable Length However, here is a chart that matches cable gauge size to total amperage: In the case of the charge wire, you really can’t go too large. 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. However, Powermaster and other alternator manufacturers do recommend you replace both the ground straps and charge wire. What Gauge Wire Do You Need?Ī performance alternator really doesn’t require much in the way of modifications. So high-output alternators will not harm your components or charging system, no matter how high you go with the amps. That is, your alternator will only supply the amount of amperage a particular component demands-and no more. And it basically operates off of supply and demand. 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:Īmperage is basically the amount of electrical current your alternator can supply. For that reason, we’ve supplied a list of some common accessories and their amp draw:Įlectrical Load of Common Vehicle Accessories Accessory That depends on the current draw, along with any future accessories you plan to add. 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. One final way to estimate your vehicle’s electrical load is to check the accessory fuses. The output should be 50 percent greater than the draw. Add up the total electrical draw and compare with your alternator’s rated output. 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. You can also check your electrical load using an ammeter. 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. 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. For example, you may have a custom-built street rod with a unique combination of accessories. ![]() However, many of our readers don’t have a stock vehicle. These alternators also typically come with a 10 to 15 percent reserve to handle additional accessories. 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. If you’ve got a basic, stock vehicle, chances are you don’t need a high-output alternator. This starts with the most basic of questions: Do You Really Need a High-Output Alternator? 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. Luckily, we’ve got smart friends to help us with our studies, so you can ace the topic. 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. So you’re considering swapping out your stock alternator.
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