You need an alternator for your ride, so you should be able to just buy one that matches the year/make/model, right? Well, not exactly, because you need to look at the pulley ratio for your particular application to ensure the alternator is functioning properly. We talked with the team at Powermaster to learn more about alternator pulley ratios.

The alternator pulley ratio is something that most people don’t consider when they’re putting the finishing touches on a build. What you need to keep in mind is that an alternator has a rpm sweet spot where it optimally functions. If you spin the alternator harder than the optimal rpm level it won’t increase output, it could actually damage the alternator. On the other end of the spectrum, if the alternator isn’t being spun hard enough, it won’t be able to produce the correct amount of current to charge your battery.

So, what is pulley ratio? Pulley ratio is the comparison of the crank pulley diameter with that of the alternator pulley. You calculate the ratio by dividing the crankshaft’s pulley diameter by the diameter of the alternator pulley.  Here’s a basic example: a 6” crank pulley used with a 2” alternator pulley comes out to a 3:1 ratio.

When you push an alternator to its max rpm there will be a lot of heat, it won’t be efficient, and the bearings are at their limit. So, if you’re revving and running your engine at high rpm, you might want to think about moving to a larger alternator pulley to slow it down. You can apply that logic to a vehicle that sees a lot of street driving, and use a smaller pulley to keep the alternator in its happy place for optimal current generation.

Make sure you’re doing the simple math to find the best alternator pulley size for your application. By doing this your alternator will last longer, and it will produce the correct amount of juice to keep your battery charged.