What is My Pulley Ratio, and
Why Does it Matter?

   Your pulley ratio is the difference between the crank pulley diameter and the alternator pulley diameter. Example: Your crank pulley is 6.5” in diameter and your alternator’s pulley is 2.5” in diameter. This gives you a  pulley ratio of 2.6 to 1 (6.5 divided by 2.5). This simply means that alternator will spin 2.6 times faster than your crank pulley. In other words if your engine is turning 1000rpms, your alternator is turning 2600rpms.

   This is important for several reasons. First and foremost, it  allows you to figure out at what rpm your alternator is turning at your engine  idle. To do this calculation simply multiply your engine idle by your pulley  ratio.

   We rate our alternators, as do all quality shops, by the  rotor rpm of the alternator, not engine rpm. This allows you the customer, to see what the alternator will produce in your application at idle.

   It is possible to modify this ratio by changing either the  alternator pulley size or the crank pulley size. The alternator’s pulley being the least expensive of the two. This is also why if you have done this already via “Power Pulleys”, you are likely having charging problems at low rpm with a stock alternator.

   Typically you want the lowest possible ratio possible. This is because an alternator requires approximately 1hp for every 22.5amps of  output, times the pulley ratio. So the lower the pulley ratio, the less the hp the alternator needs to take from your engine.

   One thing you should never do though is sacrifice system  voltage for a lower pulley ratio. As you can see quickly doing some playing  around with your system requirements, there is not a whole lot of hp to gain by  lowering your pulley ratio. Example: a 100amp load on an alternator with a 3 to 1 ratio has a load of 13.3hp on the motor. Reducing the pulley ratio to 2 to 1,  only will save 4.4hp. Which is a chunk of hp. However if doing so decreases your system voltage from 14.9 to 12.9 volts, you will likely lose a noticeable percentage of overall engine hp. This is simply because your electronic ignition  requires voltage and amperage to run, if it doesn’t see what it needs, then its  performance is effected. See the article on “System Voltage”  for a full explanation. The short story is, simply put, having enough amperage  at idle is more important than saving a couple of HP by reducing the pulley  ratio.

   Alternators have fixed charged curves. The only way to adjust this curves is to replace the alternator with a different one. Your engine idle is a fairly fixed part of the equation as well. It can be adjusted slightly up  or down, but no drastic changes are possible. By far the easiest thing to adjust  is the pulley ratio. If you need to move higher up into an alternators charge  curve to meet your system requirements, you simply put a smaller pulley on the  alternator.

   This however has some limitations. The first is it may not be possible to but a small enough pulley on the alternator to get enough at idle  amperage. In this case you need to get a higher at idle output alternator. You also have to bare in mind the maximum speed the alternator can turn. Typically  big body (big truck, EMS) alternators will not survive over 8000rpm constant rpm. Automotive units are around 16,000rpm, but double roller units can survive  momentary bursts higher than that. However even double roller bearing units that  see over 20000 rpm should have there bearings checked frequently, and have only  top quality bearings installed. We do not recommend any units with needle  bearings to see in excess of 18,000rpm.

   Now back to deciding on what pulley ratio to use. You can’t really until you decide on what alternator you are going to use. You only need  to consider it when searching for a new unit. Most automotive type alternators  start charging around 1500 rotor rpm. They start producing usable amperage just  over 2000 rotor rpm, and are well into their output curve by 3000 rotor rpm.

   Typical pulley ratios vary from 1.5 for high rpm motors, 2-2.5 for drag cars, 3 for street driven cars. This will put you in the ballpark, i.e. give you an idea when looking at alternators what one is likely going to work in your application. The pulley ratio then can be tailored to fit  your setup. But if your engine idles at 750rpm, and your system at idle requirements is 100amps, there is no point in looking at alternators that  produce less than 100amps at 2250 rotor rpm, for you cannot adjust the pulley  ratio to meet the requirements. You can consider 3.35 to be likely the highest pulley ratio available, for most cars.

   Big trucks and EMS applications have to bare in mind that the pulley ratio is not near as adjustable as in automotive applications. Further the maximum rpm is very crucial, and cannot be pushed. This is because the units  have much larger rotors, that can easy fly apart because of momentum. The larger rotors make the alternator turn on at a lot lower rotor rpm, and the low end performance is greatly increased.

   The last thing to consider about pulley ratio is belt wrap. The smaller the diameter of alternator pulley, the less contact the belt has with the pulley. In high output applications it is important to use as large as pulley as possible. Or insure 180 degrees of belt wrap. This will limit the  amount of belt slippage.

While on the subject of pulleys, did you know that a serpentine pulley has a lot more contact surface than the older “V” belts. There are very few applications in the automotive industry where alternators with over  95amps of output only had a single groove “V” belt. So if you have not got one  already, we suggest purchasing a 6groove serpentine belt system for any high out put alternator application. If you are going over 200amps, an 8 groove system is a must.