Alternator Pulley Size, How it Affects Your Charging System

6 Groove Pulleys shown above , L-R: 2-3/8" diameter, 2-1/4", 2", and 1-7/8"
Yes, you can increase your alternators overall output as well as increase power output at engine idle speeds.
Check the size of the pulley on your alternator. If you are having problems with overall alternator output or charging at engine idle, maybe a smaller pulley is all you need. The most common pulleys on GM, Ford and Daimlier/Chrysler vehicles are the 6-groove 2-7/16, 2-3/8 and 2-1/4 inch pulleys, by reducing the pulley size to 2" or 1-7/8" your alternator will start producing power sooner and produce more power cruising down the boulevard or on the highway. Also, because your alternator is spinning faster the fan will move more cooling air through the alternator adding to component life, to little air flow can burn up an alternator.

Alternator pulley belt wrap, what does that mean? Belt wrap is the contact area the belt has with the pulley. If the belt were to wrap completely around the pulley that would be 100% belt wrap. having 100% belt wrap completely around the alternator would be impossible but the greater the belt wrap the longer belt and pulley life can be expected. Alternators take quite a bit of power to run so good belt contact with the pulley is very important. If you have poor belt wrap or your pulley becomes "glazed" a glazed pulley looks very smooth it may be time to replace the pulley.

Quality alternator pulleys are very important. Many OE manufactured pulleys are made from stamped sheet metal steel. These pulleys have a tendency to heat up and warp which will cause premature belt wear, pulley "glazing" and belt failure. The better pulleys are ones that are made from billet steel, these are machined from solid chunks of steel and have a much greater mass which means they will not warp and they cool much better..

Caution must be noted: Changing the pulley size on your a lternator may void the manufacturers warranty. Spinning your Alternator faster will increase brush and bearing wear. Using a smaller pulley on a high revving engines can cause component failure from extreme speeds.Alternators are only good for about 15,000 continuous RPM's, the lower the alternator RPM's the longer your alternator will last. how to calculate your alternators RPM's you measure your crank shaft pulley size and then your alternator pulley size. Divide your alternator pulley size into your crank pulley size to get the ratio. Example: If you have a 6" crank pulley and a 2" alternator pulley. 6/2=3. You have a 3 to one ratio, you multiply your engine RPM by 3 so if your engine is turning 1500 RPM's your alternator is turning 4500 RPM's. Note: you do not count the diameter of the other pulleys on you engine, just your crank pulley and alternator pulley.