Pulley Sizing and Pulley Ratio

Pulley Sizing and Pulley Ratios- The majority of alternators on the market have a maximum rotor speed of 18,000rpm to 21,000rpm. Exceeding the maximum rotor speed can damage your alternator and result in costly rebuilds, but to insure the best low-speed performance out of your alternator you want to be sure that your unit is spinning as fast as possible at low speed. This is where proper pulley sizing becomes critical. To determine your rotor speed you need to know 3 things: 1. Crankshaft pulley diameter 2. Alternator pulley diameter 3. Maximum engine speed (redline or rev limiter). To calculate pulley ratio simply divide the crankshaft pulley diameter by the alternator pulley diameter (Ex: 6” crank / 2” alt = 3 for a 3:1 pulley ratio). Now to calculate the rotor speed you divide the crankshaft pulley diameter by the alternator pulley diameter, then multiply the result by your maximum engine speed (In this example we will use a 7,000rpm max engine speed. Ex: 6”crank / 2” alt = 3. 3 x 7,000 = 21,000). New cars have lower idle speeds these days, many under 600rpm while having a 7,000rpm+ redline, making it vital to determine the proper pulley sizing to insure that the rotor speed fast enough for good low-speed performance while preventing damage from excessive rotor speed. Underdrive pulleys can slow down the rotor speed causing very poor low-speed and creating the need for smaller alternator pulleys. Smaller pulleys also offer less belt traction and if too small will reduce the output of your alternator due to excessive belt slip so calculating the proper pulley size is very important. Overdrive pulleys like those used on supercharged cars such as the Ford Mustang Cobra, Pontiac Grand Prix GTP and many others can spin the alternator too fast by altering the pulley ratio. On these cars you may need a larger diameter pulley to prevent alternator damage, but this isn’t a bad thing as it increases belt traction for your high-output alternator.