Bigger than you think! Select an alternator based on its performance and reliability first, cosmetics second. Chrome won’t get you home – certainly not when it comes to alternators. You should choose your alternator based on a criteria that most of the mainstream alternator manufacturers don’t talk much about – output at idle. Look at it like this – you really need your alternator the most when you’re idling at a traffic light. For alternators, idle is defined as 800 engine RPM and it should be pullied to spin at 3X crankshaft speed for correct operation. It is not uncommon to require 100 to 125 amps (or even more) at idle if you’re running electric fans, an electric fuel pump, EFI, etc. Running too small of an alternator is the #1 mistake made by enthusiasts. This process is outlined fully in Chapter 5 of Automotive Wiring and Electrical Systems, Volume 2: Projects.
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