If you’ve read any of owner Tony Candela’s books, you’re well aware of the importance of a properly sized alternator for your vehicle. What makes one company’s 150 Amp alternator different from another company’s 150 Amp alternator? Why is one company’s 150 Amp alternator twice as expensive as another’s? The answers are simple: quality and performance.

Understanding Alternator Ratings

The Big Number – this is the one most companies put out there: 140 Amps, 150 Amps, 200 Amps, etc. When shopping for alternators, you simply cannot make an educated decision with only this number!

What You Really Need to Know

  • Idle Output – This is defined as how many Amps the alternator can output at 800 engine RPM with the alternator spinning at 3x crankshaft speed.*
  • Cruise Output – This is defined as how many Amps the alternator can output at 2,000 engine RPM with the alternator spinning at 3x crankshaft speed*.

*A properly pullied alternator should spin at three times the speed of the crankshaft; i.e., at 800 engine RPM, the alternator will be spinning at 2,400 RPM.

So, when you’re looking at that advertisement for a 150 Amp alternator, you’re looking only at its Cruise Output rating. Here in the real world, we enthusiasts spend an awful lot of time at stop signs, stop lights, and just idling. Idle Output is the REAL measure of an alternator’s performance and is a reflection of how well the alternator will perform under the toughest of conditions; it’s what separates the players from the posers! Alternators with excellent Idle Output are more expensive to manufacture and therefore more costly.

Here are some of the pitfalls of choosing an alternator that cannot keep up with demands at idle:

  • Overheating
  • Poor idle characteristics
  • Dead battery
  • Dim headlights
  • Short alternator life

Choosing an alternator for your vehicle with an Idle Output rating that exceeds the total current requirements of all of your accessories (electric fans, fuel pumps, water pumps, ignition system, EFI, etc.) is the key to building a reliable charging system that won’t ever let you down! This process is described in great detail in Automotive Wiring and Electrical System Volume 2: Projects and EFI Conversions.

We’ve sold and installed hundreds of alternators from all of the best companies over the years. We proudly offer Mechman alternators.

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