Engine speed vs. Alternator output

[TheBrian]

This isn't Subaru-specific, but I thought I'd ask it here, where the gearheads are. Besides, it was an old-gen thread that got me thinking about this.

 

The alternator spins at different speeds based on how fast the engine is spinning. How do Subaru engineers cope with that?

Does it produce a whole lot more power at high speeds than low ones?

What happens when the alternator doesn't need to produce much power, but it's spinning fast anyway?

 

How much of the alternator's power output is used by the ignition system, which obviously uses more power at higher revs?

[MorganM]

Voltage regulator keeps it from sending too many volts and amps through your electrical system. At idle it should produce 12-13volts and at 2000+ RPMs it should not exceed 15-16 volts. Dunno max amps at 12 volts the system can take. I'm guessing well over 100 amps @ 12 volts since people run 100 amp GM alternators on Subarus.

 

Stock ignition system on a Subaru 4 cyl isnt going to use much of your amps from the alternator. Coil will prolly take the most for the ignition system. All your accessories are gonna drain more amps than anything. Say having on your head lights, radio, auxilary electric fan, etc.

 

Hope that answers your questions.

[bratman2]

Most alternators produce maxium output at just a few hundred rpms over idle. For example I have a 25% underdrive pulley on my pickup truck that has been converted to twin electric cooling fans. The underdrive pulley is rated to reach maxium alternator output at app. 950 rpm, the 42% underdrive reaches it at 1150 rpm. Stock idle is 650 rpm which at that rpm output is reduced, I offset this by porting, polishing and half shafting the throttle body which raised idle to 750rpms. Hope this clears up some of your questions.