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I’m getting a new accessory drive setup and the include alternator is a 240 amp one wire unit, I currently have a 200amp 3g unit and with all of my load at idle voltage wasn’t always over 14 volts, it would’ve been nice to see, I’ve got, large fuel pumps, a stereo system and a Mark VIII fan along with a couple other items, anyhow, is there an issue with a one wire setup at idle, that’s my main concern, will it output enough juice while at idle with all of my stuff on?
 

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is there an increase in current at idle speed with the 240?

i dont think so.

you need to compare rated output at idle speed.
 

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Most one wire alternators have a minimum shaft rpm before they will self excite and start charging. You would want to check with your manufacturer as to their spec for exciting speed and charging rate.

A big issue is accessory drive pulley configurations. Underdrive pulleys can be a big issue here. From my earlier searching, I found having a 3:1 Alternator to Crankshaft RPM ratio is desired on a street car, with a 2,400 minimum alternator shaft RPM and an 18,000 maximum alternator shaft RPM. I have an older Powermaster 130 amp alternator in the Delco 12si case and getting the correct pulley setup installed (removing my underdrives) was a huge help with charging at idle. I had an ASP 1.87" alternator overdrive pulley, but I ended using a 2.0" pulley from Powermaster to keep the alternator shaft speed below 18,000 at 6,000 crankshaft RPM.

Pulley speed formula: Crank Pulley OD / Alternator Pulley OD X RPM of Tach (1000, 2000, 3500...)= Alternator RPM
Online pulley speed calculator: Pulley Calculator. RPM, Belt Length, Speed, Animated Diagrams
Supporting tech info at: Power Pulleys and Which Alternator?

All that to say, give your accessory drive pulley configuration a hard look.

(Edited for typos)
 

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If you use a gm 1 wire alternator and it has a minimum rpm to self excite, it can be overridden by providing power to one of the terminals on it with a second wire. Then it charges at any rpm.
 
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