question on a 90 amp vs 120 amp alternator

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my68barracuda

my68barracuda

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Would going from a 90 amp to a 120 amp nipendenso style externally regulated alternator produce more current at idle and low rpm’s? Or is the boost in amperage just at the rpm’s that produce near peak alternator output?

And a second question, at what engine rpm should an alternator like those mentioned be producing near peak power?

The associated wiring has already been upgraded, I would need to up the fuse on the alternator output lead.
 
Those are recalled a little bit of research that I did when I was trying to up the amps was that very thing that you're talking about. I stepped up to the 90 AMP but noticed the going much higher wouldn't get me anything where I really need it when the engine was idling. I mean at some point you can start just recharging the battery faster I reckon... But the problem is when the temperature is hot and I'm running the AC and I'm running the electric fans and I'm at a stop I went ahead and not only got the 90 AMP but got the yellow top optima battery as well...
 
I have a Powermaster 120 amp internally regulated and it is great at idle. No flickering lights but I also installed crackedback’s headlight conversion wiring.
 
GM powermaster alternator output graph. Note shaft RPM NOT engine RPM. Look like they ramp up pretty fast.



CS144.jpg


Here is an upgraded Denso chassis style high output. Alternator shaft rev is 2.0-2.05 x engine RPM>

1080005_140amps_graph.jpg
 
j par said:
Those are recalled a little bit of research that I did when I was trying to up the amps was that very thing that you're talking about. I stepped up to the 90 AMP but noticed the going much higher wouldn't get me anything where I really need it when the engine was idling. I mean at some point you can start just recharging the battery faster I reckon... But the problem is when the temperature is hot and I'm running the AC and I'm running the electric fans and I'm at a stop I went ahead and not only got the 90 AMP but got the yellow top optima battery as well...
Click to expand...
I have a 90 amp alternator in there now, and the scenario that you mentioned, everything in the car running and stuck in an idle situation is what I am thinking about. That is why I was considering picking up a 120 amp alternator probably at a pull a part,, but only if that would help a idle rpm’s
 
my68barracuda said:
I have a 90 amp alternator in there now, and the scenario that you mentioned, everything in the car running and stuck in an idle situation is what I am thinking about. That is why I was considering picking up a 120 amp alternator probably at a pull a part,, but only if that would help a idle rpm’s
Click to expand...
I never in my life have paid more than $40 for a reconditioned battery LOL... I just had had enough of it and thought I'm going to buy a bigger alternator which for me that was a 90 AMP and I figured a red top battery at first but then as I got to reading about the yellow top being a hybrid with deep cycling capability I figured that out of keep everything running while it's idling for a few minutes until I can get going and get it charged back up. So far so good...
 
You'd have to find the specs on the specific alternators you are interested in. Just being bigger is no guarantee that it's better at low RPM. However I'd have to say that "most" newer designs like ND are likely better than most any of the 60's-70's stuff

Frankly I'm surprised that a ND 90 does not satisfy you. You might want to do some troubleshooting first, like:

What size are the pulleys and determine if it's spinning fast enough

Put it under heavy load, like, idle, with all accessories, heater, headlights, etc running. Measure voltage between alternator output stud and the battery, and see what the voltage drop is on the charge wire?
 
I run the power master 95 amp external reg runs the sniper , msd , thermo fan, min 13.4 @ idle the whole box and dice. No issues here . Sorry to add Crackedbacked head light setup aswell .
 
thanks for the replies, as always great information here at FABO. My inquiry was more of a 'what if scenario'. Gives me something to think about.
I do plan to go through the system and check for voltage drops.
 
put a smaller pulley on it. at idle you will be rewarded. BUT be sure that the shaft RPM of the alternator will not be greater than the max rating for the alternator at your max engine RPM.

I went from about a 3in dia to about 2.5in dia (closer to what the FSM says should be on my dart) and my discharge at idle with the brakes on is almost non existent
 
Dana67Dart said:
put a smaller pulley on it. at idle you will be rewarded. BUT be sure that the shaft RPM of the alternator will not be greater than the max rating for the alternator at your max engine RPM.

I went from about a 3in dia to about 2.5in dia (closer to what the FSM says should be on my dart) and my discharge at idle with the brakes on is almost non existent
Click to expand...
Great idea, where does on find smaller diameter pulleys? Do you scrounge around pull a parts or is that a catalog item?
 
Bigger crank pulley is the easiest, like Kim said. But if you're stuck like me (fluidampr, only one pulley that fits and it's an under drive) then there are options.

If you have the mid size denso, 90A from a dodge truck, it uses a 17mm shaft. I ordered this pulley, but it required modification, bored out to 17mm, machining to clear the housing and a shim behind the pulley. If you have access to a lathe it's a fairly simple fix.

Unfortunately, in the 17mm shaft size, there are limited options for smaller diameter pulleys

This gave me 14.1v at 850rpm

http://www.metroautoinc.com/new/index.php?route=product/product&product_id=1007
 
Might be able to get a pulley from a commercial "power transmission" supply store. I go to the local store, and they have been able to order pulleys for me.
Miller Bearing
Also try a company that rebuilds alternators/starters.
 
I would guess "you won't." When I was in that business that stuff was all "U.S." sizes, fractional or decimal. The shaft/ bearings on alternators is almost universally metric---a 203 bearing.

Although you can get some taper lock products in metric sizes
 
17mm was the same size as my treadmill motor shaft. seems a pretty common size for modern imported alternators and motors. I took the pulley off a Dodge minivan alternator and used it on the 90V DC treadmill motor to reduce the ratio for a bandsaw build. That and a 10.5 inch pulley brought the speed down and the torque up.
 
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