Roundback vs. Squareback alternators?

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halfafish

halfafish

Damn those rabbits, and their holes!
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It's time for another question to put to rest some idle thinking in my part of the world. I see in various restoration threads that folks are specifically wanting either a round back or square back alternator. I get the obvious with the different shape of the back side, but otherwise what's the difference in the two, and why would someone want one or the other?

Inquiring minds want to know...
 
Get the Promaster square back one-wire. Be happy!

39E5ABF6-190B-4B28-8226-1D8F706C7346.jpeg
 
The Chevy one wire is less than half price and much more easy to find.. simple lower bracket mod and some spacers..
 
My understanding, in short is:
Round back is original, for lets say 69, and takes a single field wire, used with points ignition.
While square back is original for later years and has 2 field wires, for use with electronic ignition.
 
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Joey4speed said:
My understanding, in short is:
Round back is original, for lets say 69, and has a single field wire, used with points ignition.
While square back is original for later years and has 2 field wires, for use with electronic ignition.
Click to expand...

Close... But the round back continued to be used in 70 & 71 but was changed to a two field wire configuration... Those are the oddballs that some folks seek out since 70 & 71 are considered by many as the most desirable years for collector types...

The Squarebacks have a better design of diode bridge, it runs cooler & takes more abuse...
 
Joey4speed said:
While square back is original for later years and has 2 field wires, for use with electronic ignition
Click to expand...
round back = electro mechanical VOLTAGE REGULATOR.

Square Back = electronic VOLTAGE REGULATOR.

some say that the Square back has better low RPM output.

Most round back are 1 field wire. For a year or so (70, 71?) There were 2 field (isolated field) round back alts.

Square backs are 2 field wire (isolated field) but can be wired (internally or externally) as a single field wire.
 
The square charges better at low RPM. The one wire versions are not always the best thing to do. I put a square back on Vixen. She's a 64 Valiant and came with a round back. I could tell a noticeable difference in idle charging from the round back. As long as the stock system is working well, I don't see the need to go changin it to a one wire system. I've seen Del say it can do some funny stuff.
 
Order a Roundback from your Parts store and (picture on website shows roundback style) and a Squareback style comes in. I ordered several last Fall from Autozone and they all came in with a Squareback style.
 
gary h said:
Order a Roundback from your Parts store and (picture on website shows roundback style) and a Squareback style comes in. I ordered several last Fall from Autozone and they all came in with a Squareback style.
Click to expand...
The square back is an upgrade anyway, so use it.
 
I contacted the rebuilder who advertised a round back but delivered a square back and they td me the don't have round back cores. He said all the major rebuilders are in the same boat.

I disagree that the square back charge better at low RPM. Maybe they do maybe that don't.

I do know that the correct size pulley had more to do with idle charging.

My own testing showed that a 3.0" pulley charges for ****, square back or round at idle.

A 2.5 pulley on a square or round charges great at idle.

And I have posted video proof.
 
Dana67Dart said:
I contacted the rebuilder who advertised a round back but delivered a square back and they td me the don't have round back cores. He said all the major rebuilders are in the same boat.

I disagree that the square back charge better at low RPM. Maybe they do maybe that don't.

I do know that the correct size pulley had more to do with idle charging.

My own testing showed that a 3.0" pulley charges for ****, square back or round at idle.

A 2.5 pulley on a square or round charges great at idle.

And I have posted video proof.
Click to expand...
Now, I totally agree about the pulley size, but for me, the square back charged better at low RPM, same size pulley. Now, I will say maybe the original round back might have had some issues at low RPM, but it was working and never let the battery discharge. I simply chose to make the change. Everything I have read and heard though says the square back has better charging characteristics at low RPM, though.
 
Dana67Dart said:
round back = electro mechanical VOLTAGE REGULATOR.

Square Back = electronic VOLTAGE REGULATOR.

some say that the Square back has better low RPM output.

Most round back are 1 field wire. For a year or so (70, 71?) There were 2 field (isolated field) round back alts.

Square backs are 2 field wire (isolated field) but can be wired (internally or externally) as a single field wire.
Click to expand...

No, see post 5. All round back means is that it is the earlier "rounded corners" case with individual diodes. They came both single field (grounded field) and isolated field, incorrectly called 'dual field'. Isolated field was used 70 and later, and sometime around 72-ish?? Ma converted to the newer square back design. It is a well known fact that SOME models of the squareback charged heavier at lower RPM.
 
Not to mention the square back is the easier of the two to find. That's indisputable.
 
67Dart273 said:
No, see post 5. All round back means is that it is the earlier "rounded corners" case with individual diodes. They came both single field (grounded field) and isolated field, incorrectly called 'dual field'. Isolated field was used 70 and later, and sometime around 72-ish?? Ma converted to the newer square back design. It is a well known fact that SOME models of the squareback charged heavier at lower RPM.
Click to expand...
I know mine sure beats the PANTS off the old round back at idle.
 
The round back was like either 36 or 42 amp max. With a reman' you don't know which you have. The square back starts at 60 amp.
 
Back in the day, we would be disappointed if a scrap car we bought had a roundback (isolated field). A shitload got scrapped when I moved away from home. I think I have one 1971 left.
 
Round back uses individual diodes that must be pressed out and back in and you had to do a lot of soldering to rebuild them. The square back had an easily replaceable diode pack and no soldering required. The one and two wire distinction is only because on the round back the second wire was simply grounded directly at the case where on the square back the new electronic regulator they wanted in the ground end of the alternator so they did not ground it but put a terminal so it could be run to the regulator to allow that to ground it to control regulation. The physics of the two are the same, just a different implementation. Any better functionally is because of pulley configurations to spin it faster or changes in the number of turns in the field or stator windings, not really inherent to the difference between the two implementations.
 
halfafish said:
I get the obvious with the different shape of the back side, but otherwise what's the difference in the two, and why would someone want one or the other?
Click to expand...
An isolated ground alternator can not be used with the ground controlling regulator (although some rebuilders hack them).
See Identifying Chrysler Alternators (1960-1976)

The squareback is easier to service yourself, and more likely to produce more power at lower rpm.
With a parts store or aftermarket alternator there is no way to know for sure what the output potential is.
The revised squareback is heavier, draws more field current, and is clocked differently which can put a strain on the wiring.
RedFish said:
The round back was like either 36 or 42 amp max
Click to expand...
Those might be the test specs at 1250 rpm.

By Chrysler's rating method, in 1966 they were 30, 37, 46, 60 amp.
see TSB 66-78 page 2 https://www.hamtramck-historical.com/images/TSBs/1966/66-78_page2.jpg
A-bodies did not normally get a "60 amp".
upload_2022-1-21_8-48-53.png



Chrysler's in vehicle service test was usually done at 1250 engine rpm, at 15 V.
That's the only real output number that I'm aware of.
For example a "30 amp" can produce 26 amps at 1250 rpm and 15 Volts.
upload_2022-1-21_9-17-55.png



Nobody reveals what rpm or voltage alternator amp 'ratings' actually are based on, and there is no industry standard. Probably something close to maximum, and rounded off as convenient.

A bigger list here for 1970 Dodge trucks with the new isolated field alternators The 1970 Hamtramck Registry - 1970 Dodge Truck Service Highlights Slideshow (Chapter 4)
RedFish said:
With a reman' you don't know which you have
Click to expand...
Yep!
RedFish said:
The square back starts at 60 amp.
Click to expand...
I've seen that Cardone and others rebuilders make that rating the common offering.
By Chrysler's naming/rating 34, 41 and 50 amp were generally supplied.
upload_2022-1-21_8-42-32.png


I'd take an original '50 amp' Chrysler alternator any day over a 'remanufactured' '60 amp' if given such a choice.
Generally there is little choice for every day users.
 
67Dart273 said:
No, see post 5. All round back means is that it is the earlier "rounded corners" case with individual diodes. They came both single field (grounded field) and isolated field, incorrectly called 'dual field'. Isolated field was used 70 and later, and sometime around 72-ish?? Ma converted to the newer square back design. It is a well known fact that SOME models of the squareback charged heavier at lower RPM
Click to expand...
I See your "No, see post 5" and raise you a "we are saying the same thing, reread my post 6"
:poke::rofl:
 
Effective pulley diameter is very important, especially at low rpm.
Try to stick with originals, and save yours if getting a replacement alternator.
 
It is really too bad that Ma cheap'd out and did not provide a dedicated "sense" wire for the 70/ later systems.
 
Mattax said:
An isolated ground alternator can not be used with the ground controlling regulator (although some rebuilders hack them).
See Identifying Chrysler Alternators (1960-1976)

The squareback is easier to service yourself, and more likely to produce more power at lower rpm.
With a parts store or aftermarket alternator there is no way to know for sure what the output potential is.
The revised squareback is heavier, draws more field current, and is clocked differently which can put a strain on the wiring.

Those might be the test specs at 1250 rpm.

By Chrysler's rating method, in 1966 they were 30, 37, 46, 60 amp.
see TSB 66-78 page 2 https://www.hamtramck-historical.com/images/TSBs/1966/66-78_page2.jpg
A-bodies did not normally get a "60 amp".
View attachment 1715856596


Chrysler's in vehicle service test was usually done at 1250 engine rpm, at 15 V.
That's the only real output number that I'm aware of.
For example a "30 amp" can produce 26 amps at 1250 rpm and 15 Volts.
View attachment 1715856607


Nobody reveals what rpm or voltage alternator amp 'ratings' actually are based on, and there is no industry standard. Probably something close to maximum, and rounded off as convenient.

A bigger list here for 1970 Dodge trucks with the new isolated field alternators The 1970 Hamtramck Registry - 1970 Dodge Truck Service Highlights Slideshow (Chapter 4)

Yep!

I've seen that Cardone and others rebuilders make that rating the common offering.
By Chrysler's naming/rating 34, 41 and 50 amp were generally supplied.
View attachment 1715856595

I'd take an original '50 amp' Chrysler alternator any day over a 'remanufactured' '60 amp' if given such a choice.
Generally there is little choice for every day users.
Click to expand...
I was forced to get a " lifetime" one. China Remann. Ate one already. Less than a year.... but yeah, all are 60 it seems
 
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