Roundback or Squareback ?

[DM70GTX]

1970 Plymouth GTX 440ci Super Commando (automatic, non-ac).

What is the correct alternator and amperage for this car? The build sheet says "78" with regards to alternator. Squareback or roundback?

I appreciate the help,

D

 

[pnora]

From the factory 1970 parts manual.

 

[69Bee]

1) Alternator Stamped Number PN#3438178, Pulley PN#2875711, Double Groove, 50A, Round Back
2) Alternator Stamped Number PN#3438378, Pulley PN#2444478, Single Groove, 34A, Round Back

 

[dadsbee]

Round back in 1970 / 71... square there after..

 

[dadsbee]

PN#34383178, Pulley PN#2875711, Double Groove, 50A, Round Back

 

[69Bee]

Damn cut-n-Paste...

 

[Nacho-RT74]

70/71 are specific units, being still roundback like earliers but both brushes isolated ready for electronic regulator like squarebacks (they began on 72)

They look exactly like this

HOWEVER, if you want to make the propper investment and not looking for a “correct resto job matching numbers for a trailer Queen”, but enjoy the car, get a more powerful unit (no matter if you car is non A/C, but still with the heater blower) able to source no less than 45-50 amps at iddle.

There IS NO factory alternator up to late 70s or 80s, able to feed that. You can get aftermarket HiPo units (95-100 amps) on US$200-250 rate (still being roundback and ready with isolated brushes) or get an 80s 78/80 Amps alt which are squareback revised versions (a bit wider than the first version) on $70-100 rate from parts dealers. These are couple of millimeters wider and depending on engine setup, fitment can be a deal to deal with (BTDT). These are also tipically fitted with a smaller pulley which makes to give us more juice (or sooner into the RPMs curve) from it.

Some other option is the NipponDenso units, which are tipically on 60 amps max output rate BUT way better iddle rate than factory Mopar units (this is due the diff stator winding design). These require a diff bracket setup thought (available around from several dealers offering the conversion)

I allways advice NO MATTER THE CAR, YEAR OR ACCs, our cars require NO LESS than 45 amps alt capability at iddle. Remember alt will provide the juice requested for, having a 300 amps alt doesn’t mean will provide 300 amps. Just IS ABLE to do it if requested.

Having the source able to feed any requested power from the car at any engine speed will get you on a RELAXED charging system, STILL on original wiring since a discharged battery won’t be anymore a problem which will increase the power demand constantly.

(Although a small wiring upgrade is good)

 

[Nacho-RT74]

Although I’m fan of TuffStuff products, PowerMaster catalog website offers more info.

Roundback, ready for dual field (both isolated brushes) single pulley, original cast finish

65 amps at iddle (able to give)
75 average
95 max output

TuffStuff offers the first squareback version (not the revised one like PowerMaster), which is the correct look to my 74, hence I went with TuffStuff

 

[DM70GTX]

Thanks for the info, very helpful. I have a 34A roundback on the shelf but seems like it won't be "ideal" as far as amperage output goes. The car is pretty much stock with the exception of a Mopar electronic ignition upgrade. The car has all new stock/repro wiring harnesses from YearOne.

What is the largest alternator (max amperage output) that my electrical system can safely handle without needing any upgrades to wires and such? Will I have any issue with a 60A-95A alternator with my existing wiring?

Thanks

 

[Nacho-RT74]

As mentioned MY OPINION is the best is get an alt able to provide something around 45 amps at idle. That guarantees all operational system of your car including blower heater high speed, wipers and lights could be covered by the alt at the same time at any alt speed and the batt will never (or barely) requested for power, so no discharge at all. You amm will (if any) barely moves out of center which is the goal on this.

Your car will ALLWAYS request the same power no matter the power source. All the deal here is centralize the power source on alt side leaving the batt out of the game as much as possible.

Does your wiring hold the extra power capability from the source?… YES. The wiring is set depending on the device demand, not by the power source, and you have not changed anything on that.Things change when you begin to add accesories.

A 34 ALT would barely reach maybe 20 amps at idle. Dunno what was thinking Chrysler with that alt capability, except a constant discharged battery and dimming lights.

NOW is true that there is a weak spot on the charging wiring system which is the packard terminals, when this began to burn and melt down all around. Packard terminals are able to hold maybe barely 25 amps for some time. When an extra power running through the charging system for battery recharge was a constant demand due the lack of power coming from alt at low speed this increases the load going throught the packard terminals way higher that its limits as soon you gives throttle increasing the alt output. HOWEVER is good to save this problem to the future.

PERSONALLY I think with just run a parallel wire between alt stud and amm stud should be enough (at least at first instance) to save this soon to come huge problem.

You can go further forward on more wiring upgrades, but WITH JUST THIS, you should be already safe in case the batt gets some unexpected discharge for whatever reason adding this extra load out of the regular car demands but now for the batt recharge.

Now the bulkhead amm-batt path… will be good also to save it HOWEVER, being with a new powerful last you battery will barely enter be on the game, you STILL could be safe on the actual stage without any other job on wiring being fresh.

MANDATORY: NEVER FEED ANY EXTRA ACCESORY IF ADDED to the batt. This is WRONG when you have an ammeter as a measure gauge. ALT side is the correct procedure to feed ANYTHING into the original wiring design.

There are way more info on a stickied thread on board, where 72RoadRunnerGTX post very good info with videos AND I post there the link of all this info I made back in 2007 on another board with several diagrams explaining this and showing the ammeter reaction to every stage. In my basic English LOL. Also several ideas for wiring upgrades.

Could be nice also check the ammeter conditions INSIDE the cluster. This gauge is tipically found been abused for years with the constant back and forth charging and discharge stages for long time periods. Also incorrectly sourcing added accesories along the years. Loosen studs at internal shunt from previous overheating procedures, broken isolation etc…

NEED TO ADD… MY 74 (A/C CAR) WHEN IN ONE PIECE YET (LOL) WAS A REAL DAILY DRIVER AND I PROCEEDED WITH THE ALT UPGRADE WITH NO OTHER MOD THAN THE PARALLEL TO THE BULKHEAD PATHS STILL KEEPING AMMETER AND ADDING RELAYS AS OTHER UPGRADES (For halogen sealed beams,for example)

¡NOT A PROBLEM AT ALL, NO HEATING ANYWHERE, AND AMM BARELLY MOVING!

 

[451Mopar]

Thanks for the info, very helpful. I have a 34A roundback on the shelf but seems like it won't be "ideal" as far as amperage output goes. The car is pretty much stock with the exception of a Mopar electronic ignition upgrade. The car has all new stock/repro wiring harnesses from YearOne.

What is the largest alternator (max amperage output) that my electrical system can safely handle without needing any upgrades to wires and such? Will I have any issue with a 60A-95A alternator with my existing wiring?

Thanks

The factory wiring under the dash to/from the Ammeter, Ignition Switch, and Fuse Block is 12 AWG wire size, and the terminal connections are the Packard 58 series (56 series is a common replacement.) The connectors and wiring were designed when the alternators pot out less than 40 Amps.
72 RoadRunner has some Youtube videos on this here: 1972 Road Runner GTX
He has some videos on how the factory upgraded the wiring for higher output alternators that retains the Ammeter.
I don't care to modify the dash wiring, and I don't need to retain the Ammeter function (I leave it alone, nothing done under the dash.)
To do that change, I just remove the 12 AWG (Black) Alternator wire from the bulkhead connector and Alternator stud, and run a new large 6 AWG or larger wire from the alternator to the battery.
On the bulkhead connector, I connect the Terminal the Alternator used to connect to, to the terminal the battery power comes into the bulkhead connector in parallel. both terminals/wires protected by the single 16 AWG fusible link, and the current is shared by both packard terminals. This protects the dash wiring 9and bulkhead terminals), and if you overload the dash wiring it should burn the fusible link.
To reduce under dash current load, I put the headlights and other high current loads on relays. Individual relays/fuses get to be a mess, so I use Power distribution fuse/relay modules as it cleans up the wiring. The Eaton Bussman 1530X series are decent: https://assets-fe147638ed.cdn.insitecloud.net/d6119cbcbed99f7/46343.pdf