1967 Barracuda alternator upgrade question
All good points. Maybe it will help to explain alternator performance.
1. Alternator output will equal demand, as long as demand does not exceed the alternator's capability.
examples. Lets say the ignition draws 3 amps, and the alternator needs about 2 amps for the field circuit, so the alternator output is 5 amps.
2. Potential alternator power goes up with rpm. At a certain rpm, it reaches maximum.
Ratings can be done any way the manufacturer wants. It can be 90, 95%, or 100% max or whatever is convenient.
Here's some examples of the rpm curve for AC-Delco 15Si alternator. (I've not found a published curve for our alternators)
Alternator RPM on the graph is approximately 3 times crank rpm for a AMC Jeep 360, one application the 15 SI was used on.
However notice these graphs indicate they are for amperage out at 12 Volts. If that's true, then the amps on a system regulated at 14 volts will be less. In other words, at any given power output, the amps available can be made to look higher by rating at a lower voltage.
So an alternator rated at 12 Volts will have higher amp rating than if it was rated at 14 volts.
To give you some idea of the Chrysler roundback rating vs outputs -
The tests in the service manual are done at 15 Volts, engine speed 1250 rpm.
37 amp unit should put out 34.5 amps.
46 amp unit should put out 41 amps.
60 amp unit, should supply 51 amps.
At idle speeds, they will supply less power. At slightly lower voltage, they will supply a little more current.
The ammeter will indicate whenever the battery has to take over supplying power because the alternator can't keep up.* If you are seeing discharge on the ammeter under your normal driving conditions, then a higher rated unit may be better. If it only can't keep up at low rpm, then the max rating is less important than what the unit can do at low rpm. Sometimes higher rated units do worse at low rpm. In general, the newer (square backs for example) tend to be a little better at low rpm than the roundbacks.
The ammeter itself should only be seeing current flow when starting and when the battery is recharging. It is reasonably well protected by the fusible link. The link will melt first in any major overload. Problems occur with repeated moderately high current flows because the battery is doing extra work (that the alternator should be doing) and poor connections.
*Understanding Charging Systems with Ammeter
All good points. Maybe it will help to explain alternator performance.
1. Alternator output will equal demand, as long as demand does not exceed the alternator's capability.
examples. Lets say the ignition draws 3 amps, and the alternator needs about 2 amps for the field circuit, so the alternator output is 5 amps.
2. Potential alternator power goes up with rpm. At a certain rpm, it reaches maximum.
Ratings can be done any way the manufacturer wants. It can be 90, 95%, or 100% max or whatever is convenient.
Here's some examples of the rpm curve for AC-Delco 15Si alternator. (I've not found a published curve for our alternators)
Alternator RPM on the graph is approximately 3 times crank rpm for a AMC Jeep 360, one application the 15 SI was used on.
However notice these graphs indicate they are for amperage out at 12 Volts. If that's true, then the amps on a system regulated at 14 volts will be less. In other words, at any given power output, the amps available can be made to look higher by rating at a lower voltage.
So an alternator rated at 12 Volts will have higher amp rating than if it was rated at 14 volts.
To give you some idea of the Chrysler roundback rating vs outputs -
The tests in the service manual are done at 15 Volts, engine speed 1250 rpm.
37 amp unit should put out 34.5 amps.
46 amp unit should put out 41 amps.
60 amp unit, should supply 51 amps.
At idle speeds, they will supply less power. At slightly lower voltage, they will supply a little more current.
The ammeter will indicate whenever the battery has to take over supplying power because the alternator can't keep up.* If you are seeing discharge on the ammeter under your normal driving conditions, then a higher rated unit may be better. If it only can't keep up at low rpm, then the max rating is less important than what the unit can do at low rpm. Sometimes higher rated units do worse at low rpm. In general, the newer (square backs for example) tend to be a little better at low rpm than the roundbacks.
The ammeter itself should only be seeing current flow when starting and when the battery is recharging. It is reasonably well protected by the fusible link. The link will melt first in any major overload. Problems occur with repeated moderately high current flows because the battery is doing extra work (that the alternator should be doing) and poor connections.
*Understanding Charging Systems with Ammeter