I forgot to mention that all this was data was taken with the car fully warmed up, in a garage at idle.
Ron
Ron
Real Data on alternator size
5
There is nothing like working on real systems to get your head calibrated correctly on issues like alternator size.
Apologies in advance to those I have lead to believe that 100A or so is adequate.
My Chevelle is a 69 that has been "updated" in a bunch of ways. Some of those are electrical.
I have a Spal 3200 CFM single fan, an MSD 6AL, a FAST ECM, and a G2XPro data Aq system for track days. I don't have dual fans (to be specific) nor do I have a big stereo amp (yet).
I have a 12Si 94 amp alternator on the car, and from posts on this forum and other comments from club members, friends and acquaintances I have been informed that this is probably adequate for the application.
The wiring has been upgraded also to take care of the increased output of the alternator. The fan wiring, ECM and MSD wiring are not hooked to any of the original electrical system that GM designed and installed.
The first Oscillograph below shows the nominal output of the alternator at idle with no additional loads like head lamps, A/C, fans, etc.
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Everything looks good. The car takes about 22A to run.
Now we will increase the load by turning on the head lights on high beams>
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Notice that the current went up by 20A. The battery voltage is still where it was under the first oscillograph at about 14.6 to 14.8V
Now we will add wipers and the heater blower on high>
![]()
We are now on the edge of something that is acceptable. If I were in the rain (wipers on), heater running (it was cold), on a slick hiway (going slow) with little change in rpm, I am just on the edge of keeping up with the loads.
A few short burst of even a little RPM would help me out as is shown below:
![]()
But if it were stop and go (especially if I had an automatic) and it was a stop and go freeway on a real hot day (A/C and cooling fan running) with a little rain (wipers again) I would have this:
![]()
A dead battery in short order.
Now that I have the data, it seems I will need an alternator that has an output of 100A at idle to tack care of another fan (20amps) the A/C compressor clutch (7A???, didn't measure it yet) and the stereo 9A.
If you have a carbureted engine you can probably get 10Amps or so off of what I am showing as you don't have to drive the fuel injectors.
Oh yea, FAST Systems don't like low buss voltages, they get erratic when the voltage drops to 12 or so. Because of this you get an interaction between the ECM trying to maintain idle and the alternator trying to drag down the rpm.
It would appear that a CS144 series, 140A alternator would better serve my needs. It has an output of 80A at 1500 alternator RPM, about where my engine idle is.
Bottom line is that most of us are better off considering what the alternator is rated for at idle rather than it's full tilt output capability.
Ron
Apologies in advance to those I have lead to believe that 100A or so is adequate.
My Chevelle is a 69 that has been "updated" in a bunch of ways. Some of those are electrical.
I have a Spal 3200 CFM single fan, an MSD 6AL, a FAST ECM, and a G2XPro data Aq system for track days. I don't have dual fans (to be specific) nor do I have a big stereo amp (yet).
I have a 12Si 94 amp alternator on the car, and from posts on this forum and other comments from club members, friends and acquaintances I have been informed that this is probably adequate for the application.
The wiring has been upgraded also to take care of the increased output of the alternator. The fan wiring, ECM and MSD wiring are not hooked to any of the original electrical system that GM designed and installed.
The first Oscillograph below shows the nominal output of the alternator at idle with no additional loads like head lamps, A/C, fans, etc.
Everything looks good. The car takes about 22A to run.
Now we will increase the load by turning on the head lights on high beams>
Notice that the current went up by 20A. The battery voltage is still where it was under the first oscillograph at about 14.6 to 14.8V
Now we will add wipers and the heater blower on high>
We are now on the edge of something that is acceptable. If I were in the rain (wipers on), heater running (it was cold), on a slick hiway (going slow) with little change in rpm, I am just on the edge of keeping up with the loads.
A few short burst of even a little RPM would help me out as is shown below:
But if it were stop and go (especially if I had an automatic) and it was a stop and go freeway on a real hot day (A/C and cooling fan running) with a little rain (wipers again) I would have this:
A dead battery in short order.
Now that I have the data, it seems I will need an alternator that has an output of 100A at idle to tack care of another fan (20amps) the A/C compressor clutch (7A???, didn't measure it yet) and the stereo 9A.
If you have a carbureted engine you can probably get 10Amps or so off of what I am showing as you don't have to drive the fuel injectors.
Oh yea, FAST Systems don't like low buss voltages, they get erratic when the voltage drops to 12 or so. Because of this you get an interaction between the ECM trying to maintain idle and the alternator trying to drag down the rpm.
It would appear that a CS144 series, 140A alternator would better serve my needs. It has an output of 80A at 1500 alternator RPM, about where my engine idle is.
Bottom line is that most of us are better off considering what the alternator is rated for at idle rather than it's full tilt output capability.
Ron
1 - 15 of 15 Posts
M&H Darren is correct, the factory harness and the aftermarket harnesses are not capable of handling 100A.
If you run a larger gauge, like on my car (below), you can run the big wire (8 AWG in my case) over to the horn relay in front of the radiator support. If you really are going to run runs and a big stereo with A/V stuff you might want to consider using a 6AWG) and a bigger alternator.
Please excuse that "nick" in the insulation, very bad form.
The reason for running it to the horn relay is to get the output of the regulator to the point where the alternator senses it's output voltage (point of regulation). This point should always be at 14.4V or so when the car is running. The battery voltage will always be less, 13.2 to 13.8V.
From the buss on the horn relay you can add additional circuits by putting an additional set of fuses to run out to the other loads you are adding. In my case I take off power for a large Spal fan (26-28 amps) and the FAST fuel injection (10 amps). In this way the cars wiring is stock, except for the new loads. The down side is that the wire from the battery to the horn relay is too small. It should be made 8 awg also.
We are fixing this in another project car.
We have been working on another Chevelle that has dual Derale fans (22A each, 44A total), with aftermarket power window openers for front and back (25A front and 25A rear) and a moderate stereo (10A). We have moved the point of regulation to a new panel with a fan controller, 100A buss bar and relays and got rid of the horn relay. From the 100A bus bar we run the new circuits and the headlamps rather than from the new American Autowire Fuse Box we are installing. This additional 76 amps comes from the Bus bar through a Maxi fuse and 8AWG wire through a bulkhead feed through to a Bluesea fuse Box under the dash.
This new panel is located under the front fender next to the battery. This makes all of the high power wiring from battery and alternator to the fan shorter; less loss.
Headlamp current is also taken off the bus bar through a fuse and relays so that the factory fuse box has very little current taken from it which improves the voltage available for the stuff under the dash (wipers, interior lamps, running lights, brake lights, etc).
When I get some pictures I will post them.
There is no one "right" way to do this but there are ways that are safe with good performance and ways that are unsafe and dangerous.
Ron
1 - 15 of 15 Posts
