Whats the difference when putting a FD Alternator into an FC? Car juss gets more Energy out of the Engine?

i keep seein the FD alternator thing on here and didnt know what it did.

 

Higher amps than a FC alternator. I just put a new S4 alternator in my car, and it flow tested at 80 amps, but the FD one tests at over 120 I believe..

 

yeh so how does the higher amps help the car? if it could run fine with 80 why is it better with more?

 

The alternator can't handle the full electrical load of a FC, especially with aftermarket stereos or headlights or car computers or such.

The car will run fine with the FC alternator, but there may be times when the electrical load is more than it can handle. Winter driving comes to mind, and is one of *many* reasons I don't like to drive my FC in the snow.

My normal snowstorm running load:
Engine, ignition, fuel, etc.
Headlights (more powerful low beams than stock)
Wipers on low or high
Heater blower on full
A/C compressor on (to dry the defrost air)
Rear window defroster
Stereo

Basically every electrical load the car has, and sustained low RPM/idle operation. The voltages I see indicate that I'm actually running partly on battery - it's below the no-load voltage of the battery.

The FC alternator is fine for most stuff, especially if the car sees mostly light electrical load/higher RPM operations (summer car, highway cruiser, etc). BUT, if the car is under a heavier electrical load, the FC alternator just doesn't cut it.

-=Russ=-

 

hmm ic that explains it a lot.

seeing that i dont have a/c or a radio installed righ tnow... not necessary.

 

... So, I was curious, bored, & felt like taking the FC out for a bit of a spin.

My car: 1988 SE, stock S4 alternator, upgraded headlights (though I used high beams for the tests, which should be reasonably similar - just my low beams draw more), stock stereo. Voltage numbers were pulled from the readout on my SAFC-II, which are a few tenths lower than multimeter-on-battery-posts voltage. Also, I took all these numbers after I had gone for a spin, so the battery was topped off & the idle was down at it's proper 750 RPM.

Full load is as defined above: Headlights, wipers, blower on max, A/C on, stereo on, rear window defroster on.

12.2v: Voltage when I got in, car hadn't been run for several days, ignition on, no other loads.
12.5v: Voltage with the engine shut off after a 20 minute run (and about a minute to let the battery settle down some)
13.9v: Voltage at idle, no load (other than engine systems)
13.9v: Voltage at 2500 RPM, no load (other than engine systems)

From this, I'd say it's safe to assume that 13.9v (as read on the SAFC... it's closer to 14.5v at the terminals) is the regulated voltage, and what the system is designed to be putting out. Now for the load tests.

12.2v: 750 RPM at full load
13.3v: 2500 RPM at full load

At idle, the alternator is definitely not providing enough power to run everything. The car is running partly off the battery, and if left like this for a long time would drain the battery even with the engine running. At 2500 RPM, the voltage is better, but still lower than the designed voltage, indicating that the alternator is running out of capacity.

This is with a more or less stock FC. Now, imagine what the load will be with other fairly common items: A stereo amp or two, a car computer/laptop, an inverter, etc. It's quite clear that in slow stop & go traffic, the FC alternator flat out can't keep up, and even at higher RPM it's still having a lot of trouble. Putting in a more powerful alternator fixes this problem.

-=Russ=-

 

If you have a lot of power options, (power top, a/c, power windows, steering, stereo) it would still benefit you greatly. Also if you are running an e-fan, I would highly suggest the s6 alt due to the extra load placed on your electrical system during idle, when the fan would be running.

 

Ah! I didn't even think about an electric fan! Some of those draw a LOT of power, and it would indeed be when you weren't moving fast.

-=Russ=-

 

so i understand. its a great dead battery saver. great upgrade. got it thanx

 

Its not necessary to upgrade the alternator unless you have a stereo system or other upgrades that put a large draw on your battery.

Let's see, I'm on my 4th FC over the past 18 years and have yet to upgrade the alternator. Now, if my alternator fails, you betcha since it is easy to do and not expensive.


BTW, almost all cars are unable to provide sufficient current at idle to power EVERY possible electrical item.

I only drive my FC for pleasure and that sure as hell doesn't include traffic (stop and go driving)

 

Yea its a pain to try and put up both windows and the sunroof at the same time. Ill be getting an FD alt as soon as I can get the money.

 

The stock FC alternator, in most cases, is jus underpowered. Whle the S4 alternator is rated at 70A, many struggle to produce 60A (the poster that tested to 80 had an unusually strong alternator, and I doubt it could do 80A for long before burning up). The altarnator is underpowered from the factory, and under high load (ie. heater, defog, headlights...that's almost a 50A load!) cannot supply the necessary juice. Add a stereo and electric fan, and you have issues. S5 alternator is about the same, but I believe it is rated at 80A.

The FD alternator is around 100A, which can properly supply current and keep the battery charged under almost any load. If you need more then 100A, you can pick from any number of "one wire" Chevy aftermarket alternators and adapt to fit.