alternator question
- Thread starter Johnny
- Start date
Lee_in_KC
Active Member
Add up the amperages of all your electrical loads in the "worst-case, steady-state" scenario, i.e. electric radiator fan running, electric fuel pump running, headlights on high-beam, etc. The amperage of your alternator needs to at least meet this load or you will be draining the battery during this scenario.
Example:
radiator fan = 30 amps
fuel pump = 30 amps
high beams = 20 amps
other loads (running lights, instrument lights, etc.) = 10 amps
Total = 90 amps
If you have a mechanical fan and fuel pump you save yourself 60 amps right there, so you could get away with a smaller alternator. I wouldn't go less than 60-amp.
Example:
radiator fan = 30 amps
fuel pump = 30 amps
high beams = 20 amps
other loads (running lights, instrument lights, etc.) = 10 amps
Total = 90 amps
If you have a mechanical fan and fuel pump you save yourself 60 amps right there, so you could get away with a smaller alternator. I wouldn't go less than 60-amp.
Hotrod46
Member
It should work fine. Most of these cars don't have a lot of electrical load. I would guess that the alternator you have is about 50 - 60 amps. If it's a stock GM unit, it should have the amperage rating stamped into the casing.
I'm using a 55 amp Nipondenso unit from a Geo Metro and have no voltage issues. Runs a minimum of 14 volts with all loads on (lights, trailer lights,MSD ignition and electric fuel pump). I don't have an electric fan, though.
I would use what you have and check the battery voltage with all loads on. As long as you have a minimum of 13.5 volts, you should be good to go. If not, change it out for a higher amp rating.
One thing about the high amp alternators is that they don't always put out their full rated amps at low speed. Also, the one wire units have to be turned up to high RPM's on start up to get them charging.
Mike
I'm using a 55 amp Nipondenso unit from a Geo Metro and have no voltage issues. Runs a minimum of 14 volts with all loads on (lights, trailer lights,MSD ignition and electric fuel pump). I don't have an electric fan, though.
I would use what you have and check the battery voltage with all loads on. As long as you have a minimum of 13.5 volts, you should be good to go. If not, change it out for a higher amp rating.
One thing about the high amp alternators is that they don't always put out their full rated amps at low speed. Also, the one wire units have to be turned up to high RPM's on start up to get them charging.
Mike
ellis8500
Member
Looks like you are adding the fuse amp ratings. The real loads are much smaller than the fuse ratings. Fuse amp ratings are based on the wire size being used so that a short circuit blows the fuse rather than start the wires on fire. Bright headlights are usually 55 to 60 watts which is about 5 amps each. We use 16 ga wire which is good for 20 amps. Hence, a 20 amp fuse.
Lee_in_KC
Active Member
Sorry Bill (and others), I didn't intend for my numbers to be used literally. I just pulled some numbers out of my a$$ to illustrate the point. You're right that steady-state the loads will be well be below what the circuit is wired and fused for. You did it right looking up the ratings for each of your components.
fordsbyjay
Active Member
A lot of aftermarket fans can draw up to 25 amps. Thirteen has to be one of the lowest I have ever seen but lower the better. That being said to answer the OP, 60 amp should be enough but if it was me I would do at least 100 amp.
2old2fast
Active Member
On startup , I think my cooling fan must draw at least 25 amps, it cooked a 14 ga. fusible link in real hot weather last year so I put in a 30 amp fuse instead , seems to be working fine , also at the same time it cooked a 30 amp relay , so i replaced that w/ a 40 amp , again , seems to be o.k.
dave
dave
benT
Active Member
I don't see the need for a 100a alt on a T-bucket. Old fords had 65 amp alts and had a bunch more power drain than most buckets-- just my humble opinion.
Both my dad and I run 50a chrome mini alternators (Nippendenso) for space reasons and have a 14" spal electric fans with no charging issues. I don't run an electric pump or a radio however. My dad does have a Holley electric pump and it is still not an issue. There is no problem staying above 13a with the lights on. The mini alts are readily available on ebay for about $75 bucks unchromed and $125 chromed.
Both my dad and I run 50a chrome mini alternators (Nippendenso) for space reasons and have a 14" spal electric fans with no charging issues. I don't run an electric pump or a radio however. My dad does have a Holley electric pump and it is still not an issue. There is no problem staying above 13a with the lights on. The mini alts are readily available on ebay for about $75 bucks unchromed and $125 chromed.
Lee_in_KC
Active Member
We need to keep in mind that there are different load states for various electrical components. I've been using the term "steady-state" to indicate the load once the component has spun up or achieved equilibrium. Many dynamic loads have a high amperage draw when they first come on line or spin up. Once they are stabilized, the load is significantly reduced. Something like a radiator fan takes a lot of amperage to come up to speed, but once it's there it draws a lot less. In the case of a fuel pump, the electrical load is going to depend on how hard the pump has to suck to get the fuel out of the tank and how much resistance to flow it is seeing at the delivery end. You don't necessarily need to size the alternator for the transients... you just need to understand what the total draw on your battery will be during the transients.
fordsbyjay
Active Member
Load can also vary between brands as well as age. Newer pumps and motor can be a lot more efficient then older technology which explains why fuel pumps have dropped their current draw over the years. All of my Spal fans require a 30 amp relay so if you can find one that only uses 14 amps that is great. I have never measured the actual current draw, I just ran the recommended wire size and appropriate relay. Stuck on the side of the road on fire is not the time to find out how much money you saved buying too small of wire.
I do like those small alternators though, do they come in a higher amperage?
I do like those small alternators though, do they come in a higher amperage?
CrAlt
New Member
+1
The higher the amprage the less efficent the alternator is at idle. That is because there needs to be thicker wire in the windings. Thicker wire = less windings in the same space. Its a trade off.
My plan is to use an alt off a '89 firebird...because i have a few spares. Its a 85amp unit. The car has electric fans, A/C, electric fuel pump and EFI, a massive rear defroster, motorized headlighs, 6 tail light bulbs, etc.. And even with my aftermarket stereo/amp and MSD the voltage holds steady. My feeling is its going to be overkill for my bucket.
