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Username Post: How to size an alternator for my restomod        (Topic#277736)
57TOMMY 
Frequent Contributor
Posts: 1216
57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-10-12 07:44 AM - Post#2200720    

Hey Guys
I have added items to my car , 1000 watt amp 2 10 inch subs, colling fan to my radiator & wanted to figure out if my 100 watt alternator is big enough. Is there a simple way to calculate the best size alternator I need?
Thanks for your help
Tommy

 
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acardon 
Senior Member
Posts: 9942
acardon
Loc: DFW TEXAS
Reg: 03-25-05
03-10-12 07:57 AM - Post#2200730    
    In response to 57TOMMY

A 1000 watt amplifier will draw 83 amps when it's wide open. Fans can draw 30 to 40 amps. Oops, ran out of juice already. The best thing to do is measure the current draw with everything turned on that you use constantly. In other words, don't count door locks or power windows as they are not on long enough to matter. A auto electric shop should be able to measure the current if you don't want to buy a clamp on ampmeter. Their kinda expensive.
Don
66 Corvair (driving)
57 2dr HT (driving)
56 2dr HT (waiting to be restored)


 
57TOMMY 
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57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-10-12 09:39 AM - Post#2200766    
    In response to acardon

Thank you Don
Stand up for what is right ,even if you stand alone.



 
beejay 
Ultra Senior Member
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beejay
Age: 78
Loc: Pflugerville, Texas
Reg: 06-01-04
03-10-12 11:58 AM - Post#2200810    
    In response to 57TOMMY

Tommy,

I just bought a 140A from Summit for my Elky. It has the new-fangled lights, H3 fog lights, and two electrice fans. That's plus a low-wattage radio (Sony) with two rear, 7" speakers in the rear and two small tweeters in the dash. Reason I bought it? At night, with brights on AND fans off, the lights were flickering on and off (on relays, same as fans). On dims, the lights work OK. I can just imagine what ruckus would be raised if I were on brights, and the fans kicked on, or added to that the AC was put on. Well, so much for an 80-something Amp alternator. Time to swap. Incidentally, you didn't say if you have AC, and use it.
Bruce

'56 4-door BelAir, 350, Holley 600, Eddie intake, TKO 600, CPP P/S and A arms, Sierra Gold & Adobe Beige
2010 VW Jetta S/W, 2.5, 5-cyl,6-spd auto.
'87 Elkie, 350 with 700r4 tranny B&M floor shift
http://www.picturetrail.com/beejay3/"


 
triplefive 
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triplefive
Loc: Brisbane Australia
Reg: 07-08-11
03-10-12 05:46 PM - Post#2200938    
    In response to beejay

Audio equipment is notorious for being over-rated.

These days, most amplifiers are rated at their peak output (top of the sine waves) instead of RMS (root mean square, which is average - or true output). And then most of them just advertise a lot of downright BS on top of that. They have to do it because everyone else does it.

I have a 1000 watt Yamaha P.A. system for musical instruments and vocal. When you look at the input requirements in amperage and times it by the voltage, it actually puts out to the speakers about three times what it takes from the powerpoint. This, as we all know, is against the laws of physics, but why let the truth interfere with a good yarn. Almost all sound systems are peddled this way. There are some exceptions in the very expensive high-end studio and opera house stuff and, I think, many good quality guitar amps use RMS. That's why a good 100 watt guitar amp has no problem competing with a 1000 watt PA.

If your 1000 watt amp actually puts out 1000 watts RMS (extremely unlikely), you would need about 1500 watts from the alternator due to losses in the amplifier, but I'd be very, VERY dubious.

I'm not in any way being critical of the system you bought. It could be a great amp, but 100 TRUE watts in a car with the right speakers will almost blow you out of the seatbelt.

I've used a 300 watt sound system (150 RMS or true watts per side) for vocals and piano at a wedding in a large hall with a 25 piece bigband and it kept up no probs without being driven too hard.

If I were you, I'd allow about 10 amps for the sound system (20 max) and I think you'll have it covered. The size fuse on the + lead might be a fair guide to the real story. I bet it's not 83 amps.
Cheers.
Mike.
Mike.

Procrastination; the one thing I never put off until tomorrow.


 
57TOMMY 
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57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-10-12 07:04 PM - Post#2200969    
    In response to triplefive

Oh. I know it's not putting out 1000 watts , thats just the amp size. I thought a 140 alternator would do the trick. But can you go overboard with an alternator. If I go to 160 or 200 will it do any harm?
Stand up for what is right ,even if you stand alone.



 
triplefive 
Contributor
Posts: 695
triplefive
Loc: Brisbane Australia
Reg: 07-08-11
03-10-12 07:39 PM - Post#2200990    
    In response to 57TOMMY

  • 57TOMMY Said:
But can you go overboard with an alternator. If I go to 160 or 200 will it do any harm?



Absolutely no problem. You can go 500 amps if you want. the output will only ever be what's needed, and the load on the engine will only reflect what you are using at the time.

160 or 200 might not actually be necessary, but it can't do any harm and at least you know you won't be left short
Mike.
Mike.

Procrastination; the one thing I never put off until tomorrow.


 
57TOMMY 
Frequent Contributor
Posts: 1216
57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-11-12 08:07 AM - Post#2201139    
    In response to triplefive

thank you all
Tommy
Stand up for what is right ,even if you stand alone.



 
MikeB 
Senior Member
Posts: 9533
MikeB
Loc: Plano, TX
Reg: 08-28-03
03-12-12 10:58 AM - Post#2201589    
    In response to triplefive

  • triplefive Said:
Audio equipment is notorious for being over-rated.

These days, most amplifiers are rated at their peak output (top of the sine waves) instead of RMS (root mean square, which is average - or true output). And then most of them just advertise a lot of downright BS on top of that. They have to do it because everyone else does it.



Well said. Custom Autosound is a big-time offender. Their overblown "specs" make them sound better than RetroSound, which uses conservative RMS specs. Kinda funny because the RetroSound probably has a much better amplifier section. One of the cleanest sounding car audio systems I ever had used a "little" 25W x 4 channel Sony amp.

1982 C-10 SWB pickup -- Unmolested, original paint, 250 six, 3-on-the-tree
(Soon to be 350 w/Vortec heads & TH350)

Sold my 55 sedan in June 2014, having owned it 11 years.


 
triplefive 
Contributor
Posts: 695
triplefive
Loc: Brisbane Australia
Reg: 07-08-11
03-12-12 05:40 PM - Post#2201739    
    In response to MikeB

If that was RMS, I bet that 'little' 4x25 had some punch to it.

I'm not one for over-regulation, but in this case, I wish someone would legislate to make it illegal to advertise other than demonstrated proven RMS output only. I know output in watts is not the only factor to consider, but It would level up that part of the playing field at least, and you could then look at what else you want in a sound system. Much better than checking what size the input fuse is and trying to figure it out from there.

Sorry about the thread drift. I just have a thing about it that's all.
Mike.
Mike.

Procrastination; the one thing I never put off until tomorrow.


 
jk56chevy 
"11th Year" Silver Supporting Member
Posts: 848
jk56chevy
Age: 58
Loc: maryland
Reg: 02-27-05
03-13-12 08:13 AM - Post#2201972    
    In response to triplefive

Tommy:

I have 2 Arc audio amps, 600 & 400 amps, 2 10 inch subs, 2 electric fans and I went with the MSD 160 amp alternator. No problems at all for the last couple of years.
Keep in mind that MSD told me that alternator needs to excite and with everything on I need to keep the RPMS at 1000 to 14+ volts on the volt guage. I did put the smaller March Pulley on the alternator to get it to spin faster. They said it can handle 30,000 rpms.

Good Luck!
John
John
1956 Chevy Belair


 
1960_Guy 
"3rd Year" Platinum Supporting Member
Posts: 81
1960_Guy
Loc: New Orleans LA
Reg: 01-23-12
03-13-12 09:46 AM - Post#2202007    
    In response to jk56chevy

As stated before the current drawn from the Alternator will just be the connected load, so bigger only creates one issue, you need to size the output wire to handle the maximum alternator current or install a fuselink that will protect the output wire against handling more current than it is rated for, otherwise in the case of an overload or fault on the output wire you could do damage to it and create a fire potential.

 
57TOMMY 
Frequent Contributor
Posts: 1216
57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-14-12 05:37 PM - Post#2202686    
    In response to 1960_Guy

thanks guys .looking at a 160 altenator ,see how that works
Stand up for what is right ,even if you stand alone.



 
busyguy8 
Dedicated Enthusiast
Posts: 5665
busyguy8
Age: 44
Loc: SF Bay Area, CA
Reg: 11-21-04
03-14-12 08:45 PM - Post#2202777    
    In response to 57TOMMY

Tommy, are you having problems currently? I would think the 100 amp is enough. Look up the RMS of your amp online to see if it's very high.

TOMMYYYYYYYY!!!!!!!!!!!! :-)
Aquired a '56 2dr hdtp BelAir on 6/9/05!
Link to my pics!
-bill


 
2Loose 
"10th Year" Platinum Supporting Member
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2Loose
Age: 72
Loc: Sandwich Isles
Reg: 03-17-03
03-15-12 08:56 AM - Post#2202957    
    In response to busyguy8

Interesting thread. I just got my 55 "gasser" running, and put on a fairly new single wire alternator I had laying around, not sure what it was rated at.

Lots of MSD ignition equipment, very large fuel pump, and two large fans on the radiator is about all I have, haven't run at night yet so don't know what the headlights will draw.

When driving around yesterday, fans had not come on yet, the volt meter was showing a full 13 volts. It dropped to 12 suddenly and when I stopped and checked, the fans had come on. So now I can tell when the fans are coming on and off, as the voltage shows 13 or 12 volts.

I plan on adding some sound equipment (what, isn't the blower whine and the gear whine enough???) and am thinking I don't have enough alternator output if it drops to 12 volts when the fans come on. What if I was cruising at night with the lights on and a stereo blasting away?

Is there any way to find out the output rating of an alternator when it is already installed?

Aloha,
Willy

 
acardon 
Senior Member
Posts: 9942
acardon
Loc: DFW TEXAS
Reg: 03-25-05
03-15-12 10:40 AM - Post#2202991    
    In response to 2Loose

Many alternators have the output stamped on the case, but if it's been chromed or polished, it's probably been ground off. An auto electric shop can measure the alternator output and can also determine the total current required by all your devices.
What size is the alternator to battery wire? 12 volts is too low. You have to have at least 13 to begin chargeing a battery.
Don
66 Corvair (driving)
57 2dr HT (driving)
56 2dr HT (waiting to be restored)


 
2Loose 
"10th Year" Platinum Supporting Member
Posts: 4108
2Loose
Age: 72
Loc: Sandwich Isles
Reg: 03-17-03
03-15-12 11:18 AM - Post#2203000    
    In response to acardon

I am suspicious that my dash mounted voltmeter might not be that accurate. I connected my timing light, with has voltage indication on it, and it registered higher than the dash gage by about one volt. So 14 to 13 instead of 13 to 12. If I am really getting the higher voltage, and know what it really is, I'm ok with that and will just leave the dash gage alone. Have a very heavy cable from the alternator to the batt, and back up to the front from the batt in the trunk. Elec cut off switch on the rear cuts off everything, including the alternator.
Willy

 
57TOMMY 
Frequent Contributor
Posts: 1216
57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-15-12 05:58 PM - Post#2203173    
    In response to 2Loose

it runs a little over 13 on my amp guage, is that about where it should be? 100 alternator right now. No problems right now Busy just going over the car early for hot August nights & i'll get a bigger alt if I need one. Last battery crapped out on me but it was 5 years old.
Tommy
Stand up for what is right ,even if you stand alone.



 
56BUDDY 
Senior Member
Posts: 804

Loc: Union City, CA
Reg: 12-31-02
03-15-12 08:17 PM - Post#2203231    
    In response to 57TOMMY

Tommy,
Did you get your tranny dialed in?
Mel




 
57TOMMY 
Frequent Contributor
Posts: 1216
57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-16-12 05:34 PM - Post#2203587    
    In response to 56BUDDY

Hey Mel
It's better with the new govenor. Have a 4th to 2nd downshift now. wil try the 6 cyl gov when the weather gets better. Been too busy lately to work on the car
See you soon
Tommy
Stand up for what is right ,even if you stand alone.



 
Bruces 57 
Frequent Contributor
Posts: 2001

Reg: 01-03-07
03-17-12 04:13 AM - Post#2203687    
    In response to 57TOMMY

Hello;
All this talk of alternators gets me to thinking.
My electrical system was re wired with a EZ harness. One of the features of this harness is a wire going directly to the harness from the alternator (exciter). I have a single wire on my 32 Hot Rod and it always seemed not to have much output. This car had a lot of electrical gremlins as it is a fiberglass car and the grounding was really crappy! I didn't want that on my 57. So, I read lots of articles on this subject and found that if I hooked up my alternator 3 wire, the voltage was continious at 14. Another area of thought was that (mentioned earlier) if you change the pulley on the alternator (single wire) the voltage comes up more earlier and stays higher through out the rpm range of the engine.
Single wire alternators need RPM to excite the regulator in them to put out voltage while 3 wire hook ups are at peak voltage right from start up.
I tend now to think that any internally regulated alternator works best hooked up 3 wire more than a single wire.

Bruce

 
57TOMMY 
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Posts: 1216
57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-17-12 07:01 AM - Post#2203732    
    In response to Bruces 57

I am hooked up with 1 wire but might try the 3 wire to see if there is a difference. good idea
Thanks
Stand up for what is right ,even if you stand alone.



 
2Loose 
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03-17-12 09:46 AM - Post#2203777    
    In response to 57TOMMY

A very interesting comment. I did not know that a one wire alternator could be reconnected as a three wire. How do you do that?
Willy

 
57TOMMY 
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Posts: 1216
57TOMMY
Age: 56
Loc: Bay Area , Calif
Reg: 08-19-03
03-17-12 12:02 PM - Post#2203815    
    In response to 2Loose

My wiring harness came with both 1 & 3 wire hookups
Stand up for what is right ,even if you stand alone.



 
beejay 
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beejay
Age: 78
Loc: Pflugerville, Texas
Reg: 06-01-04
03-17-12 01:39 PM - Post#2203839    
    In response to 2Loose

Willy,

A "dual" alty has a rubber plug covering the two-connector where a plastic plug goes into it. One side goes to the alternator post (red wire), and the other wire goes to the an ignition switch connection to excite the process. Sooo, you have an exciter connection, a connection from the same connector to the post connection, and a connection from the post to the +. Hence a 3-wire.
Bruce

'56 4-door BelAir, 350, Holley 600, Eddie intake, TKO 600, CPP P/S and A arms, Sierra Gold & Adobe Beige
2010 VW Jetta S/W, 2.5, 5-cyl,6-spd auto.
'87 Elkie, 350 with 700r4 tranny B&M floor shift
http://www.picturetrail.com/beejay3/"


 
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