Previous Thread
Next Thread
Print Thread
alternator power loss question #2687934
08/15/19 07:55 AM
08/15/19 07:55 AM
Joined: Oct 2003
Posts: 3,784
South Park, Pa.
68LAR Offline OP
master
68LAR  Offline OP
master

Joined: Oct 2003
Posts: 3,784
South Park, Pa.
How much power does an alternator pull from an engine ?


4 speed street legal. Best time 10.99 @ 124 mph on 93 octane pump gas @ 3926# total weight
Re: alternator power loss question [Re: 68LAR] #2687940
08/15/19 08:16 AM
08/15/19 08:16 AM
Joined: Feb 2005
Posts: 14,928
PA.
pittsburghracer Offline
"Little"John
pittsburghracer  Offline
"Little"John

Joined: Feb 2005
Posts: 14,928
PA.
Originally Posted by 68LAR
How much power does an alternator pull from an engine ?




Probably less than running with a over worked battery. Been there done that and I Will never race without one again

Last edited by pittsburghracer; 08/15/19 08:17 AM.

1970 Duster
Edelbrock headed 408
6.143@110.85 mph
9.73@135.05 mph shifting at 6700 RPM
422 Indy headed small block
5.982@112.56 mph
9.42@138.27

Livin and lovin life one day at a time




Re: alternator power loss question [Re: 68LAR] #2687959
08/15/19 09:10 AM
08/15/19 09:10 AM
Joined: Jan 2005
Posts: 1,906
Milwaukee WI
T
TRENDZ Offline
top fuel
TRENDZ  Offline
top fuel
T

Joined: Jan 2005
Posts: 1,906
Milwaukee WI
Calculate with an ohms law chart.
P=watts
1 hp = 745 watts

F4F17143-C306-4242-9DF9-259C63F3536D.jpeg
Last edited by TRENDZ; 08/15/19 11:44 AM. Reason: Mo info

"use it 'till it breaks, replace as needed"
Re: alternator power loss question [Re: 68LAR] #2688029
08/15/19 12:23 PM
08/15/19 12:23 PM
Joined: Jan 2003
Posts: 26,327
Oregon
A
AndyF Offline
Too Many Posts
AndyF  Offline
Too Many Posts
A

Joined: Jan 2003
Posts: 26,327
Oregon
Not very much. I'm pretty sure a 1 hp electric motor would spin an alternator at full charge. Normal use the alternator only puts out a few amps. Even with a big fuel pump and big radiator fan you should only be pulling 30 or 40 amps max.

Re: alternator power loss question [Re: AndyF] #2688042
08/15/19 12:52 PM
08/15/19 12:52 PM
Joined: Jan 2003
Posts: 2,032
NE Ohio
DoubleD Offline
top fuel
DoubleD  Offline
top fuel

Joined: Jan 2003
Posts: 2,032
NE Ohio
Somewhere in the back of my head - I seem to remember 1HP per 25 amps as a rough estimate - so figure for a 100 amp alternator at full output at about 4 HP - pretty much insignificant draw on total HP

Re: alternator power loss question [Re: 68LAR] #2688050
08/15/19 01:09 PM
08/15/19 01:09 PM
Joined: Aug 2003
Posts: 33,765
Bend,OR USA
C
Cab_Burge Offline
I Win
Cab_Burge  Offline
I Win
C

Joined: Aug 2003
Posts: 33,765
Bend,OR USA
Dyno testing years ago on a motor with battery only(two fully charged dyno batteries) feeding the MSD 7Al2 ignition made less power than with a alternator mounted and working on the same motor twenty minutes later shruggy(2 or3 HP,CRS)
I saw the same thing at the track with my alternator gutted by bypassing the blue Holley fuel pump using the Carter NASCAR Hemi mechanical fuel pump only, MSD race ignitions need and like good voltage and current work
As already pointed out the cars current requirements will have a large effect on any testing, especially with large draw electric water pumps and fans work

Last edited by Cab_Burge; 08/15/19 01:10 PM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: alternator power loss question [Re: 68LAR] #2688051
08/15/19 01:15 PM
08/15/19 01:15 PM
Joined: Jan 2003
Posts: 26,327
Oregon
A
AndyF Offline
Too Many Posts
AndyF  Offline
Too Many Posts
A

Joined: Jan 2003
Posts: 26,327
Oregon
Originally Posted by 68LAR
How much power does an alternator pull from an engine ?


Describe your setup and we can tell you. Electric fuel pump? Big MSD ignition system? Electric water pump? Electric fans? Anything else?

Re: alternator power loss question [Re: AndyF] #2688082
08/15/19 02:43 PM
08/15/19 02:43 PM
Joined: Oct 2003
Posts: 3,784
South Park, Pa.
68LAR Offline OP
master
68LAR  Offline OP
master

Joined: Oct 2003
Posts: 3,784
South Park, Pa.
Originally Posted by AndyF
Originally Posted by 68LAR
How much power does an alternator pull from an engine ?


Describe your setup and we can tell you. Electric fuel pump? Big MSD ignition system? Electric water pump? Electric fans? Anything else?



Nothing to extravagant, MSD 6AL, and electric fuel pump. That"s it unless I run at night, then we got to consider head lights.

Thanks for all the replies, by the way. I appreciate it.


4 speed street legal. Best time 10.99 @ 124 mph on 93 octane pump gas @ 3926# total weight
Re: alternator power loss question [Re: 68LAR] #2688193
08/15/19 06:29 PM
08/15/19 06:29 PM
Joined: Jan 2003
Posts: 26,327
Oregon
A
AndyF Offline
Too Many Posts
AndyF  Offline
Too Many Posts
A

Joined: Jan 2003
Posts: 26,327
Oregon
You are probably at less than 1 hp if the power draw is only 15 or 20 amps.

Re: alternator power loss question [Re: 68LAR] #2688209
08/15/19 07:30 PM
08/15/19 07:30 PM
Joined: Oct 2007
Posts: 3,352
New York
polyspheric Offline
master
polyspheric  Offline
master

Joined: Oct 2007
Posts: 3,352
New York
Math is fictional.
What matters is how you feel about things.


Boffin Emeritus
Re: alternator power loss question [Re: polyspheric] #2688246
08/15/19 09:04 PM
08/15/19 09:04 PM
Joined: Jan 2003
Posts: 26,327
Oregon
A
AndyF Offline
Too Many Posts
AndyF  Offline
Too Many Posts
A

Joined: Jan 2003
Posts: 26,327
Oregon
The formula assumes zero friction. I don't know how much power the v belt saps but I'm sure there is some. Same with the bearings inside the alternator and the little cooling fan and all that jazz. I don't have any numbers for all the little friction loads but I'm sure it adds up. My guess is the alternator costs about 1 hp even if it is only charging 20 or 30 amps. Bottom line I don't think it matters much. If I was building a bracket car to go rounds I'd put an alternator on it. It is only a few pounds of weight and a very small power drain but it cuts down on the monkey motion in the pits. I'd rather not have to hook up the generator every time I pull back into the pits. It is just one more thing to remember to do.

Re: alternator power loss question [Re: AndyF] #2688248
08/15/19 09:17 PM
08/15/19 09:17 PM
Joined: Feb 2005
Posts: 14,928
PA.
pittsburghracer Offline
"Little"John
pittsburghracer  Offline
"Little"John

Joined: Feb 2005
Posts: 14,928
PA.
I bought my stock Chevy one wire off a table at Norwalk over five years ago for 25.00. It puts out 50-60 amps. It runs my taillights, BG400 fuel pump, miesere water pump, two fans, racepak, MSD7al, electric shifter, and probably a couple of other things. I race sometimes several times a week and charge my battery once a month to top it off.


1970 Duster
Edelbrock headed 408
6.143@110.85 mph
9.73@135.05 mph shifting at 6700 RPM
422 Indy headed small block
5.982@112.56 mph
9.42@138.27

Livin and lovin life one day at a time




Re: alternator power loss question [Re: pittsburghracer] #2688259
08/15/19 09:52 PM
08/15/19 09:52 PM
Joined: Jun 2003
Posts: 51,356
Romeo MI
MR_P_BODY Offline
Master
MR_P_BODY  Offline
Master

Joined: Jun 2003
Posts: 51,356
Romeo MI
I had my alt hooked up so I could turn the field on and off... set up that way with a charged batt there
was less than .001 difference on the track.. back to back runs
wave

Re: alternator power loss question [Re: 68LAR] #2688391
08/16/19 11:27 AM
08/16/19 11:27 AM
Joined: Mar 2007
Posts: 2,909
West Coast, USA
jbc426 Offline
master
jbc426  Offline
master

Joined: Mar 2007
Posts: 2,909
West Coast, USA
On a side note, a single V-belt's ability to transfer power is about 5hp max. I found that out when I was upgrading my alternator to increase the amp output at idle. Had to go with a GM CS-144 for that duty, as it puts out over 115 amps at a low idle. Bolted right in too.

Last edited by jbc426; 08/16/19 05:17 PM.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 3.91's (closest thing to an automatic 5 speed going)
Re: alternator power loss question [Re: AndyF] #2688392
08/16/19 11:27 AM
08/16/19 11:27 AM
Joined: Jan 2003
Posts: 6,109
Minnesota
Hemi_Joel Offline
master
Hemi_Joel  Offline
master

Joined: Jan 2003
Posts: 6,109
Minnesota
Originally Posted by AndyF
The formula assumes zero friction. I don't know how much power the v belt saps but I'm sure there is some. Same with the bearings inside the alternator and the little cooling fan and all that jazz. I don't have any numbers for all the little friction loads but I'm sure it adds up. My guess is the alternator costs about 1 hp even if it is only charging 20 or 30 amps. Bottom line I don't think it matters much. If I was building a bracket car to go rounds I'd put an alternator on it. It is only a few pounds of weight and a very small power drain but it cuts down on the monkey motion in the pits. I'd rather not have to hook up the generator every time I pull back into the pits. It is just one more thing to remember to do.



Plus the power required to overcome the inertia of the armature as the engine rpm climbs in each gear. I suspect that in the lower gears, this could be more than the power required for producing electricity.

Last edited by Hemi_Joel; 08/16/19 11:29 AM.
Re: alternator power loss question [Re: AndyF] #2688488
08/16/19 04:19 PM
08/16/19 04:19 PM
Joined: May 2008
Posts: 5,204
Aurora, Colorado
451Mopar Offline
master
451Mopar  Offline
master

Joined: May 2008
Posts: 5,204
Aurora, Colorado
Originally Posted by AndyF
The formula assumes zero friction. I don't know how much power the v belt saps but I'm sure there is some. Same with the bearings inside the alternator and the little cooling fan and all that jazz. I don't have any numbers for all the little friction loads but I'm sure it adds up. My guess is the alternator costs about 1 hp even if it is only charging 20 or 30 amps. Bottom line I don't think it matters much. If I was building a bracket car to go rounds I'd put an alternator on it. It is only a few pounds of weight and a very small power drain but it cuts down on the monkey motion in the pits. I'd rather not have to hook up the generator every time I pull back into the pits. It is just one more thing to remember to do.


Like Andy mentioned, the loss to electrical load alone is pretty small. Once the battery is fully charged the alternator need to supply the amperage to run whatever electric items the car is using, maybe 20-30 Amps unless you have a killer fuel pump or cooling fan. Note that the electrical load loss in not related to RPM.
The other alternator losses like the cooling fan, (drag force) increase with RPM.
The mass of rotating parts and friction will create a load when changing speed (acceleration) or F=Ma
As I understand it, In theory friction force does not increase with velocity, but I think real friction forces (viscous and unbalance of rotating parts) increases with both Velocity and Acceleration.

Re: alternator power loss question [Re: 451Mopar] #2688964
08/18/19 04:56 AM
08/18/19 04:56 AM
Joined: Dec 2018
Posts: 49
MASSACHUSETTS
C
Chip Offline
member
Chip  Offline
member
C

Joined: Dec 2018
Posts: 49
MASSACHUSETTS
Sorry to jump off subject alittle here...

But could I get some guidance on my set up? How much alternator do I need to drive the following electrical loads in my drag car?

Magnafuel 500 fuel pump at 13-amps
Meziere water pump at 12-amps
Duel fans on my radiator 30-amps each to start and 20-amps to run each fan
MSD 7AL 3 ignition with a MSD HVC-2 Coil
No lights except gauges

I have installed a East Coast Mini Alternator (50-55-amp) and a Optima Yellow Top Battery with a #8 gauge wire from the trunk located battery to the alternator.


One friend tells me I should be OK

Another says I need a bigger alternator and larger wire from battery?

Thanx, Chip

Re: alternator power loss question [Re: Chip] #2688987
08/18/19 07:58 AM
08/18/19 07:58 AM
Joined: Feb 2005
Posts: 14,928
PA.
pittsburghracer Offline
"Little"John
pittsburghracer  Offline
"Little"John

Joined: Feb 2005
Posts: 14,928
PA.
Double check those fans as that seems awful high.

Last edited by pittsburghracer; 08/18/19 07:59 AM.

1970 Duster
Edelbrock headed 408
6.143@110.85 mph
9.73@135.05 mph shifting at 6700 RPM
422 Indy headed small block
5.982@112.56 mph
9.42@138.27

Livin and lovin life one day at a time




Re: alternator power loss question [Re: pittsburghracer] #2689070
08/18/19 11:57 AM
08/18/19 11:57 AM
Joined: Oct 2004
Posts: 2,963
back in Georgia
dthemi Offline
master
dthemi  Offline
master

Joined: Oct 2004
Posts: 2,963
back in Georgia
People that race heads up are mostly on lithium batts that deliver full voltage during discharge without an alt. Bracket racing you'd always be better off with an alt. Repeated cranking, ect without an alt will leave ya stuck in the lanes one day. Worst case you need a jump, but once running you're fine. I run alts on my sp bracket car and foot brake car. The other 2 are lith without alt, but they both push to the line.


Advertisement
Sponsored Link
Moparts Newest Topics
2007 Dodge 2500 electrical issues
by hulmule. 12/07/19 11:47 PM
dealer stock searching
by crowbait. 12/07/19 11:21 PM
oil pan gasket replacement '93 Dakota V6
by Satilite73. 12/07/19 10:44 PM
issues with fresh 440
by stinger. 12/07/19 06:46 PM
Moparts Recent Posts
Alcohol Carb
by rick4106. 12/08/19 05:52 AM
NOS 77/78 Truck grill on Ebay
by chargervert. 12/08/19 05:14 AM
Possible to convert 6 volt gen to 12 volt?
by Cab_Burge. 12/08/19 02:19 AM
1969 Daytona 727 transmission
by NANKET. 12/08/19 12:40 AM

Powered by UBB.threads™ PHP Forum Software 7.7.1