alternator power loss question

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

Calculate with an ohms law chart.
P=watts
1 hp = 745 watts

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.

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

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 (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
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

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.

You are probably at less than 1 hp if the power draw is only 15 or 20 amps.

Math is fictional.
What matters is how you feel about things.

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.

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.

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

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.

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.

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.

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

Double check those fans as that seems awful high.

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.