Alternator Cutoff

[RednBlack]

Anyone know how much HP an alternator pulls? And would it be worthwhile to have a cutoff switch and run on battery for the track?

[SappySE107]

well, compared to the water pump and power steering...nothing. I wouldn't do that at all, as the loss of voltage will probably hurt your times more than the alt draw would. Injectors and coil packs don't really care for 12 volts.

[RednBlack]

Just wondered. Old school really differs. Back when we had 3 belts, water pump was the only one left on.

[SappySE107]

You didnt have fuel injection or tons of sensors and electrical devices in the car either:P

[Lyle's GTP]

Well, 120 Amps x 14.5 Volts = 1740 Watts (@ peak efficiency) 1 HP = 746 Watts

1740 Watts/ 746 Watts = 2.33 HP

Now you have to consider efficiency losses, and static mechanical losses. A rough guesstimate would point to about 3.5 - 4 HP lost at peak alternator output. I'm not sure about the efficiency of these things or static load. So maybe someone else could elaborate more.

Lyle

[SappySE107]

stock alt is 105 amps, and its 14.4 volts optimal with the car running.

[Gemmer]

I wonder about the friction on the pullys bearings. How many RPMs does the pully turn per crank rotation?

maybe a larger pully would be in order?

[Juglenaut]

Well, 120 Amps x 14.5 Volts = 1740 Watts (@ peak efficiency) 1 HP = 746 Watts

1740 Watts/ 746 Watts = 2.33 HP

Now you have to consider efficiency losses, and static mechanical losses. A rough guesstimate would point to about 3.5 - 4 HP lost at peak alternator output. I'm not sure about the efficiency of these things or static load. So maybe someone else could elaborate more.

Lyle

That is just HP equivalent to Watts produced, it has nothing or little to do with lag on the belt and other systems.

You would have to figure the dampener pully to alt pully ratio and find the drag at the dampener. since the dampener is larger and the alt pully is smaller the effect of the alt would be decreased proportionally.

Since the unregulated, unrecitifed Voltage Peak to peak is quite abit so the load has to load down the regulator before it starts lagging the winding armuture.

[firechiken]

Do you guys have all this in your heads? When ever I need to figure something like this out, I have to consult numerous books and such. Guess I should have laid off the green stuff when I was younger!

[Lyle's GTP]

Yep, up here in the old melon.

Hey Jugs , what you said is what I meant by static load. The load of the friction of the pulley system, bearings, etc. Thanks for elaborating.

Lyle

[TheProfessor]

someone said recently in another thread that you can use a 3800 alternator pully to underdrive a 660 alternator i wonder if thats so

[Lyle's GTP]

I personaly feel that under driving an alternator is a bad idea. In fact under-driving any of the accesories is a bad idea. (in my opinion) When I changed my alternator to high output it changed my output curve signifigantly. I get shit for voltage/current at stock idle. I actually had to "overdrive" my alternator with a smaller pulley.

My lights dim all the time at idle, the power windows suck, the turn signal slows down, etc. I have actually heard that it fucks withe the things you don't know about. For example all your relays, ignition coils, doorlock selonoids, power window motors. All that shit suffers prematur failure when used at a low voltage. Not too mention you rack and pinion system fucks up when you don't have correct hydraulic pressure. As far as the water pump goes, I would imagine that you car would try to run hotter.

Something to consider. If it were for race purposes only, go for it, short term won't hurt anything.

Lyle

[butchville]

or run a 16 volt battery an alternatoran cut the alter ,off to run 4hp is alot in a close race