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Old 10-17-2005, 08:12 PM
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How big an alternator do I need?

I'm trying to figure out if I can get away with running a mini-alternator (designed primarilly for racing applications) on my project car.

I have a severe space problem and running a mini alternator would make my life a lot easier.

I'm running a Ford 302 in a pretty agressive state of tune, I have no AC, no stereo, what sort of rating on an alternator am I looking for that will keep the battery charged?

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Old 10-17-2005, 08:32 PM
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You need to add up the total load of all of the circuits in the car. If you run a small 35 amp alternator and your fuel pump, ignition, headlights etc draw 42 amps then you're gonna drain your battery. I had that problem with the factory ~63 amp unit in my Blazer, I had to step up to a 100 amp piece.
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Old 10-17-2005, 11:57 PM
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Strip Vs Street

Doc here,

Is this going to be used as a Strip only application? Not a streeter?

If it is strip only , you need like Blazin' said to add up your current needs and upgrade accordingly, Ignition allow about 6 to 10 amps, fuel pump 7 to 25 amps (depending on what pump you use) Gauges, about 5 amps, and any accessories you run in addition.

If it is to be both..

Then you need to do a lot more math, Lighting, 60 amps, brake lamps 30 amps, turnsignals 15 to 25 amps, heater 30 amps, wipers 15 to 25 amps, power accessories (like seats, radio/cd , amplifiers, and the like..)

You need to decide which this is to be..and then you have 3 choices how to approach it..

If it is strip only, you can just match the draw as said.

If it is to be strip / street, you MUST total up your draw , match the alternator to the load and a little more..

Then, if you wish, you can with the aid of a battery switch separate the main power lead to the "A" side of the switch (the whole car) and then on the "B" side of the switch run only that which is required ONLY to race the that the load on the Alternator is minimal...hence less crank snout draw..

Or, as I have done on one of the Vettes that I use as a "weekend warrior"

I have a proper matched alternator, (for the total system) and using a magnetic window sensor from ADT, have it mounted to trigger at WOT , on throttle linkage..

In this manner, the system is online and charging until WOT,where the mag switch is tripped by the magnet on the throttle linkage, it fires a relay and that opens the Field line on the draw mechanically (minus the belt and bearing friction)..hence added HP..

If your out for every ounce of HP, this is a way to go if you still need to charge a battery for daily use..

Remember, for every 746 watts of power consumed, equals 1 horsepower at the crank snout..and you don't have to remember to throw switches to reconfigure for charge...

There are lots of ways to go at it, if your all out racing, I'd assume a Magneto, so no ignition power required, and if it is all out racing, why run an alternator at all? , you could trickle charge it between heats, and on a good battery, It should hold a charge enough for a Saturday's set of passes...and just carry a jump case..

If it's to be both, THEN you must design for total safe street operation first, and design around it for strip use..

Which is yours to be?

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Old 10-18-2005, 09:54 AM
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My intention is to design for safe street operation.

As I mentioned above this isn't really about horsepower, I have a packaging problem, making a fullsized alternator fit is going to be tricky to say the least. But if that's what I require then that's what I'll make work.

60 amps for lights? that's a lot mor than I'd have guessed, guess I should go look at the lights and get a real figure.
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