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Old 06-02-2007, 09:47 AM   #1
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Default cut-off switch

On our old car when we go to the track,we install a cut-off switch on the alternator,because at hight rpm the alternator rob horsepower.We make the 1/4 on the battery,can we make thet on a ssei 1994???On a old chevy 350 the rpm at idle raise 200 rpm .If we can make it on a ssei 1994,which wire we install the switch??just for the run at the track.
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Old 06-02-2007, 09:51 AM   #2
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No, most fuel injected GMs from like '86+ rely highly on the alternator, and the car will most likely stall out or die when you cut the alternator off, as the PCM needs a signal from the alternator.
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Old 06-02-2007, 09:59 AM   #3
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the signal is not from the battery?but from alternator??if the alternator fail the car stall immediatly???we can't drive on the battery for few distance??
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Old 06-02-2007, 10:07 AM   #4
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Pretty much from experience, when I disconnected the positive cable from the alternator on my '94, the car stalled with a known, good alternator.
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Old 06-02-2007, 10:32 AM   #5
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This is why a little battery corrosion on the cables can kill your car.

You don't want to anyway. Your ignition coils need the highest input possible to produce the hottest spark possible. With a battery only, you'll have a weak spark.
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Old 06-02-2007, 10:38 AM   #6
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I just ask this because we can make that on our old car ...we installed a 1000 amp bat and we make the run without alternator (I think the car needed a litlle voltage because we don't have pcm) it was just for 1/4 ......sorry for my english
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Old 06-02-2007, 10:39 AM   #7
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Amps will get you nowhere for spark. You step up the voltage through a coil (transformer) and the current drops proportionally to the voltage gain.

Voltage is what allows you to jump a spark gap, not current (amps).
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Old 06-02-2007, 10:51 AM   #8
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Let'* put this another way. Based on a known coil:

The MSD 8224 coil has a turns ratio of 80:1

At 12VDC that equates to a spark voltage of 960 V.
At 14VDC (typical alternator output) that voltage is 1,120.


15-20% hotter spark. I'll keep my alternator.
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Old 06-02-2007, 11:19 AM   #9
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Let'* look at this.

200A * 14v = 2800W = 3.75Hp

So even a 200A alternator at full load is pulling less than 4Hp.
Even the hottest MSD ignitions are pulling less than 10A at 6000rpm.
During a run at the track (lights, AC, stereo all off) the alternator has less than 50A load.

50A * 14v = 700W = 0.94Hp

This is less that the run to run variation you will see on a dyno.

The extra weight of the bigger battery costs as much as the gain from the alternator shut off.
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Old 06-02-2007, 11:21 AM   #10
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The ICM boosts the voltage going to the coils, so spark output is a lot more than 1,000 volts. Somewhere around 30,000 volts IIRC. So at 15% you'd be more like 30,000 vs 34,500 volts.
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